Nelfinavir mesylate is found to contain ethyl methanesulfonate.
A process-related impurity found in the HIV drug nelfinavir mesylate (Viracept, Pfizer) has led to a Dear Healthcare Professional letter from Pfizer. Ethyl methanesulfonate, a potential human carcinogen, may also be teratogenic and mutagenic. As such, the FDA has asked Pfizer to implement new procedures to limit the presence of the chemical in the drug. In the meantime, pediatric patients already taking nelfinavir may continue to receive the medication. Pediatric patients and pregnant women initiating HIV therapy should not start a regimen containing nelfinavir until further notice. Pregnant women currently taking the drug should be switched to an alternative antiretroviral drug. More information can be accessed at