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Cited by 223Inflammatory mediators and NSAID effectsPain Mechanisms and TreatmentsAnesthesia and Pain Management

Paracetamol (acetaminophen): A familiar drug with an unexplained mechanism of action

Open access
Ayoub, Samir S·2021·Temperature
DOI 10.1080/23328940.2021.1886392PMC PMC8654482
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Abstract

Paracetamol (acetaminophen) is undoubtedly one of the most widely used drugs worldwide. As an over-the-counter medication, paracetamol is the standard and first-line treatment for fever and acute pain and is believed to remain so for many years to come. Despite being in clinical use for over a century, the precise mechanism of action of this familiar drug remains a mystery. The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to the inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted by paracetamol and on the nature of the molecular interactions with these enzymes. Paracetamol has been proposed to selectively inhibit COX-2 by working as a reducing agent, despite the fact that in vitro screens demonstrate low potency on the inhibition of COX-1 and COX-2. In vivo data from COX-1 transgenic mice suggest that paracetamol works through inhibition of a COX-1 variant enzyme to mediate its analgesic and particularly thermoregulatory actions (antipyresis and hypothermia). A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules. AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems. The current review gives an update and in some cases challenges the different theories on the pharmacology of paracetamol and raises questions on some of the inadequately explored actions of paracetamol.List of Abbreviations: AM404, N-(4-hydroxyphenyl)-arachidonamide; CB1R, Cannabinoid receptor-1; Cmax, Maximum concentration; CNS, Central nervous system; COX, Cyclooxygenase; CSF, Cerebrospinal fluid; ED50, 50% of maximal effective dose; FAAH, Fatty acid amidohydrolase; IC50, 50% of the maximal inhibitor concentration; LPS, Lipopolysaccharide; NSAIDs, Non-steroidal anti-inflammatory drugs; PGE2, Prostaglandin E2; TRPV1, Transient receptor potential vanilloid-1

AI snapshot

Written from the paper’s full text.

TL;DR

This review argues that paracetamol remains a first-line analgesic and antipyretic despite more than a century of use, because its molecular mechanism is still unresolved. It weighs competing COX-centered explanations against serotonergic, AM404/endocannabinoid/TRPV1, and COX-1-variant theories, favoring a central COX-1-derived target for analgesia and thermoregulation.

Key findings

  • In vitro COX inhibition by paracetamol is weak and inconsistent: one study reported IC30 values of 2.7 µg/ml for COX-1 and 20 µg/ml for COX-2, while another reported COX-2 selectivity with IC50 values of 25.8 µmol/L for COX-2 versus 113.7 µmol/L for COX-1.
  • The proposed COX-3/COX-1 variant was inhibited by paracetamol with an IC50 of 460 µM; in the author’s acetic acid writhing model, paracetamol’s analgesia and central PGE2 reduction were lost in COX-1 knockout mice but not COX-2 knockout mice, with ED50 values of 149 mg/kg for iloprost-induced and 172 mg/kg for acetic acid-induced constriction.
  • For thermoregulation, paracetamol-induced hypothermia and antipyresis, along with hypothalamic PGE2 reduction, were attenuated in COX-1−/− mice but not COX-2−/− mice; selective COX-1 inhibition with SC560 and COX-2 inhibition with celecoxib did not produce hypothermia.
  • AM404 was detected in human CSF in 14 of 26 patients at 5.1–57.4 nmol/L after 1 g intravenous paracetamol, but conversion rates were very low (about 0.009% in CSF and 0.0013% in rat plasma), below concentrations needed for several proposed in vitro targets; serotonergic involvement is indirect because paracetamol does not bind serotonergic receptors.

Methods

  • Narrative review synthesizing historical, preclinical, and clinical literature on paracetamol pharmacology, with particular emphasis on the author’s own rodent and cell-based experiments.
  • Evidence discussed includes COX-1 and COX-2 knockout mice, acetic acid-induced writhing and iloprost nociception models, LPS-induced fever, body temperature and PGE2 measurements, human CSF/plasma AM404 sampling, and cited randomized trials.
  • Main analysis is qualitative comparison of competing mechanistic theories rather than a systematic review, meta-analysis, or new pooled quantitative synthesis.

