Coadministration has not been studied. Mefenamic acid is metabolized by CYP2C9 and glucuronidated by UGT2B7 and UGT1A9. Emtricitabine and tenofovir are eliminated renally. No interaction is expected with emtricitabine. However, mefenamic acid could potentially decrease the renal elimination of tenofovir as in vitro data suggest that mefenamic acid inhibits the renal transporter OAT1. Coadministration could potentially result in increased risk of nephrotoxicity. The risk is increased if an NSAID is used for a long duration, if the patient has a pre-existing renal dysfunction, has a low body weight, or receives other drugs that may increase tenofovir exposure. Cases of acute renal failure after initiation of high dose or multiple NSAIDs have been reported in patients treated with tenofovir-DF and with risk factors for renal dysfunction. Alternatives to NSAIDs should be considered in patients at risk for renal dysfunction. If emtricitabine/tenofovir-DF is co-administered with an NSAID, renal function should be monitored adequately.