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

The DOI matches its Crossref record; 182 of the 182 references it cites were found in a registry or an index; 30 of the 149 sources its claims cite were checked for support.

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

Open access
Ayoub, Samir S·2021·Temperature

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

Overview

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.

Verification

Claims the paper makes

58 No full text of the sources · 14 Own interpretation · 13 Partly supported · 7 Supported · 7 Sources not found · 6 Own result · 5 No basis given · 4 Supported so far · 4 Not checked yet · 1 Not supported · 1 Couldn’t confirm · accuracy not yet measured

Verify all 1 references — free account

Show all 81 claims

  • Probably not a substantial claim. 11
  • It may state two separate claims as one. 31
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

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8 of 8

Citations

Cites

Show all 178 references

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

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Metadata

Authors
Ayoub, S. S.
Affiliations
University of East London (GB)
Corresponding author
Ayoub, Samir S
Venue
Temperature
Published
2021-03-16
Volume
8
Issue
4
Pages
351-371
ISSN
  • 2332-8940 (ISSN-L)
  • 2332-8959
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 (medical) › Inflammatory mediators and NSAID effects
  • Health Sciences › Medicine › Physiology (medical) › Pain Mechanisms and Treatments
  • Health Sciences › Medicine › Anesthesiology and Pain Medicine › 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

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