Limitations

  • The paper states that several hypotheses remain inadequately tested, including the lipid hydroperoxide/reducing-agent theory, which it says has not been tested in vivo; it also notes that AM404’s clinical relevance is unclear because most evidence comes from animals.
  • Evident from the text: as a narrative review, it does not describe systematic search criteria or formal bias assessment, and its interpretation often favors the author’s COX-1-variant hypothesis.
  • Evident from the text: many mechanistic conclusions rely on animal models, knockout mice, or in vitro systems, and some cited clinical findings are indirect or limited.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1080/23328940.2021.1886392

Cites

182 of 182 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

30 of 149 cited sources checked · 14 supported · 1 flagged · accuracy not yet measured

  • Findingcitation

    The authors' study reported a relatively similar conversion rate of plasma paracetamol to AM404 CSF (0.009%).

    “Our study reported on a relatively similar conversion rate of plasma paracetamol to AM404 CSF (0.009%) [106].”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites [106] source

      The Discussion (section 5) reports a conversion rate of 0.0013%, not 0.009%. The Results (section 4) measure AM404 in rat brain tissue, not cerebrospinal fluid (CSF). (LLM, conf=0.90)

      Checked by a reasoning model — may be wrong.

      Experimental — accuracy not yet measured
  • Premisecitation

    Using functional magnetic resonance imaging in humans, paracetamol was shown to reduce firing within the spinothalamic tract in response to thermal noxious stimulation.

    “Using functional magnetic resonance imaging in humans, paracetamol was shown to reduce firing within the spinothalamic tract in response to thermal noxious stimulation [26].”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites [26] source

      The Results (fMRI results) report reduced pain-related BOLD activation in several brain regions during noxious thermal stimulation after paracetamol. The Discussion suggests this may reflect inhibition of spinothalamic tracts, but the study did not measure firing within those tracts directly. (LLM, conf=0.90)

      Experimental — accuracy not yet measured
  • Premisecitation

    The analgesic action of paracetamol was shown not to be linked to inhibition of prostaglandin synthesis in the periphery.

    “In addition, the analgesic action of paracetamol was shown not to be linked to inhibition of prostaglandin synthesis in the periphery [21,23,27,28].”
    Quote in the paperSubstantial claim1 of 4 sources checked
    • cites [21] source
    • cites [23] source
    • cites [27] source

      The Discussion reports no significant effect of acetaminophen on PGE2 in gingival crevicular fluid and says this supports the theory that it does not block prostaglandins peripherally. But the Discussion also states that the study did not evaluate pain relief, so it did not directly show that acetaminophen’s analgesic action is independent of peripheral prostaglandin inhibition. (LLM, conf=0.85)

      Experimental — accuracy not yet measured
    • cites [28] Seppälä E, 1983, Int J Clin Pharmacol Res
  • Premisecitation

    Ex vivo animal studies have detected levels of AM404 in whole brain tissue equating to the low nanomolar range following systemic administration of standard analgesic doses of paracetamol in rats (300 mg/kg).

    “Ex vivo animal studies have detected levels of AM404 in whole brain tissue equating to the low nanomolar range following systemic administration of standard analgesic doses of paracetamol in rats (300 mg.kg -1 ) [85].”
    Quote in the paperNot a substantial claim1 of 1 sources checked
  • Premisecitation

    In 2005 Högestätt identified a new metabolite for paracetamol, AM404.

    “nonetheless in 2005 Högestätt identified a new metabolite for paracetamol [85].”
    Quote in the paperSubstantial claim1 of 1 sources checked
  • Classified as probably not a substantial claim. 11
  • May combine two claims. 31

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems.

    “AM404 as a metabolite for paracetamol has been proposed to activate the endocannabinoid and the transient receptor potential vanilloid-1 (TRPV1) systems.”
    Quote in the paperNot a substantial claim1 of 2 sources checked
    • cites [85] source
    • cites [86][87][88][89][90]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Premisecitation

    Mallet et al. (2010) showed attenuation of paracetamol-induced analgesia in the formalin, tail immersion and von Frey tests of nociception in mice lacking FAAH and TRPV1 and that intracerebroventricular administration of the TRPV1 channel antagonist capsazepine abolishes the analgesic action of paracetamol.

