CHICAGO (Reuters) - Stimulants used to treat attention deficit hyperactivity disorder do not increase the risk of heart attacks, strokes or sudden death, U.S. researchers said on Monday, in a finding that should reassure millions of parents whose children take the drugs.

Researchers studied the medical records of more than 1 million children and young adults aged 2 to 24 who were taking or had taken stimulants such as Ritalin or Adderall and found no sign of increased risk of heart problems.

"We don't see any evidence of increased risk," said Dr. William Cooper of Vanderbilt University, whose study was published in the New England Journal of Medicine.

The study in children is the first of three commissioned by the U.S. Food and Drug Administration to understand the potential heart risks of the drugs after U.S. and Canadian regulators received a number of reports in 2006 of heart attacks, strokes and sudden cardiac arrest in children taking the medications.

The reports prompted several FDA advisory committee hearings on heart problems, and Health Canada temporarily suspended marketing of ADHD drugs.

The concerns also prompted the American Heart Association to issue guidelines suggesting that children who were just starting to take the drugs should be tested for potential underlying heart problems.

"There was a lot of concern and confusion among families and providers about what the best approach would be to treating kids who had ADHD and who might benefit from these medicines," Cooper said by phone.

COMMON CHILD DISORDER

ADHD is one of the most common child mental disorders, affecting around 3 to 5 percent of children globally.

Children with ADHD are excessively restless, impulsive and easily distracted, and often have trouble at home and in school. There is no cure, but the symptoms can be kept in check by a combination of behavioral therapy and medication.

In the United States, some 2.7 million people have prescriptions for ADHD drugs including Novartis' Ritalin, known generically as methylphenidate, Johnson & Johnson's Concerta, Shire's Adderall and Vyvanse and Eli Lilly's Strattera.

For the study, Cooper's team used prescription data from four large, geographically and demographically diverse health plans on 1,200,438 children and youth, including both current and past users of the drugs. They checked health records for evidence of heart problems, including heart attacks, strokes and sudden cardiac deaths, in children who were currently taking the drugs or who had taken them.

The team found no increased risk of heart problems for either current or past users of the drugs. Yet because there were so few cases of serious heart problems -- just 81 -- the study may not have been large enough to detect it.

But even if there were a risk of heart problems, it is extremely slight, Cooper said.

FDA spokeswoman Sandy Walsh said in an e-mail the FDA continues to recommend that the drugs not be used in patients with serious heart problems.

The agency is expected to release fuller guidance later on Tuesday.

(Editing by Philip Barbara)