The U.S. National Institutes of Health said on Tuesday it had stopped a trial of the drug sildenafil, saying it caused serious medical problems in sickle cell patients.

The trial was looking at the safety and effectiveness of sildenafil, sold by Pfizer Inc. under the brand names Revatio and Viagra, for pulmonary hypertension in patients with sickle cell disease, the National Heart, Lung, and Blood Institute (NHLBI) said.

Patients without sickle cell disease taking the drug - which is approved under the Revatio brand name for pulmonary hypertension - are not at risk, the NHLBI said. Sildenafil is sold under the name Viagra as a treatment for erectile dysfunction or impotence.

The researchers found that 38 percent of patients taking Revatio had serious adverse effects such as painful sickle cell “crises”, compared to 8 percent of patients getting a placebo.

“No deaths have been associated with the drug in the clinical trial,” the NHLBI said in a statement.

“The increase in sickle cell medical problems is concern enough for us to stop this clinical trial to protect the safety of our participants,” added NHLBI Director Dr. Elizabeth Nabel.

“We will continue to look into the possible causes of these preliminary results,” she said.

“In the meantime, we encourage patients with sickle cell disease who are taking sildenafil for pulmonary hypertension to talk with their physicians about the potential risks and benefits of the medication and what actions they should consider, including whether to taper off this medication and how to best manage both sickle cell disease and pulmonary hypertension.”

Pulmonary hypertension is the medical term for high blood pressure in the arteries that carry blood to the lungs. It can cause heart failure and death.

About 30 percent of sickle cell disease patients develop pulmonary hypertension, which can kill them suddenly.

Sildenafil treats pulmonary hypertension by relaxing the blood vessels in the lungs to allow blood to flow more easily.

It is not approved by the U.S. Food and Drug Administration to treat pulmonary hypertension in patients with sickle cell disease, so the NHLBI was doing a small trial to see if it would help these patients.

WASHINGTON (Reuters)