Pfizer

(PFE) - Get Report

said Monday that the Food and Drug Administration has approved a new treatment for a rare blood-vessel disease, endorsing a drug that has the same main ingredient found in the impotence product Viagra.

The agency's support of the drug Revatio marks the second pill approved for the rare condition known as pulmonary arterial hypertension, or PAH. The disease is characterized by dangerously high blood pressure in blood vessels leading from the heart to the lungs. Without treatment, patients could die within three years of diagnosis, Pfizer says. Symptoms include fatigue, dizziness and difficulty breathing.

Revatio should reach the market in mid-July. "Revatio demonstrates our commitment to developing treatments for unmet medical needs, including rare conditions such as PAH, regardless of commercial potential," said Dr. Joe Feczko, Pfizer's chief medical officer. Pfizer's shares rose 3 cents to $27.99.

The company spent six years testing Revatio for PAH. The transforming of a compound for erectile dysfunction into a treatment for PAH isn't such a stretch, because both Viagra and Revatio work by improving blood flow by relaxing blood vessels. After all, when Pfizer started studying the drug that eventually became Viagra, the goal was to find a new blood-pressure treatment.

Although Pfizer says PAH affects about 100,000 people worldwide, the market is crowded with treatments and experimental products. The other PAH pill on the market is Tracleer from the Swiss company

Actelion

.

In addition,

United Therapeutics

(UTHR) - Get Report

and

GlaxoSmithKline

(GSK) - Get Report

make PAH drugs injected under the skin or administered intravenously. And

CoTherix

(CTRX)

received FDA approval in late December to market an inhalable PAH drug called Ventavis. The drug is licensed from the German drug company

Schering AG

( SHR).

Companies developing experimental oral PAH treatments include

Myogen

( MYOG) and

Encysive Pharmaceuticals

( ENCY).

Two weeks ago, Encysive filed its PAH drug application with the FDA, seeking approval for Thelin, a once-a-day pill. Myogen is conducting two late-stage clinical trials of its PAH drug Ambrisentan.

Revatio was approved by the FDA on the basis of a clinical trial involving 277 people that measured their ability to exercise after taking the drug vs. people taking a placebo. The Revatio patients received three different dosage strengths.

Although each of the groups taking Revatio showed "highly significant improvement" vs. placebo patients, there was no difference among the Revatio patients. Therefore, the FDA has approved the drug at its lowest dosage -- 20 milligrams three times daily.

The most prominent side effects are similar to those among patients taking Viagra such as headache, stomach distress and flushing. Revatio wasn't studied on patients currently taking Tracleer.