Comparing How Aspirin and Acetaminophen Work

Aspirin and acetaminophen are two popular over-the-counter medications that share a similar indication for use: the relief of pain. However, each medication has additional indications that allow the average consumer to choose whether aspirin or acetaminophen will best address their needs. The different indications for use are a reflection of differences in how aspirin and acetaminophen actually work.

Acetominophen is commonly known by the brand name Tylenol. This otc pain relieving agent is used for mild to moderate pain. Tylenol is also used as an antipyretic (fever reducer) and is preferred over aspirin for use in children. Although aspirin can be used to relieve fevers in adults, it can cause a potentially lethal condition called Reye's syndrome if it is given to children with viral infections.

Aspirin is also known as acetylsalicylic acid. In addition to it's use as a pain reliever, aspirin is often used for inflammation, acute therapy for heart attacks, unstable angina and transient ischemic attacks. It is also used as prophylaxis (prevention) of heart attacks and strokes in some individuals.

Acetaminophen and aspirin both work by inhibiting the enzyme cyclooxygenase resulting in decreased production of prostaglandins, key players in inflammation, pain and fever. However, acetaminophen reversibly inhibits cyclooxygenase (COX), while aspirin irreversibly inhibits this enzyme.

Aspirin and Acetaminophen both work as fever reducers by interfering with prostaglandin E2 (PGE2) synthesis in the hypothalamus of the brain. Without the interference from these pain relieving agents, the hypothalamus would respond to the cytokine interleukin-1 (IL-1) by resetting the body's thermostat at a higher temperature.

Additional aspects of the mechanism of action of aspirin include:

-Anti-inflammatory: Aspirin decreases the ability of white blood cells known as granulocytes to adhere to damaged vasculature, inhibits migration of white blood cells and stabilizes lysozymes.

-Antiplatelet: Aspirin decreases platelet production of thromboxane A2 (TXA2) which is a platelet activator and vasoconstrictor. This OTC antiplatelet medication also decreases endothelial cell production of prostaglandin I2 (PGI2) which is a known inhibitor of platelet aggregation and vasodilation. Because endothelial cells can resynthesize the COX enzyme, but platelets cannot, the net effect of aspirin is anti-platelet.

Acetaminophen does not have the anti-inflammatory and anti-platelet effects of aspirin. These important differences between acetaminophen and aspirin depend on the ability of acetaminophen to work better in the central nervous system (brain and spinal cord) than in peripheral tissues. Acetaminophen may be inactivated by enzymes called peroxidases that are released during the inflammatory response.

While Aspirin and Tylenol share many uses, there are significant differences in the ways in which these medications work. As a result, aspirin is the better of the two if an anti-inflammatory or anti-platelet effect is desired. On the other hand, acetaminophen is particularly effective for pain accompanied by fever. Individuals should always speak with their physician about any questions or concerns they may have regarding medications.

Source: Johannsen, E. MD. Pharmcards. 2nd Ed. LW&W. 2002.