WASHINGTON, Oct 18 (Reuters) - US regulators on Thursday added new warnings about the potential risk of sudden hearing loss to best-selling impotence drugs Viagra, Cialis, and Levitra, used by millions of men.

The US Food and Drug Administration said a published report of a man taking Pfizer Inc's Viagra, who suffered from sudden hearing loss, prompted it to explore a possible association.

A further review of the FDA's side effect data found 29 cases of sudden hearing loss with a relationship to the three drugs.

In one-third of the cases the hearing loss was temporary, the agency said, the remaining cases were described as ongoing at the time of the report or the outcome was not described.

Eli Lilly sells Cialis and GlaxoSmithKline Plc sells Levitra. Pfizer's Revatio, a version of Viagra used for a lung condition, will also carry a new warning.

Erectile dysfunction drugs are a huge business, with Pfizer reporting Viagra sales of US$450 million in its most recent quarter. The drugmakers are all cooperating with the FDA and defended the safety of their drugs.

The new label warnings follow a 2005 label change noting cases of sudden vision loss in some patients.

Reports of serious side effects to the FDA's safety data-base are widely viewed as representing only a fraction of actual problems with drugs and medical devices.

Drugmakers Pfizer and Lilly both said their data did not show any causal relationship between hearing loss and the drugs. The FDA said no causal link has been established.

Pfizer vice president for medical affairs, Dr. Ponni Subbiah, said hearing loss was included in Viagra's adverse events section of its label upon its 1998 FDA approval. It occurred in less than 2 percent of patients in clinical trials, which she said was statistically comparable to those in a placebo group.

About 30 million men have taken Viagra since its approval in 1998 as the first widely-used drug for erectile dysfunction, according to Pfizer.

Lilly spokeswoman Keri McGrath said a recent Lilly review found about 1.1 incidents of sudden hearing loss per million Cialis patients, which she said was lower than the incident rate in the general population. Nearly 12 million men have been prescribed the drug, the company said.

Schering-Plough Corp. and GlaxoSmithKline co-market Levitra in the United States, while Germany's Bayer AG sold the drug originally.

Schering-Plough spokesman Lee Davies said the company would comply with the FDA's request and said all the hearing loss cases were temporary.

About 4,000 new cases of sudden hearing loss occur in the United States annually, according to the National Institute on Deafness and Other Communication Disorders. It typically occurs in one ear, and symptoms include ringing in the ear and dizziness.

The condition resolves itself in some cases, but in 15 percent of cases, it gets worse over time, according to the institute.