The U.S. Food and Drug Administration has approved labelling changes for three kinds of erectile dysfunction drugs to display more prominently the potential risk of sudden hearing loss.

The FDA has urged patients who experience hearing problems, hearing loss or ringing in the ears to call their doctors and stop taking impotence drugs.

The FDA on Thursday asked manufacturers of Viagra, Cialis and Levitra to revise product labelling after a very small number of patients taking the drugs reported sudden hearing loss, sometimes accompanied by ringing in the ears and dizziness.

It's not clear if the drugs actually trigger hearing loss, but the agency decided to act after counting 29 reports of hearing problems since 1996 among users.

Dr. Robert Boucher, an agency ear, nose and throat specialist, said in reviewing the reports he noticed that the hearing loss occurred within hours to two days of taking one of the drugs.

"We don't know enough to say that it's ironclad caused by the drugs, but we see enough to say we can't ignore it either," he said.

The reports involved hearing loss in one ear, which in a third of cases was temporary.

The FDA has urged patients who experience any hearing problems, loss or ringing in the ears to promptly call their doctors and stop taking the impotence drugs.

Because some level of hearing loss is usually associated with the aging process, patients taking these drugs (phosphodiesterase type 5 inhibitors) may not think to talk to their doctor about it, said Dr. Janet Woodcock, the FDA's chief medical officer.

With files from the Associated Press