    “Mallet et al. (2010) showed attenuation of the paracetamol-induced analgesia in the formalin, tail immersion and von Frey tests of nociception in mice lacking FAAH and TRPV1 and that intracerebroventricular administration of the TRPV1 channel antagonist capsazepine to abolish the analgesic action of paracetamol [88].”
    Quote in the paperSubstantial claim1 of 1 sources checked

  • May combine two claims.

    Interpretationcitation

    AM404 has been demonstrated in in vitro studies to inhibit reuptake of anandamide, block TRPV1 and inhibit COX-1 and COX-2 activities in the high nanomolar to low micromolar concentration range, well above the concentrations the authors were able to detect in human CSF.

    “AM404 has been demonstrated in in vitro studies to inhibit reuptake of anandamide, block TRPV1 and inhibit COX-1 and COX-2 activities [86,98,[102][103][104][105] in the high nanomolar to low micromolar concentration range, well above the concentrations were able to detect in human CSF [101].”
    Quote in the paperSubstantial claim1 of 2 sources checked
    • cites [86,98,[102][103][104][105]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [101] source

  • May combine two claims.

    Findingcitation

    Paracetamol-induced hypothermia in normothermic mice and concomitant reduction in brain PGE2 synthesis were significantly attenuated in COX-1-/- mice compared to wild-type mice but not in COX-2-/- mice.

    “Previously, we showed that the paracetamol-induced hypothermia in normothermic mice and concomitant reduction in brain PGE 2 synthesis (Figure 6) were significantly attenuated in COX-1-/mice in comparison to wild-type mice but not in COX-2-/-mice [22].”
    Quote in the paperSubstantial claim1 of 1 sources checked

  • May combine two claims.

    Premisecitation

    Activated vascular endothelial cells respond through the induction of COX-2 (and not COX-1) that produces PGE2, which in turn resets the thermostatic control to a higher temperature.

    “These activated vascular endothelial cells respond through the induction of COX-2 (and not COX-1) that produces PGE 2 , which in turn resits the thermostatic control to a higher temperature [112,116,118].”
    Quote in the paperNot a substantial claim1 of 3 sources checked
    • cites [112] source
    • cites [116] Matsumura K, Cao C, Ozaki M, et al. Brain endothelial cells express cyclooxygenase-2 during lipopolysaccharide-induced fever: light and electron microscopic immunocytochemical studies. 1998;18(16):6279–6289. source

      The Abstract and the section “Timing of COX-2 expression and its physiological relevance” report COX-2 induction in brain endothelial cells during LPS-induced fever and propose that it promotes PGE2 production, which acts on CNS neurons to evoke fever. The paper treats endothelial PGE2 production as a hypothesis still requiring confirmation; it does not establish the claimed exclusion of COX-1 or describe a reset of thermostatic control. (LLM, conf=0.87)

      Experimental — accuracy not yet measured
    • cites [118] source

  • May combine two claims.

    Premisecitation

    The evidence supporting centrally mediated paracetamol-induced hypothermia, antipyresis and analgesia comes from a large volume of published literature spanning 6 decades, including direct in vivo experiments correlating pharmacological actions with reduction in CNS PGE2 synthesis.

    “The evidence that support the paracetamolinduced hypothermia and indeed the paracetamolinduced antipyresis and analgesia are centrally mediated is supported by a large volume of published literature that spans 6 decades [18][19][20][21][22][23]27,28,96,[131][132][133][134][135], which comes from direct in vivo experiments in which the pharmacological actions of paracetamol are directly correlated”
    Quote in the paperNot a substantial claim1 of 3 sources checked
    • cites [18] Feldberg W, 1972, BrJ Pharmacol source

      Split into 2 atoms — 0 confirmed, 0 partially supported, 0 disputed, 0 misattributed, 2 unverifiable. Expand the claim to see per-atom verdicts.

      Experimental — accuracy not yet measured
    • cites [19] Yaksh TL, 1993, Agents Actions
    • cites [20] source

  • May combine two claims.

    Findingcitation

    Paracetamol-induced hypothermia in normothermic animals and humans is a reversible and nontoxic action that temporally correlates with the pharmacokinetic profile of paracetamol.

