The National Heart, Lung and Blood Institute of the NIH has stopped a clinical trial testing a drug treatment for pulmonary hypertension in adults with sickle cell disease nearly one year early due to safety concerns. In an interim review of safety data, researchers found that participants taking sildenafil (Revatio) were significantly more likely to have serious medical problems. The most common problem was episodes of severe pain called sickle cell crises. No deaths have been associated with the drug in the clinical trial.