Mechanism of action

The main mechanism of action of paracetamol is considered to be the inhibition of cyclooxygenase(COX), and recent findings suggest that it is highly selective for COX-2. While it has analgesic and antipyretic properties comparable to those of aspirin or other NSAIDs, its peripheral anti-inflammatory activity is usually limited by several factors, one of which is high level of peroxides present in inflammatory lesions. However, in some circumstances, even peripheral anti-inflammatory activity comparable to other NSAIDs can be observed.

Because of its selectivity for COX-2 it does not significantly inhibit the production of the pro-clottingthromboxanes.

The COX family of enzymes are responsible for the metabolism of arachidonic acid toprostaglandin H₂, an unstable molecule that is, in turn, converted to numerous other pro-inflammatory compounds. Classical anti-inflammatories such as the NSAIDs block this step. Only when appropriately oxidized is the COX enzyme highly active.

Paracetamol reduces the oxidized form of the COX enzyme, preventing it from forming pro-inflammatory chemicals. This leads to a reduced amount of Prostaglandin E2 in the CNS, thus lowering the hypothalamic set-point in the thermoregulatory centre.

Paracetamol also modulates the endogenous cannabinoid system. Paracetamol is metabolized to AM404, a compound with several actions; what is most important is that it inhibits the uptake of the endogenous cannabinoid/vanilloid anandamide by neurons. Anandamide uptake would result in the activation of the main pain receptor (nociceptor) of the body, the TRPV1 (older name: vanilloid receptor). Furthermore, AM404 inhibits sodium channels, as do the anesthetics lidocaine and procaine. Either of these actions by themselves has been shown to reduce pain, and are a possible mechanism for paracetamol. However, it has been demonstrated that, after blocking cannabinoid receptors with synthetic antagonists, paracetamol's analgesic effects are prevented, suggesting its pain-relieving action involves activation of the endogenous cannabinoid system.

The exact mechanisms how COX is inhibited in various circumstances is still subject of discussion. Because of differences in the activity of paracetamol, aspirin, and other NSAIDs, it has been postulated that further COX variants may exist. A recently discovered COX-1 splice variant termed COX-3 was considered to explain some of the knowledge gap but newer findings do not support the hypothesis that it plays any significant role in the functioning of paracetamol.

Aspirin is known to inhibit the cyclooxygenase (COX) family of enzymes and, because paracetamol's action is partially similar to aspirin's, much research has focused on whether paracetamol also inhibits COX. It is now clear that paracetamol acts via at least two pathways.

One theory holds that paracetamol works by inhibiting the COX-3 isoform of the COX family of enzymes. When expressed in dogs, this enzyme shares a strong similarity to the other COX enzymes, produces pro-inflammatory chemicals, and is selectively inhibited by paracetamol. However, some research has suggested that, in humans and mice, the COX-3 enzyme is without inflammatory action.Another possibility is that paracetamol blocks cyclooxygenase (as in aspirin), but that is in an inflammatory environment where the concentration of peroxides is high, and the high oxidation state of paracetamol prevents its actions. This would mean that paracetamol has no direct effect at the site of inflammation, but instead acts in the CNS where the environment is not oxidative, to reduce temperature, etc.The exact mechanism by which paracetamol is believed to affect COX-3 is disputed.