    “The reported paracetamol-induced hypothermia in normothermic animals [22,38,135] and humans [140][141][142][143][144][145] is a reversable and nontoxic action that temporally correlates with the pharmacokinetic profile of paracetamol [22,135]”
    Quote in the paperSubstantial claim3 of 5 sources checked
    • cites [22] source

      The Results report that hypothermia in mice resolved within 4–5 hours and correlated with acetaminophen plasma concentrations. The Discussion describes human hypothermia as short-lived and generally not presenting serious problems, but also notes concerns about adverse consequences in young children. It does not establish that the effect is nontoxic across animals and humans. (LLM, conf=0.86)

      Experimental — accuracy not yet measured
    • cites [38] source

      The Discussion (section 4; page unavailable) reports acetaminophen-induced hypothermia in mice and humans and says its duration approximately coincided with the drug’s plasma half-life. It describes the effect as acute, which is consistent with reversibility, but does not establish that the hypothermia is nontoxic. (LLM, conf=0.84)

      Experimental — accuracy not yet measured
    • cites [135] source
    • cites [140] Van Tittelboom T, 1989, Vet Hum Toxicol

      Not checked in the free run.

    • cites [141] source

      The Results (Secondary outcomes) report hypothermia after acetaminophen in 11 children, supporting the human-hypothermia portion of the citation. The Methods (Subjects) specify that these children were febrile, not normothermic. The sentence attaches separate citations to the reversible, nontoxic, pharmacokinetic characterization, so that characterization is not attributed to this source. (LLM, conf=0.86)

      Experimental — accuracy not yet measured

  • May combine two claims.

    Findingcitation

    The dose of paracetamol used by Fischer et al. (1981) to study the pharmacokinetics of a toxic dose in mice was 500 mg/kg, well above the 200 mg/kg and 300 mg/kg doses used by us.

    “The authors also go on reference Fischer et al. (1981) in which the dose of paracetamol used to study the pharmacokinetics of a toxic dose of paracetamol in mice was 500 mg/kg [157], well above the 200 mg/ kg and 300 mg/kg doses used by us [22,100,135].”
    Quote in the paperNot a substantial claim1 of 4 sources checked
    • cites [157] Fischer LJ, 1981, J Pharmacol Exp Ther source
    • cites [22] source
    • cites [100] source
    • cites [135] source

  • May combine two claims.

    Findingcitation

    Diclofenac inhibited the acetic acid-induced constriction with an ED50 value of 16 mg/kg, but had no effects on the iloprost-induced constriction.

    “In contrast, diclofenac, a peripherally active NSAID [46,47], inhibited the acetic acid-induced constriction with an ED 50 value of 16 mg/kg, but had no effects on the ilorpost-induced constriction [23].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [23] source

  • May combine two claims.

    Findingcitation

    In the acetic acid-induced abdominal constriction model, the analgesic action of paracetamol was accompanied by reduction of prostaglandin synthesis in the brain and spinal cord, but not in the peritoneum; effects were abrogated in COX-1 homozygote knockout mice, but not in COX-2 homozygote knockout mice.

    “Using the acetic acid-induced abdominal constriction model of acute nociceptive pain, we showed that the analgesic action of paracetamol was accompanied by reduction of prostaglandin synthesis in the brain and spinal cord, but not in the peritoneum; effects that were abrogated in COX-1 homozygote knockout mice, but not in COX-2 homozygote knockout mice [23].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [23] source

  • May combine two claims.

    Findingcitation

    The hypothermic action of paracetamol in mice is not mediated through AM404 and is not dependent on activation of the endocannabinoid or TRPV1 pathways.

    “Using similar experimental approaches to those reported above, we showed that the hypothermic action of paracetamol in mice is not mediated through AM404 and is not dependent on activation of the endocannabinoid or TRPV-1 pathways [100].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [100] source

  • May combine two claims.

    Findingcitation

    The hypothermic action of cannabinoid receptor-1 and TRPV1 agonists is not mediated through the inhibition of prostaglandin synthesis, demonstrating parallel rather than interdependent pathways.

    “In addition, we showed that the hypothermic action of cannabinoid receptor-1 and TRPV-1 agonists is not mediated through the inhibition of prostaglandin synthesis demonstrating parallel rather than interdependent pathways [100].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [100] source

  • May combine two claims.

    Findingcitation

    In the same cell line, paracetamol did not inhibit the catalytic activity induced by the pro-inflammatory stimulus LPS, with the intracellular lipid hydroperoxide tone remaining unchanged after the addition of LPS.

