COX-1 is an enzyme which is normally present in a variety of tissues in the body, including sites of inflammation and the stomach. Some of the prostaglandins made by COX-1 protect the inner lining of the stomach. Common NSAIDs such as aspirin block both COX-1 and COX-2 (see below). When the COX-1 enzyme is blocked, inflammation is reduced, but the protection of the lining of the stomach also is lost. This can cause stomach upset as well as ulceration and bleeding from the stomach and even the intestines.
The other cyclooxygenase, COX-2, also produces prostaglandins, but the COX-2 enzyme is located specifically in areas of the body that commonly are involved in inflammation but not in the stomach. When the COX-2 enzyme is blocked, inflammation is reduced; however, since the COX-2 enzyme does not play a role in protecting the stomach or intestine, COX-2 specific NSAIDs do not have the same risk of injuring the stomach or intestines.
Older NSAIDs (for example, aspirin, ibuprofen, naproxen, etc.) all act by blocking the action of both the COX-1 and COX-2 enzymes. COX-2 inhibitors selectively block the COX-2 enzyme and therefore have a lower risk of causing ulcers of the stomach or intestine.
What are side effects of COX-2 inhibitors?
Common side effects of COX-2 inhibitors include:
COX-2 inhibitors may increase the risk of serious, even fatal stomach and intestinal adverse reactions, such as ulcers, bleeding, and perforation of the stomach or intestines but to a lesser extent than other nonselective NSAIDs that block both COX-1 and COX-2. These events can occur at any time during treatment and without warning symptoms.
People allergic to sulfonamides, for example, trimethoprim (Trimpex, Proloprim, Primsol) and sulfamethoxazole (Bactrim), aspirin or other NSAIDs may experience allergic reactions to COX-2 inhibitors and should not take them. Serious allergic reactions have occurred in such patients.
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