NSAIDs may be grouped according to their preference for COX-1 and COX-2 enzymes. Those that favor COX-1 are more likely to cause gastrointestinal side effects. Those that favor COX-2 have a higher risk of cardiovascular effects but less gastrointestinal effects. Higher dosages of NSAIDs tend to result in more COX-2 enzyme inhibition (and more cardiovascular side effects), even in those NSAIDs traditionally seen as low risk (such as ibuprofen). NSAIDs with higher activity against COX-2 enzymes should be used with caution in people with cardiovascular disease or at increased risk of cardiovascular disease.
Formulations of topical diclofenac, ibuprofen, ketoprofen, piroxicam, and indomethacin demonstrated significantly higher rates of clinical success (more participants with at least 50% pain relief) than matching topical placebo (moderate or high quality data ). Benzydamine did not. Three drug and formulation combinations had NNTs for clinical success below 4. For diclofenac, the Emulgel® formulation had the lowest NNT of (95% CI to ) in two studies using at least 50% pain intensity reduction as the outcome . Diclofenac plasters other than Flector® also had a low NNT of ( to ) based on good or excellent responses in some studies. Ketoprofen gel had an NNT of ( to ), from five studies in the 1980s, some with less well defined outcomes. Ibuprofen gel had an NNT of ( to ) from two studies with outcomes of marked improvement or complete remission. All other drug and formulation combinations had NNT values above 4, indicating lesser efficacy .
NSAIDs have anti-inflammatory (reduce inflammation), analgesic (relieve pain) and antipyretic (lower temperature) effects. Although different NSAIDs have different structures, they all work by blocking cyclooxygenase (COX) enzymes. There are two main types of COX enzymes: COX-1 and COX-2. Both types produce prostaglandins; however, the main function of COX-1 enzymes is to produce baseline levels of prostaglandins that activate platelets and protect the lining of the gastrointestinal tract, whereas COX-2 enzymes are responsible for releasing prostaglandins after infection or injury. Prostaglandins have a number of different effects, one of which is to regulate inflammation. Most NSAIDs inhibit both enzymes, although a few are available that mainly inhibit COX-2. The pain-relieving and anti-inflammatory effects of NSAIDs are mainly due to inhibition of COX-2, and their unwanted side effects are largely due to inhibition of COX-1.