    “In the same cell line, paracetamol did not inhibit the catalytic activity induced by the pro-inflammatory stimulus LPS with the intracellular lipid hydroperoxide tone remaining unchanged after the addition of LPS [71].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [71] source

  • May combine two claims.

    Findingnone given

    Paracetamol is able to induce hypothermia and reduce brain PGE2 synthesis in the absence and presence of fever within 30 minutes after administration.

    “given the fact that paracetamol is able to induce hypothermia and reduce brain PGE 2 synthesis in the absence and presence of fever within 30 minutes after administration (insufficient time for the induction of COX-2)”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    Therapeutically administered paracetamol induced potent hypothermic and antipyretic actions in COX-1+/+ mice with established pyrexia, which were partly lost in COX-1-/- mice.

    “a notion that is supported by the finding that therapeutically administered paracetamol-induced potent hypothermic and anti-pyretic actions in COX-1+/+ mice (with established pyrexia), which were partly lost in COX-1-/-mice.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [135] source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingcitation

    Bashir et al. (2020) present no data to support their argument challenging the notion that paracetamol-induced hypothermia is mediated through inhibition of a COX-1 variant.

    “Bashir et al. (2020) attempt to the challenge the notion that the paracetamol-induced hypothermia is mediated through the inhibition of a COX-1 variant (and other COX enzymes including COX-1 and COX-2), but present no data to support this argument [155].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [155] source

  • May combine two claims.

    Premisecitation

    Paracetamol showed selectivity for inhibition of COX-3 (IC50 = 460 μM) over COX-1 and COX-2, which was dependent on the concentration of the substrate arachidonic acid suggesting competitive blockage at the active site.

    “Paracetamol showed selectivity for inhibition of COX-3 (IC 50 = 460 µM) over COX-1 and COX-2, which was dependent on the concentration of the substrate arachidonic acid suggesting competitive blockage at the active site [36].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [36] source

  • May combine two claims.

    Premisecitation

    A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules.

    “A separate line of research provides evidence on potentiation of the descending inhibitory serotonergic pathway to mediate the analgesic action of paracetamol, but with no evidence of binding to serotonergic molecules.”
    Quote in the paperNot a substantial claim0 of 4 sources checked
    • cites [72] source
    • cites [73] source
    • cites [74][75][76][77]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [84] source

  • May combine two claims.

    Interpretationreasoning

    Paracetamol's lipid solubility enables it to rapidly penetrate cellular membranes and readily cross the blood-brain barrier.

    “Its lipid solubility enables it to rapidly penetrate cellular membranes and to also readily cross the blood-brain barrier.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Several reports disputed the expression of a fully functional COX-3 protein in human and rodent tissues as retention of intron-1 in the mRNA sequence introduced a stop codon early in the translational process resulting in a truncated protein with no catalytic activity.

    “Several reports disputed the expression of a fully functional COX-3 protein in human and rodent tissues as retention of intron-1 in the mRNA sequence introduced a stop codon early in the translational process resulting in a truncated protein with no catalytic activity [37][38][39][40][41].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [37][38][39][40][41]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Classified as probably not a substantial claim.

    Premisecitation

    The reduction of central nervous system PGE2 synthesis by paracetamol may be mediated through inhibition of a COX enzyme yet to be discovered.

    “In this report, the authors conclude that the reduction of central nervous system (CNS) PGE 2 synthesis by paracetamol may be mediated through inhibition of a COX enzyme yet to be discovered [33], a notion that has also been predicted by others [34,35].”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [33] source
    • cites [34] Botting RM, 2000, Clin Infect Dis source
    • cites [35] source

  • May combine two claims.

    Premisecitation

    Paracetamol is a moderately lipid-soluble weak organic acid with a pka value of 9.5 and is largely un-ionized over the physiological range of pH.

    “Paracetamol is a moderately lipid-soluble weak organic acid with a pka value of 9.5 and is largely un-ionized over the physiological range of pH [29].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [29] source

  • May combine two claims.

    Premisecitation

    Despite potent inhibition by paracetamol of brain and spinal cord-derived prostaglandins in in vivo experiments, in vitro screening experiments demonstrated weak inhibitory activities on COX-1 and COX-2 enzymes by paracetamol.

    “Despite the potent inhibition by paracetamol of brain and spinal cord-derived prostaglandins in in vivo experiments, in vitro screening experiments demonstrated weak inhibitory activities on COX-1 and COX-2 enzymes by paracetamol [33].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [33] source

  • Classified as probably not a substantial claim. May combine two claims.

