The common side effects occurring during therapy with non-steroidal anti-inflammatory drugs (NSAIDs) are generally at generally gastro-intestinal; these are usually mild and reversible but in some patients peptic ulcer and severe gastrointestinal bleeding have been reported. These adverse effects on the gastrointestinal tract may be associated with inhibition of the form of cyclooxygenase known as cyclooxygenase-1 (COX-1) and NSAIDs that are highly selective inhibitors of the form known as COX-2 may have less gastro-intestinal toxicity.