Pfizer Inc. (PFE) has begun pulling two television advertisements for its Viagra impotence drug after U.S. officials said the "wild thing" ads made unsubstantiated claims about a return of sexual desire.

The Food and Drug Administration (search), in a letter released Monday, said the 30- and 15-second ads also failed to mention major side effects and why some patients should not take Viagra.

Pfizer spokesman Daniel Watts said the company had begun the process of withdrawing the commercials.

The ads, with a voice-over that includes "Remember that guy who used to be called 'wild thing'?" and later says, "He's back," additionally failed to mention the specific condition Viagra is intended for -- erectile dysfunction, the FDA said.

In a letter dated Nov. 10, the FDA said Pfizer implied that the male seen in the ads had returned to a previous level of sexual desire and activity.

"FDA is not aware of substantial evidence or substantial clinical experience demonstrating this benefit for patients who take Viagra," said the letter from the FDA's Division of Drug Marketing, Advertising, and Communications.

"If you have data substantiating this claim, please submit them to FDA for review," the letter said.

The FDA said Pfizer should immediately cease using the offending ads and any similar promotions. It asked the company to respond by Nov. 24 on its intent and plan to comply.

Watts said Pfizer was preparing that response.

Viagra has been in an advertising battle with newer impotence competitors like Cialis (search), by Eli Lilly & Co. (LLY) and Icos Corp. and Levitra (search), marketed by GlaxoSmithKline (GSK) and Bayer AG .

Viagra, containing sildenafil, can cause a dangerous lowering of blood pressure if it is taken with certain prescription drugs called nitrates.

Patients with diabetes, high blood pressure or heart disease are often prescribed nitrates.

Headache, flushing and transient abnormal vision are among adverse reactions that can be experience when taking Viagra, according to FDA-approved labeling.