    Premisenone given

    The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted and on the nature of the molecular interactions with these enzymes.

    “The oldest and most prevailing theory on the mechanism of analgesic and antipyretic actions of paracetamol relates to the inhibition of CNS cyclooxygenase (COX) enzyme activities, with conflicting views on the COX isoenzyme/variant targeted by paracetamol and on the nature of the molecular interactions with these enzymes.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    A standard therapeutic dose of paracetamol produces 80% bioavailability and reaches peak plasma level (Cmax) of 18 mg/L (120 μM) after approximately 120 minutes and a cerebrospinal fluid Cmax of 8.8 mg/ml at around 240 minutes.

    “A standard therapeutic dose of paracetamol produces 80% bioavailability and reaches peak plasma level (Cmax) of 18 mg/L (120 µM) after approximately 120 minutes and a cerebrospinal fluid (CSF) Cmax of 8.8 mg/ml at around 240 minutes [31].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [31] source

  • Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing).

    Premisecitation

    Paracetamol as a reducing agent has been proposed to work by lowering the intracellular lipid hydroperoxide tone, driving COX-1 and COX-2 enzymes into the inactive reduced state and ultimately reducing prostaglandin synthesis.

    “Paracetamol as a reducing agent has been proposed to work by lowering the intracellular lipid hydroperoxide tone driving COX-1 and COX-2 enzymes into the inactive reduced state ultimately reducing prostaglandin synthesis [65].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [65] source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Activation of the descending inhibitory serotonergic pathway by paracetamol has been proposed as a possible mechanism for the analgesic action of paracetamol in humans and experimental animals.

    “Activation of the descending inhibitory serotonergic pathway by paracetamol has been proposed as a possible mechanism for the analgesic action of paracetamol in humans [72,73] and experimental animals [74][75][76][77].”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites [72] source
    • cites [73] source
    • cites [74][75][76][77]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Premisecitation

    In cultured hippocampal slices and microglial cells, AM404 was shown to inhibit the LPS-mediated PGE2 production in a TRPV1 independent manner and was also able to decrease the expression of COX-2 protein.

    “In support of a non-TRPV1 mediated action for AM404, in cultured hippocampal slices and microglial cells AM404 was shown to inhibit the LPS-mediated PGE 2 production in a TRPV1 independent manner and was also able to decrease the expression of COX-2 protein [98].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [98] source

  • May combine two claims.

    Interpretationcitation

    Activation of the serotonergic pathway cannot fully explain the mechanism of analgesic action of paracetamol; the interaction between paracetamol and the descending inhibitory serotonergic system is indirect and perhaps a by-product of inhibition of a centrally expressed COX variant enzyme.

    “Activation of this pathway cannot fully explain the mechanism of analgesic action of paracetamol as it was shown that paracetamol did not have affinity for any of the serotonergic receptor types or subtypes or to any of the enzymes involved in the synthesis or degradation of serotonin [84]; hence, this interaction between paracetamol and the descending inhibitory serotonergic system is an indirect”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [84] source

  • May combine two claims.

    Premisecitation

    AM404 as a novel metabolite for paracetamol was shown to activate the endocannabinoid system and to also activate the TRPV1 channel, both of which are involved in the modulation of pain signaling and proposed to mediate the analgesic action of paracetamol.

    “AM404 as a novel metabolite for paracetamol was shown to activate the endocannabinoid system and to also activate the transient receptor potential vanilloid-1 (TRPV-1) channel, both of which are involved in the modulation of pain signaling and proposed to mediate the analgesic action of paracetamol [86][87][88][89][90].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [86][87][88][89][90]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationreasoning

    The target for paracetamol-induced hypothermia is not COX-1 or COX-2 and is likely to be a variant of COX-1.

    “We therefore conclude that the target for the paracetamol-induced hypothermia is not COX-1 or COX-2 and is likely to be a variant of COX-1.”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: Paracetamol reduces body temperature through the induction of hypothermia through inhibition of a constitutively expressed COX-1 variant enzyme.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Blockade of brain PGE2 synthesis by NSAIDs has been associated with the antipyretic action of these drugs, which has been suggested to be due to inhibition of inducible COX-2 produced by hypothalamic vascular endothelial cells.

