WASHINGTON -- An online advertisement with men praising Viagra to the tune of an Elvis Presley song has drawn objections from US regulators, who said drugmaker Pfizer Inc failed to list the impotence drug's risks.

Pfizer said the omission of side effects warnings was due to a technical error on CNN's Web site, http://www.cnn.com, which ran the advertisement.

The Food and Drug Administration sent a written warning to Pfizer that was made public on Monday.

"The video raises public health and safety concerns through its complete omission of risk information for Viagra by suggesting that Viagra is safer than has been demonstrated," the FDA said in its letter, dated April 16.

Viagra's prescribing instructions warn against use by men taking heart drugs known as nitrates and caution about sudden vision and hearing loss and other problems.

Pfizer spokesman Francisco Gebauer said the risk information should have appeared simultaneously in print on the computer screen. The information did not run "due to a technical error" on the Web site of CNN, a unit of Time Warner Inc, Gebauer said.

"We regret that the Internet video ran without the appropriate safety information," he said.

CNN spokesman Sal Petruzzi said the company's Web site had a "technical mishap" that was corrected.

To avoid similar errors, Pfizer has pulled all 30-second Internet video ads that require safety information to appear separately on the screen rather than within the advertisement, Gebauer said.

Viagra's worldwide sales were nearly $1.8 billion in 2007. The drug's generic name is sildenafil.