WASHINGTON — Millions of men soon will have access to the first pill to treat impotence, a long-awaited therapy that promises to be easier to use and less embarrassing than traditional treatments.

The Food and Drug Administration approved Pfizer Inc.’s Viagra on Friday, saying it helped about two-thirds of impotent men improve their sexual function.

The prescription pill becomes the first nonsurgical treatment for impotence that doesn’t have to be either injected or inserted directly into the penis. And unlike other remedies, it does not cause an erection unless the man is sexually stimulated.

Pfizer said it would begin shipping Viagra to pharmacies within two weeks, with a wholesale price of $7 per pill. The FDA said Viagra should be used only once a day, about an hour before intercourse.

Impotence specialists hope the pill will encourage more patients to seek medical help. Only 5 percent of the estimated 10 million to 20 million impotent Americans get treatment, but the pill could increase that number to 20 percent very quickly, said Dr. Harin Padma-Nathan of the University of Southern California.

But the pill also has been much-hyped as a sexual revolution for healthy men merely seeking to increase or improve their sexual activity. But it only works in men with a medical problem, stressed experts who are concerned about the potential for abuse.

“Yes, it’s an erection improver, but only in men with erectile dysfunction,” said Padma-Nathan, director of The Male Clinic in Santa Monica, Calif. “This drug does not change libido or desire … and it’s not going to have any impact on normal men.”

“This is not an aphrodisiac,” added FDA drug chief Dr. Janet Woodcock.

But it is the first of what promises to be a booming market. New York drug analyst Mariola Haggar predicts Viagra sales could hit $300 million this year alone. Padma-Nathan predicts half a dozen oral impotence medicines will hit the market in the next seven years. Now in late-stage testing are:

Tap Pharmaceuticals’ apomorphine, designed to stimulate an erection by affecting the brain chemicals responsible for sexual response.

Zonagen Inc.’s Vasomax, an oral version of an injectable blood pressure drug.

Viagra, known chemically as sildenafil, is a failed heart drug that Pfizer pursued after some heart patients unexpectedly reported having erections.

In studies of 4,000 men with varying erectile dysfunction, 64 percent to 72 percent successfully completed intercourse after taking Viagra, vs. 23 percent of men taking a dummy pill.

Viagra works by blocking an enzyme found mainly in the penis. That enzyme is responsible for quelling an erection after sex by breaking down a chemical called cyclic GMP that is produced during sexual stimulation. The longer cyclic GMP stays around, the better chance of maintaining an erection — hence Viagra’s effect.

Impotence increases with age, and is mostly caused by such medical problems as diabetes, heart disease, prostate surgery and spinal cord injury. It also can be psychological or a side effect of certain drugs.

The treatments already available have drawbacks: Penile implants require surgery; vacuum-style devices force blood into the penis but interrupt lovemaking; men are squeamish about injecting drugs or inserting suppositories into the penis — and they occasionally cause hours-long erections.

But patients say Viagra’s benefit is the naturalness. They don’t have to plan ahead for an injection, and they only get an erection if they are sexually stimulated.

“This is something you can take every night. It brings back the spontaneity in a normal marital life,” said Tolman Geffs, 64, of Trabuco Canyon, Calif., who was left impotent after prostate cancer surgery.

But just prescribing a pill isn’t proper therapy, warned Dr. James Barada, an Albany, N.Y., physician representing the American Urological Association. Patients must get treated for underlying disease — and understand that the pill isn’t a silver bullet.

“This is a great advance in the sense that it will increase awareness” of impotence treatments, Barada said. But “the pill’s not going to be for everybody.”

Heart patients taking nitroglycerin should not use Viagra because the drugs can interact to lower blood pressure, doctors said.

But other side effects were rare and mild, mostly headaches, facial flushing and upset stomach. A few men experienced strange visual effects: About 3 percent see a blue tinge while taking the recommended dose and higher doses left them unable to distinguish between the colors blue and green.