An erectile dysfunction drug that reduces by half the time patients need to take the pill before sexual activity has been approved by the US Food and Drug Administration.

Developed by Auxilium Pharmaceuticals and Vivus, the drug, Stendra, is the first FDA-approved erectile dysfunction drug that can be taken about 15 minutes prior to sexual activity.

Pfizer's blockbuster erectile dysfunction drug, Viagra, can be taken about an hour before sex.

Stendra, first approved in 2012, is available in multiple dosages (50 mg, 100 mg and 200 mg tablets) and may be taken with or without food and moderate alcohol consumption (up to three drinks), the companies said on Thursday.

It belongs to a class of drugs called phosphodiesterase type 5 inhibitors, which are used to help increase blood flow to the penis.

Vivus owns the worldwide development and commercial rights to Stendra for use in sexual dysfunction, with the exception of certain Asian countries in the Pacific Rim.

Auxilium has exclusive marketing rights to Stendra in the United States and Canada, while Sanofi SA holds a license to commercialise it in Africa, the Middle East, Turkey, and the Commonwealth of Independent States including Russia.

The drug is sold as Spedra in the European Union.

Auxilium said on Wednesday it would not withdraw its offer to buy Canadian eye drug maker QLT Inc and adopted a poison pill, a day after receiving an unsolicited buyout offer from Endo International. - Reuters