A potential human carcinogen has been found in batches of Viracept, according to a letter the drugmaker is sending to doctors. The carcinogen - ethyl methanesulfonate or EMS - is the same one that caused Roche to recall the drug in Europe, but Pfizer says levels found in its own batches were “substantially lower.”

An FDA notice called EMS a “potential human carcinogen.” Studies suggest EMS may cause cancer and birth defects in animals but no data in humans exist, and the FDA and Pfizer agreed that pregnant women and children who are starting treatment shouldn’t take Viracept until further notice. For pregnant women with no alternative treatment options, “FDA and Pfizer agree that the risk-benefit ratio remains favorable for the continued use of Viracept,” the letter said.

Viracept is an aging pill in Pfizer’s portfolio, although the drugmaker was accused earlier this year of off-label promotion by a sales rep. The same HIV division was also accused of improperly marketing Pfizer’s newest AIDS med, Selzentry, and three managers recently left the company.