A panel of outside advisers to the U.S. Food and Drug Administration voted 7 to 4, with one abstention, that the benefits of the once-daily tablet, called mirabegron, outweighed its possible risks to the heart and liver.

The FDA usually follows the recommendations of its expert panels and will make a final decision on mirabegron by June 29. The drug is already approved in Japan.

"I think the drug shows that it's comparable to what we're using today, for better or worse," said panel member Dr. Christian Pavlovich, associate professor of urology at Johns Hopkins Bayview Medical Center.

However, panelists expressed concern the drug's benefits may not make a huge difference for patients and there were uncertainties about its long-term safety.

Overactive bladder, whose symptoms include frequent urination and urgency to urinate, affects about 12 percent to 17 percent of adults in Europe and the United States and 30 percent to 40 percent of those 75 years and older, Astellas said.

Mirabegron, the first drug in its class, works by activating a protein receptor, a beta-3 adrenoceptor, in bladder muscles that help the bladder fill and store urine.

In clinical trials, a 50-milligram dose of the drug reduced the number of times a person went to the bathroom by about four times in a week, compared with placebo, and patients taking the drug had three fewer episodes of urine leakage, or incontinence, each week.

But some advisers said that may not greatly help people who often go to the bathroom at least eight times a day.

However, the FDA had said clinical trials for the drug only had to prove it was better than a placebo, prompting the majority of panelists to vote in favor of mirabegron.

"I would state that the bar was extremely low, that maybe the FDA needs to reconsider what it requires," said panel member Dr. Antonio Tito Fojo, program director at the National Cancer Institute.

"It was low enough that you could trip over it and get to the other side," he said.

FDA staff reviewers had been concerned about the drug's safety, as it raised blood pressure and heart rate in some clinical trials. There were also rare instances of liver toxicity and tumors that developed in some patients taking the drug in trials. The panelists called for the company to do more trials if the drug is approved.

Astellas, Japan's second-largest drugmaker, said mirabegron can provide another option to patients beyond current treatments for overactive bladder.

Many patients drop out of standard treatments after the first year because of side effects, such as dry mouth, or because the drug does not work for them, the company said.

Current treatments include the Astellas drug Vesicare, one of the company's biggest products, which had global sales of 86.7 billion yen ($1.1 billion) in 2010.

With mirabegron, Astellas is hoping to cement its position in the overactive bladder market, the company said. Mirabegron is approved in Japan under the trade name Betanis.

Global sales of Vesicare and mirabegron should total over 155 billion yen ($1.9 billion) in the fiscal year ending in March 2015, the company said in its annual report.

(Reporting by Anna Yukhananov; Editing by Gary Hill, Andre Grenon and Tim Dobbyn)

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