Pregnant women and children who are starting HIV therapy should not be given the drug until further notice, the FDA said.
Tests detected the presence of ethyl methanesulfonate, or EMS, a chemical formed during manufacturing.
EMS is a 'potential human carcinogen,' the FDA noted.
Studies suggest EMS may cause cancer and birth defects in animals, but no data for humans exist.
While agreeing with the FDA warning, Pfizer said EMS levels in its product were 'substantially lower' than those that prompted Roche Holding AG (to recall the same drug in Europe.
Viracept, known generically as nelfinavir, belongs to a class of AIDS drugs called protease inhibitors. It is a key part of many drug cocktails used to suppress the HIV virus that causes AIDS.
'Pfizer is working with the FDA to prospectively limit EMS levels in Viracept, while still considering the immediate needs of patients on therapy,' a company statement said.
In a letter to doctors, Pfizer said manufacturing changes would reduce EMS levels so the theoretical increased cancer risk for adults over a lifetime would be less than 1 case per 100,000 people exposed.
Pediatric patients who are stable on Viracept-containing regimens may continue to be treated with the drug, the company said. Pregnant women currently taking Viracept should be switched to an alternative medicine until the manufacturing changes are made, Pfizer said.
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.
In August, authorities suspended Roche's license to market Viracept in the European Union because of the EMS finding.