Avanafil, which will be sold under the brand name Stendra, is the first new drug in the category in a decade.

The drug -- which will compete with Pfizer Inc's Viagra, Eli Lilly's Cialis and Levitra, sold by GlaxoSmithKline and Bayer -- gives Vivus, which has struggled to bring a weight loss drug to market, a potentially lucrative commercial product.

The Vivus drug will have some attractive marketing claims as it goes up against established rivals sold by companies with serious marketing muscle.

"This is potentially the fastest acting of the four," said Dr Wayne Hellstrom, professor of urology at Tulane University School of Medicine in New Orleans.

While it is recommended that patients take the new pill 30 minutes prior to sexual activity, in clinical trials it has been shown to work in as fast as 15 minutes. Viagra typically takes about an hour to start working.

Stendra belongs to the same PDE5 inhibitor class as its rivals and also works by increasing blood flow to the penis. But researchers found it to be more selective than the older drugs, meaning it could have fewer unintended effects.

"Higher selectivity should translate into fewer side effects," said Hellstrom. "It's going to add more excitement" to the field.

ED afflicts as many as 30 million men in the U.S., Vivus said. The Vivus drug is currently awaiting a European approval decision.

Cowen and Co analyst Simos Simeonidis, in a research note, forecast annual Stendra sales of about $300 million. Viagra had sales of about $2 billion in 2011.

Vivus shares were up about 3 percent following the expected approval by the Food and Drug Administration. Investors are far more focused on the prospects for Vivus's obesity pill Qnexa, for which the FDA is expected to rule by mid July.

Doctors should prescribe the lowest dose of Stendra that provides benefit, the FDA said. It has been approved at doses of 50 milligrams, 100 mg and 200 mg.

"This approval expands the available treatment options to men experiencing erectile dysfunction, and enables patients, in consultation with their doctor, to choose the most appropriate treatment for their needs," Victoria Kusiak, deputy director of the Office of Drug Evaluation in the FDA's Center for Drug Evaluation and Research, said in a statement.

The most common side effects reported in clinical studies included headache, flushing of the face and other areas, nasal congestion, common cold-like symptoms and back pain.

The drug carries the same cautions as its rivals, including that it should not be used by men who also take nitrates due to the potential for a sudden dangerous drop in blood pressure and the now famous warning to see a doctor if an erection lasts more than four hours.

Vivus shares, which closed up 2.9 percent at $25.15 on Nasdaq, rose to $25.40 in after hours trading.

(Additional reporting by David Morgan in Washington; editing by Dave Zimmerman and M.D. Golan)

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