    “Blockade of brain PGE 2 synthesis by NSAIDs has been associated with the antipyretic action of these drugs, which has been suggested to be due to inhibition of inducible COX-2 produced by hypothalamic vascular endothelial cells [124,125].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [124] source
    • cites [125] source

  • May combine two claims.

    Premisecitation

    Mice lacking the PGE2 receptor EP3 or microsomal prostaglandin E synthase-1 enzyme were shown to have impaired febrile responses.

    “Mice lacking the PGE 2 receptor EP3 [112] or microsomal prostaglandin E synthase-1 enzyme [121,122] were shown to have impaired febrile responses.”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites [112] source
    • cites [121] source
    • cites [122] source

  • May combine two claims.

    Interpretationcitation

    The authors report a conversion rate of plasma paracetamol to AM404 of only 0.0013%, which would result in low concentrations expected to have negligible pharmacological activities.

    “However, the authors report on a conversion rate of plasma paracetamol to AM404 of only 0.0013%, which would result in low concentrations expected to have negligible pharmacological activities.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [16] source

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    NSAIDs induce antipyresis through inhibition of the inducible COX-2 enzyme expressed in hypothalamic vascular endothelial cells.

    “As NSAIDs are believed to induce antipyresis through inhibition of the inducible COX-2 enzyme expressed in hypothalamic vascular endothelial cells [112,127]”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [112] source
    • cites [127] Abe M, 2001, Brain Res source

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    The suprachiasmatic nucleus within the anterior medial zone of the hypothalamus is involved in circadian control of the sleep-wake cycle and body temperature.

    “Indeed, the suprachiasmatic nucleus within the anterior medial zone of the hypothalamus is known to be involved in circadian control of sleep-wake cycle and body temperature.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim. May combine two claims.

    Premisereasoning

    At therapeutic dose paracetamol is a safe drug, but with a narrow therapeutic window, it is easy to accidentally or deliberately over-dose.

    “At therapeutic dose paracetamol is a safe drug, but with a narrow therapeutic window, it is easy to accidently or deliberately over-dose.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    Paracetamol-induced hypothermia is not mediated by AM404 and does not involve activation of the endocannabinoid or TRPV1 systems.

    “These findings demonstrate that the paracetamol-induced hypothermia is not mediated by AM404 and does not involve activation of the endocannabinoid or TRPV1 systems.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The concentrations of paracetamol used by Bashir et al. are pharmacologically irrelevant and do not represent plasma paracetamol concentrations in humans or rodents.

    “Thus, despite being nontoxic to the cells, the concentrations of paracetamol used by these authors are pharmacologically irrelevant and do not represent plasma paracetamol concentrations in humans or rodents [31,135].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [31] source
    • cites [135] source
39 of 39

  • The authors state this as an assumption.

    Dental inflammatory reactions present low-grade inflammation.

    “The notion that paracetamol produces anti-inflammatory actions in such dental inflammatory reactions is based on the assumption that such inflammatory reactions present lowgrade inflammation, an idea that has not been tested thoroughly.”

  • The paper relies on this without support.

    The acetic acid-induced constriction model represents a human model of acute pain that shares similar pathologies to human pain.

    “The acetic acid-induced constriction model represents a human model of acute pain that shares similar pathologies to human pain.”

  • The authors state this as an assumption.

    Paracetamol reduces body temperature through the induction of hypothermia through inhibition of a constitutively expressed COX-1 variant enzyme.

    “We, therefore, make the assumption that paracetamol reduces body temperature through the induction of hypothermia through inhibition of a constitutively expressed COX-1 variant enzyme [22,135].”
3 of 3

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Figure 7 says an effect was abolished rather than partly lost

    Major

    The caption says paracetamol's antipyretic and PGE 2 effects were abolished in COX-1 knockout mice. The discussion instead describes the antipyretic effect as partly lost and the reduction in PGE 2 as significantly attenuated.

  • AM404 is said both to activate and block TRPV1

    Major

    One passage says AM404 activates the TRPV1 channel, while another says it blocks TRPV1. The text does not distinguish conditions that would reconcile these opposite actions.

  • Qin figure caption points to an Ayoub reference

    Minor

    The caption attributes the figure to Qin et al. (2005) but cites reference 43, which lists an Ayoub et al. (2008) paper.

