According to a “Dear Healhcare Professional" issued by Pfizer on Monday the pharmaceutical company warned about the presence of an impurity in its AIDS drug Viracept.

Viracept, which is produced in Europe by Roche Holding AG and in US by Pfizer, is a protease inhibitor which is used in conjunction with other antiretroviral agents to fight HIV virus.

Pfizer said it has been discovered that a potential human carcinogen, ethyl methanesulfonate (EMS), had been introduced during the production process into Viracept. Viracept comes in tablets (625 mg or 250 mg) and an oral powder.

The U.S. Food and Drug Administration warned in a notice posted on its website that there are studies that proves the teratogenic, mutagenic and carcinogenic effect of EMS on animals, but no data are available for humans.

FDA and Pfizer said that despite the presence of EMS the drug is safe for being used in HIV treatment, but the healthcare professional should not prescribe Viracept to pregnant women and children who are starting HIV therapy until further notice.

The pediatric patients who already received Viracept and are stable may continue their treatment. According to Pfizer’s letter, as a precautionary measure, pregnant women currently receiving Viracept should be switched to an alternative antiretroviral therapy while the company and FDA work to implement the long term EMS specification for Viracept.

In June, the European Medicines Agency had issued a recall of Viracept because of the same issue.