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 in strongly worded letters to the companies that 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). Some of the letters include complaints about multiple drugs. For instance, a letter to Pfizer Inc. (PFE) mentions six drugs, including its anti-smoking drug Chantix and the arthritis medicine Celebrex.

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."

The company is working closely with the FDA to resolve the situation, said spokeswoman Naomi Aoki. She said the company takes its responsibility to communicate the risks and benefits of Tysabri "very seriously."

FDA spokeswoman Rita Chappelle said 19 of the 48 drugs cited in the letters carry the agency's strongest warning, a black box, about side effects. She said in some instances the information on the ads expanded uses of the drug beyond what they are approved for.

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, Chappelle said. 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.

Google Inc. (GOOG) spokeswoman Sandra Heikkinen said the Internet-search company has been working with the pharmaceutical companies to make sure their ads align with FDA regulations. She said the companies have control over what appears in the sponsored ads.

Pfizer has received warnings for sponsored ads related to Aromasin, Caduet, Chantix, Detrol, Lyrica and Celebrex. The company said it "recognizes the importance of communicating online about medical conditions." The company is reassessing its use of sponsored links to ensure they continue to adhere to the FDA guidelines."

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), 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 many of the companies weren't immediately available for comment.

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