  • Paracetamol is described as active before its first synthesis

    Minor

    The text says paracetamol was first synthesized in 1893, but also says its antipyretic activity was found in the 1880s.

  • Figure 7 citation names different authors and year

    Minor

    The caption attributes Figure 7 to Ayoub and Flower (2019), but its cited reference 130 is Engström et al. (2013).

  • Figure 5 citation names different authors and year

    Minor

    The caption attributes Figure 5 to Ayoub et al. (2011), but its cited reference 72 is Pickering et al. (2006).

  • Muramatsu claim cites the Flower and Vane paper

    Minor

    The Muramatsu claim cites reference 16, but the reference list identifies 16 as a 1972 paper by Flower and Vane.

  • The paper's own conversion-rate claim cites Muramatsu

    Minor

    The text identifies the 0.009% conversion rate as a result of the author's study, but reference 106 lists Muramatsu et al., whose work is discussed separately in the preceding sentences.

8 of 8

Cites

Flagged first, then by citations

Research works printed but not linked: 22

  1. Von Mering J., 1893, Therap. Monatsch, 7, 558
  2. Brodie BB, 1949, J Pharmacology Exp Ther, 97, 58
  3. Wober C, 2000, Funct Neurol, 15, 89
  4. Dahl V, 2000, Scand, 44, 1191
  5. HKH L, 2008, J Emerg Crit Care Med, 15, 5
  6. Yaksh TL, 1993, Agents Actions, 41, 89
  7. Seppälä E, 1983, Int J Clin Pharmacol Res, 3, 265
  8. Botting RM, 2000, Clin Infect Dis, 31
  9. Tonussi CR, 1994, Eur J Pharmacol, 14, 73
  10. Gyires K, 1975, Pol J Pharmacol Pharm, 27, 257
  11. Pelissier T, 1996, J Pharmacol Exp Ther, 278, 8
  12. Soukupová M, 2010, Neuro Endocrinol Lett, 31
  13. Giuffrida A, 2001, J Pharmacol Exp Ther, 298, 7
  14. Walson PD, 1992, Am J Dis Child, 146, 626
  15. Abe M, 2001, Brain Res, 1, 1
  16. 10.1016/j.prostaglandins.2007.10.007 · DOI · Paper page
  17. Ryseck RP, 1992, Cell Growth Differ, 3, 443
  18. Van Tittelboom T, 1989, Vet Hum Toxicol, 31, 57
  19. Foster J, 2016, Front Pharmacol, 7, 54
  20. Fischer LJ, 1981, J Pharmacol Exp Ther, 219, 281
  21. Tittarelli R, 2017, Eur Rev Med Pharmacol Sci, 21, 95
  22. Day M, 2020, BMJ, 368:m1086

Cited by

Sorted by citations

Related

Authors
Ayoub, S. S.
Affiliations
University of East London (GB)
Corresponding author
Ayoub, Samir S
ORCID
Ayoub, Samir S
Venue
Temperature
Publisher
Informa UK Limited
Published
2021-03-16
Volume
8
Issue
4
Pages
351-371
ISSN
  • 2332-8940 (ISSN-L)
  • 2332-8959
DOI
10.1080/23328940.2021.1886392
PMC
PMC8654482
OpenAlex
W3129333098
Cited by
223
References
178
Citations by year
2026: 59 · 2025: 73 · 2024: 58 · 2023: 22 · 2022: 10 · 2021: 1
FWCI
2.88
Type
journal article
Language
en
Open access
Locations
  • Temperature (publishedVersion)
  • PubMed Central (publishedVersion)
  • PubMed (publishedVersion)
  • UEL Research Repository (University of East London) (submittedVersion, cc-by-nc)
Topics
  • Health Sciences › Medicine › Pharmacology › Inflammatory mediators and NSAID effects
  • Health Sciences › Medicine › Physiology › Pain Mechanisms and Treatments
  • Health Sciences › Medicine › Surgery › Anesthesia and Pain Management
Keywords
paracetamol, mechanism of action, cyclooxygenase inhibition, cyclooxygenase-1, cyclooxygenase-2, fever reduction, analgesia, descending serotonergic pathways, AM404, endocannabinoid system, TRPV1

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-11.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-11.

Cited sources are checked by gpt-6-sol.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

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