Earlier this month, Pfizer Inc. began shipping to pharmacies a round, white pill called Revatio, a treatment for a rare and life-threatening condition called pulmonary hypertension. The drug, which dilates some of the body's key blood vessels, was approved in June for treating adults with PH, and Pfizer is now studying whether it also can help children with the debilitating illness. In early tests, the drug helped ill children increase by about 60 percent the distance they could walk in six minutes.

But there's another side to Revatio you'd never guess by looking at it. When the same active ingredient, sildenafil, comes at higher doses in a diamond-shaped blue pill, the drug is known by a different name -- Viagra.

Pfizer says the market for Revatio will be small, even if it wins additional approval for treating children. PH strikes about two people per million population a year. That means it affects an estimated 100,000 in the U.S. and Europe -- a small fraction of the tens of millions of Viagra users.

The drug may well be as effective in relieving symptoms of pulmonary hypertension in children as it is in treating impotence in adult men. Sildenafil, as it's known generically, dilates selected blood vessels in the body, including those in the male sex organ but also those in the lungs. Pfizer is now enrolling 332 youngsters, ages 1 to 16, in a placebo-controlled trial to find out if Revatio is as safe and effective in children with the lung disease as in adults.

By doing the pediatric trial, Pfizer can expect a six-month extension on its patent on sildenafil's chemical composition, which expires in 2012. Like many drug developers, Pfizer patented the drug's chemical composition early in clinical development of the drug, before all uses were known.

Sildenafil was initially tested as a heart drug for the pain of angina. Though it failed that test, patients reported it enhanced their potency, and Viagra was born.

Pfizer then sought a use patent covering the drug as a treatment for erectile dysfunction; that patent expires in 2019 and is unchanged by the new research. Drug companies routinely seek such additional patents on a drug's particular uses. Pfizer says it hasn't filed an application seeking a second use patent as a lung treatment.

The price of Revatio won't change from the price of Viagra per pill, even though each is a different dose of the same active ingredient. A 20-milligram tablet of Revatio costs about $10, the same as Viagra pills that come in 25-, 50- and 100-milligram strengths.

One potential beneficiary of the new use of sildenafil for children is 7-year-old Connor Jacobs of Redding, Calif. Two years ago, when he was 5, he mysteriously began losing consciousness while at play. These episodes were so abrupt that onlookers at first thought he was goofing -- until they saw that he had turned blue and stopped breathing. After being resuscitated, Connor told paramedics and his mother that he had felt "a cold breath" before going down.

Doctors diagnosed primary pulmonary hypertension, in which high blood pressure of the arteries in Connor's lungs stresses his heart muscle. This can cause failure of the right ventricle, which pumps blood to the lungs to pick up oxygen. Eventually, no blood flows through the lungs, and the patient passes out.

But just over a year ago Connor began a drug regimen including Viagra -- this was before the new name and pill Revatio -- that has kept him swimming and roughhousing with his older brother.

PH runs in families. Often, as in Connor's case, no cause can be identified. In still other patients, the condition is a byproduct of heart surgery, diseases like lupus, or diet drugs like fen-phen.

People with PH tire easily, suffer shortness of breath, and have severely limited exercise capacity. Without treatment, survival was once as short as six months to three years after diagnosis. Although modern therapy has added years to life expectancy, survival varies by the severity of the disease. Even today, on modern drugs, just 50 percent to 60 percent of some people with severe cases survive five years after diagnosis. Such patients are at risk of sudden death.

This summer, researchers at Toronto's Hospital for Sick Children reported a group of 14 young people -- 5 to 18 years old -- had much improved quality of life on the drug. In that pilot study published in June in the journal Circulation, Ian Adatia and his colleagues at Toronto's Hospital for Sick Children treated the youths for 12 months. The young volunteers experienced lower blood pressure in their lungs. Despite improving on the drug for about six months during the 2001 pilot study, about a third of the study participants later died despite continued treatment amid the relentless progression of the disease, said Dr. Adatia, who now is director of pediatric cardiac intensive care at the University of California at San Francisco.

Now the findings face a tougher test as the drug undergoes the larger, randomized clinical trial. Enrollment is going slowly, Pfizer says, largely because it is hard to get parents to agree to let sick children enter a trial in which they might receive a placebo. Still, Pfizer discourages off-label use by children due to a lack of pediatric safety and efficacy data.

One researcher in that study is Jeffrey Feinstein, assistant professor of pediatrics at Stanford University and director of its center for pulmonary vascular diseases. Dr. Feinstein says PH treatment options have greatly expanded in recent years, but some treatments have drawbacks for active kids. One drug requires intravenous delivery using a catheter and a pump worn in a backpack or fanny pack. Some drugs cause liver problems, nausea, vomiting, dizziness or diarrhea.

Patients' families are counseled that Viagra, too, has side effects, which commonly include facial flushing, headache, stuffy nose, leg cramps and occasionally blue vision. They are also counseled about Viagra's ability to facilitate erections. "That's one of the first questions families ask when the giggling subsides," Dr. Feinstein says.

Actually, Viagra-style erections won't trouble most PH sufferers, since two-thirds of sufferers are female. Moreover, users, including children, don't get an erection just by taking the drug, he says. It requires physical stimulation, as well as desire for sex, to complete the reaction, he says. What's more, patients with PH have low levels of nitric oxide, a blood gas needed to have an erection.

Tamara Jacobs, Connor's mom, had a big job educating bewildered pharmacists about her son's therapeutic need for Viagra when he began using it a year ago. She also had to campaign to get her insurance carrier to cover the drug, which she says would otherwise cost about $1,000 for a 90-day supply.

Connor's fainting episodes have abated, Ms. Jacobs says, since he started taking a four-drug regimen that includes Viagra; another vasodilator, Tracleer; a diuretic; and aspirin. Connor can play baseball and jog two laps around a quarter-mile track, she says. And when he goes to Stanford for a checkup that includes a six-minute walk, she says, "he always wants to run."

Ms. Jacobs adds that Connor has learned to sit out an activity "when he felt like he was going to get 'cold breath' or collapse." However, she says, it's too early to know whether combination therapy with Viagra will extend Connor's life, or merely improve its quality.