How does Tylenol work?

Tylenol (paracetamol) has a Wikipedia entry (reference 1). Under "Mechanism of Action," you will find the following:

"Paracetamol has long been suspected of having a similar mechanism of action to aspirin because of the similarity in structure. That is, it has been assumed that paracetamol acts by reducing production of prostaglandins, which are involved in the pain and fever processes, by inhibiting the cyclooxygenase (COX) enzyme.

However, there are important differences between the effects of aspirin and those of paracetamol. Prostaglandins participate in the inflammatory response, but paracetamol has no appreciable anti-inflammatory action. Furthermore, COX also produces thromboxanes, which aid in blood clotting - aspirin reduces blood clotting, but paracetamol does not. Finally, aspirin and the other NSAIDs commonly have detrimental effects on the stomach lining, where prostaglandins serve a protective role, but paracetamol is safe.

Indeed, while aspirin acts as an irreversible inhibitor of COX and directly blocks the enzyme's active site, paracetamol indirectly blocks COX, and that this blockade is ineffective in the presence of peroxides.[2] This might explain why paracetamol is effective in the central nervous system and in endothelial cells but not in platelets and immune cells which have high levels of peroxides.

In 2002 it was reported that paracetamol selectively blocks a variant of the COX enzyme that was different from the then known variants COX-1 and COX-2.[3] This enzyme, which is only expressed in the brain and the spinal cord, is now referred to as COX-3. Its exact mechanism of action is still poorly understood, but future research may provide further insight into how it works.

A single study has shown that administration of paracetamol increases the bioavailability of serotonin (5-HT) in rats,[4] but the mechanism is unknown and untested in humans."

In other words, it probably works by blocking a COX (cyclooxygenase) enzyme, similar to the action of non-steroidal anti-inflammatory agents (NSAIDs), like "Aleve."