WASHINGTON – The government ordered Pfizer Inc. to yank cheeky television ads that promised better sex for men taking Viagra because they failed to disclose known risks associated with the drug, according to a letter released yesterday.

“The TV ads omit the indication for the drug (namely, treatment of erectile dysfunction) and fail to provide information relating to the major side effects and contraindications of the drug, as required,” Christine Hemler Smith, a Food and Drug Administration regulatory review officer, informed Pfizer in a letter posted on the agency’s Web site.

“Remember that guy who used to be called ‘Wild Thing?'” the ads say as a middle-aged couple shop, looking in the window of a lingerie store. “The guy who wanted to spend the entire honeymoon indoors?” Later, blue horns sprout from the frisky man’s head with “He’s back” written on his forehead. The horns morph into the letter “V” of Viagra.

“The TV ads claim that Viagra will provide a return to a previous level of sexual desire and activity,” Smith wrote to Robert B. Clark, a Pfizer vice president. “FDA is not aware of substantial evidence or substantial clinical experience demonstrating this benefit for patients who take Viagra,” Smith wrote.

Pfizer has already removed the TV ad clip from its Web site and is “in the process” of pulling the TV ads, said spokesman Daniel Watts.

Pfizer is also drafting a formal reply to the FDA’s letter. Viagra is among the company’s top 10 best-selling drugs, behind such products as Lipitor, Zoloft, and Celebrex. Viagra accounted for $1.87 billion in global sales last year, including $1.1 billion in sales to American men.

It’s not clear whether the agency’s regulatory action extends beyond the 15- and 30-second television ads. Viagra’s Web site, for instance, reinforces the ad campaign with a blue-horned man under the headline “Get back to mischief.”

The FDA asked the company to cease “promotional materials” similar to the television ads it described in its letter but did not include specifics.

“We’re going to look at everything we have,” Mr. Watts said, to ensure the company complies with the FDA’s request.

Men who already have heart disease can risk further heart woes when they have sex, Ms. Smith’s letter said. Viagra, approved to treat erectile dysfunction, should not be used by men with heart conditions whose doctors have warned them not to have sex. Also, patients taking drugs that contain nitrates have been warned not to take Viagra because of sudden, unsafe drops in blood pressure.

The drug’s label also warns of erections lasting longer than four hours, painful erections lasting longer than six hours, headaches, flushed skin, and vision problems.

The FDA said Pfizer’s television ads “misbrand” Viagra and demanded that the company immediately pull them. The agency is requiring a response to its letter by November. 24.

Consumer advocate Sidney Wolfe said FDA enforcement against misleading advertising dropped from 157 actions in 1998 to just 23 last year.

“If you are the industry, you cannot help but notice the FDA has largely stopped enforcing the laws and regulations on drug advertising,” Mr. Wolfe said. “Therefore, you’re almost encouraged to run misleading ads like this. By the time the FDA catches up with this kind of ad, it’s likely millions of people, or more, have seen the ad.”