Aspirin’s mechanism of action

Aspirin is a medication that has two main actions in the body.

  1. An anti-prostaglandin (anti-inflammation, fever-reducing, pain reliever) and
  2. An anti-platelet (‘blood thinner’) agent.

Both of these actions are the result of the effect of aspirin on an enzyme in the body called cyclo-oxygenase, or COX.

Mechanism of Action animation

COX and Inflammation
When the body is injured, inflammation occurs as part of the natural healing process. Inflammation is facilitated by a number of enzymes, mediators and different cells, such as white blood cells. One enzyme involved in inflammation is cyclo-oxygenase (COX). COX is responsible for the formation of a group of inflammatory mediators known as prostaglandins.

  • Aspirin inhibits COX, in turn stopping the formation of prostaglandins; hence aspirin acts as an anti-inflammatory agent in this process.

Uses of Aspirin

COX and ‘blood thinning’
COX also plays a role in cessation of bleeding. Blood clotting is the result of a complex mechanism which involves many different cells, including a type known as platelets. When blood vessels are damaged or are diseased, platelets clump together over the hole or vessel tear to facilitate repair. COX activates a chemical known as thromboxane A2 that causes platelets to stick together to form a 'plug' over the damaged area. The aggregation of platelets (plug), in concert with the clotting process, results in a fibrin clot which stops bleeding and aids repair of the blood vessel.

Aspirin inhibits COX, thereby reducing the ability of platelets to aggregate. This is why aspirin is known as a ‘blood thinner’ or anti-platelet agent. It is also why increased bleeding is a side effect of aspirin. Although major bleeding is rare, aspirin can cause this. ‘Blood thinning’ however, is useful to prevent heart attacks and strokes. It is important to talk to your doctor before starting aspirin even though you can buy it without a prescription.

Side effects of aspirin
Prostaglandins are produced by cells in the lining of the stomach and form a protective barrier against the harsh stomach acid.")

  • Aspirin inhibits the formation of prostaglandins, in turn depleting the protective barrier in the stomach, leading to stomach (or peptic) ulcers.