For the first time, the Food and Drug Administration on Friday warned 14 of the world's leading pharmaceutical companies about misleading Internet ads that appear when people do online queries for their medical products through search engines such as Google.

The FDA said the ads are misleading because they omit risk information associated with the products.

Companies that received the letters include Biogen Idec Inc. (BIIB), Sanofi-Aventis (SNY), Johnson & Johnson (JNJ) and GlaxoSmithKline (GSK).

Biogen received a warning letter for its multiple-sclerosis drug Tysabri. The ads say "A Multiple Sclerosis Treatment That's Different from the Others" or "Satisfied with your MS Medication or Looking for Something Different?" but don't include any risk information.

"Their casual approach to Tysabri treatment is extraordinary in light of the potentially lethal risks of the drug and the stringent controls over its distribution," the FDA said in its letter to Biogen on March 26. The letter was posted on the agency's Web site Friday.

Biogen's ad includes a link to the Web site for the drug, which does contain the relevant risk information. The FDA said the link "does not mitigate the misleading omission of risk information from these promotional materials."

Sanofi received a warning for ads for Plavix, a powerful anticlotting drug that is the world's second-largest drug by sales. "The sponsored links misleadingly suggest Plavix is safer than has been demonstrated," the FDA letter said.

The FDA discovered the ads as part of its routine monitoring of Internet advertising, said agency spokeswoman Rita Chappelle. She said the FDA hasn't contacted any of the search engines where the ads have appeared because the agency doesn't contact third-parties that carry ads, even if they are violative ads.

The FDA wants the companies to remove the violative ads and respond to the agency next week.

The other companies that received letter are: Forest Laboratories Inc. (FRX), Cephalon Inc. (CEPH), Bayer AG (BAYRY), Pfizer Inc. (PFE), Novartis AG (NVS), Merck & Co. (MRK), Eli Lilly & Co. (LLY), Roche Holding AG (RHBBY), Genentech Inc., and Boehringer Ingelheim Pharmaceuticals Inc. Genentech was recently acquired by by Roche.

Represenatives for the companies weren't immediately available for comment.

-By Jared A. Favole, Dow Jones Newswires; 202.862.9207; jared.favole@dowjones.com