UPDATE 2-Possible carcinogen found in Pfizer AIDS drug

(Adds Pfizer statement, closing stock price, details on drug's use, cancer risk)

By Lisa Richwine

WASHINGTON, Sept 10 (Reuters) - A potential human carcinogen has been found in batches of Pfizer Inc's (PFE.N: Quote, Profile, Research, Stock Buzz) AIDS drug, Viracept, U.S. officials and the drugmaker said on Monday.

Pregnant women and children who are starting HIV therapy should not be given the drug until further notice, Pfizer and the Food and Drug Administration said.

Tests detected the presence of ethyl methanesulfonate, or EMS, a chemical formed during manufacturing, Pfizer said in a statement.

EMS is a "potential human carcinogen," the FDA and Pfizer said.

Studies suggest EMS may cause cancer and birth defects in animals, but no data for humans exist.

EMS levels were "substantially lower" than those that prompted Roche Holding AG (ROG.VX: Quote, Profile, Research, Stock Buzz) to recall the same drug in Europe, Pfizer said.

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.