Cytochrome P450 comprises many enzymes that are involved in phase I metabolic pathways, including oxidation of endo- as well as exogenous compounds. Non-steroidal anti-inflammatory drugs (NSAIDs) are widespread therapeutic agents. Several isoenzymes, including CYP2C9, CYP2C8, CYP3A4 and CYP1A2, catalyze biotransformation of these drugs as well as many other compounds. Several NSAIDs are the substrates for CYP2C9; however, its activity can be inhibited or induced by many substances. The inhibition of CYP2C9 can decrease NSAID metabolism and lead to increased systemic drug concentration, which may cause adverse events. On the other hand, faster metabolism can increase the elimination of the therapeutic agents, and might decrease the drug concentration below the therapeutic range.