NEW YORK (Reuters) - Drugmakers Eli Lilly and Co. and Icos Corp. on Wednesday said clinical trials of their new impotence drug showed it to be safe, with only minor side effects.

The drug, Cialis, which is already sold in Europe but is awaiting approval in the United States and Canada, is expected to compete with Pfizer Inc.'s Viagra.

The new data, presented at the Canadian Urological Association meeting in Toronto, found the most common adverse side affects to be headache and upset stomach.

"This study clearly shows Cialis to be generally a safe and tolerable treatment, even when evaluated over a prolonged period of time," said Dr. Luc Valiquette, a Montreal-based researcher who participated in the 18-month study.

In April 2002 the U.S. Food and Drug Administration gave conditional approval to the drug but said final approval depended on further data.

Lilly and Icos have never said what the FDA was seeking, but analysts have speculated that it was a safety issue due to the length of time the drug remains in the body after ingestion.

While Viagra is effective for about four hours, trials of Cialis have shown it to last up to 36 hours.

As of last month, Lilly and Icos had not yet provided the information sought by the FDA.

Eli Lilly spokeswoman Carole Copeland on Wednesday declined to comment on the status of the FDA submission or whether the new data might satisfy the agency's conditions for approval. But she said Lilly and Icos still expect to launch Cialis in the United States later in the second half of this year.

Viagra had worldwide sales of about $1.74 billion last year, but this is considered a small percentage of the potential market for drugs to treat erectile dysfunction. Doctors say more than 80 percent of an estimated 40 million men in the United States suffering some degree of the disorder have yet to seek treatment.

Industry analysts have speculated that impotence remedies could reach annual sales of $6 billion in five or six years.

The 18-month Cialis study of 1,100 men with erectile dysfunction showed the side effects occurred in 10 percent to 15 percent of patients. Of those, 6 percent stopped taking the drug due to side effects.

Lilly and Icos said any single side effect, such as headaches, accounted for less than 1 percent of discontinuations.

A second study of safety and tolerability in patients who were also taking medication for high blood pressure showed no increase in adverse side effects, the companies said.

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