The U.S. Food and Drug Administration warned Pfizer Inc. on Monday for not mentioning the risks of its erectile dysfunction drug Viagra in an advertisement featuring several country musicians praising the pill.

The federal health agency sent a warning letter dated April 16 to Pfizer in which it said a video advertisement that appeared on CNN.com failed to disclose “any risk information associated with Viagra,” thus violating the agency’s advertising rules. The letter was posted Monday on the FDA’s Web site.

The 30-second video showed a group of Nashville musicians singing the refrain “Viva Viagra” in a song praising the benefits of the drug.

“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.

According to Pfizer representative Francisco Gebauer, the omission of the side effects warnings was “due to a technical error" on CNN’s Web site, which ran the advertisement. The risk information should have appeared simultaneously in print on the computer screen, he added.

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

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

The FDA’s advertising rules require drugmakers to mention a drug‘s negative side effects in any advertisement that highlights benefits. Viagra’s label warns of potential risks to patients with existing heart conditions as well as reported headaches and abnormal vision.