Drugs used to treat ADHD in adults do not increase the risk of heart attacks, strokes or sudden death, US researchers said on Monday, lifting a cloud over the popular drugs used to improve attention and focus.

People with attention deficit hyperactivity disorder, known as ADHD, are excessively restless, impulsive and easily distracted. There is no cure but symptoms can be kept in check by a combination of behavioral therapy and stimulant medications.

"There had been concern about the cardiovascular safety of these medications," says Laurel Habel of Kaiser Permanente Northern California, whose study appears in the Journal of the American Medical Association. "We didn't see any evidence that these medications increased the risk of any of these events."