| Summary: | Although it is more than a century since it was first marketed, acetaminophen remains one of the most popular analgesic agents. In addition, acetaminophen has recently been applied to multimodal analgesia in combination with non-steroidal anti-inflammatory drugs, and its consumption significantly increased during the pandemic of coronavirus disease 2019 as well as diclofenac and ibuprofen. However, the detailed mode of analgesic action of acetaminophen is still unclear. In the present study, we comprehensively discuss conventional, recognized, and postulated mechanisms of analgesic acetaminophen and highlight the current mechanistic concepts while comparing with diclofenac and ibuprofen. Acetaminophen inhibits cyclooxygenase with selectivity for cyclooxygenase-2, which is higher than that of ibuprofen but lower than that of diclofenac. In contrast to diclofenac and ibuprofen, however, anti-inflammatory effects of acetaminophen depend on the extracellular conditions of inflamed tissues. Since the discovery of cyclooxygenase-3 in the canine brain, acetaminophen had been hypothesized to inhibit such a cyclooxygenase-1 variant selectively. However, this hypothesis was abandoned because cyclooxygenase-3 was revealed not to be physiologically and clinically relevant to humans. Recent studies suggest that acetaminophen is deacetylated to 4-aminophenol in the liver and after crossing the blood–brain barrier, it is metabolically converted into <i>N</i>-(4-hydroxyphenyl)arachidonoylamide. This metabolite exhibits bioactivities by targeting transient receptor potential vanilloid 1 channel, cannabinoid receptor 1, Cav3.2 calcium channel, anandamide, and cyclooxygenase, mediating acetaminophen analgesia. These targets may be partly associated with diclofenac and ibuprofen. The perspective of acetaminophen as a prodrug will be crucial for a future strategy to develop analgesics with higher tolerability and activity.
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