WASHINGTON (Reuters) -- Viagra's first real competitor -- another pill for impotence -- went up for Food and Drug Administration approval today.

Woodlands, Texas-based Zonagen Inc. said it had submitted a New Drug Application to the FDA for Vasomax, a treatment for male erectile dysfunction.

If approved, the drug, known generically as phentolamine, would compete against the Viagra, which is made by Pfizer Inc.

Pfizer says nearly 3 million prescriptions have been written for Viagra, making it the fastest-selling new drug ever.

Zonagen and marketing partner Schering-Plough Corp. are the first to attempt to compete with Pfizer's Viagra. Like Viagra, Vasomax enhances the blood flow to the penis. It is already licensed in Mexico and sold under the name Z-max.

'Fast-track' consideration by the FDA is not considered likely, meaning approval of Vasomax could take approximately one year, barring any major problems with the application.