Drugs to treat attention deficit hyperactivity disorder, or ADHD, don't appear to put kids at higher risk of heart problems or death, scientists said Monday.

Scattered reports of sudden deaths among children on the medications have caused concern among parents and doctors in recent years, and several of the drugs now carry warnings about heart complications and behavioral side effects.

While the new study has limitations, researchers called the findings reassuring.

"The risk of death is certainly no higher in children who take ADHD medications than in children who don't," said Sean Hennessy, a pharmacist at Philadelphia's University of Pennsylvania, who led the work.

Some 2.7 million children and teens in the U.S. use the drugs, which are stimulants that can raise blood pressure and heart rate. The drugs include Novartis' Ritalin and Shire's Adderall.

The new study, funded by Shire, is based on claims data from Medicaid and a commercial insurer. It includes more than 240,000 kids ages three to 17, who received ADHD drugs and were followed for 135 days on average.

The researchers then sought to compare those children to more than 965,000, who didn't take the drugs but were of similar age and gender and came from the same states as the users.

That was easier said than done, the researchers discovered, because often the claims data didn't match the hospital records.

Based on the data they could validate, Hennessy and his colleagues calculated that there would be six sudden deaths or cardiac arrests per 1,000,000 kids taking ADHD drugs for a year.

That's slightly more than the four per 1,000,000 kids in the comparison group. But because the numbers are so small, the difference could easily have been due to chance.

There were no strokes or heart attacks in the ADHD group, and the researchers estimate it's very unlikely that the true rates would exceed 24 cases per 1,000,000 per year.

Rates of death "from any cause," which were the most reliable numbers in the insurance data, were 179 per 1,000,000 kids per year in the ADHD group and 300 per 1,000,000 in the comparison group.

"For kids who would benefit from ADHD medications, the potential cardiovascular risks should not dissuade physicians from prescribing the drugs," Hennessy told Reuters Health.

The findings, published in the journal Pediatrics, are in line with two previous reports that didn't find evidence of a link between sudden death and ADHD drugs.

However, they run counter to one small 2009 study that found stimulant use was more common (1.8 percent) in children who died suddenly from cardiac arrest than in those who died in car accidents (0.4 percent).

One expert who was not involved in the current study said the results were hard to interpret due to the small number of deaths and heart problems.

"The new findings confirm that if there is an association between stimulants and cardiac events, it is quite rare," Almut Winterstein, of the University of Florida College of Pharmacy in Gainesville, told Reuters Health.

But she added that at this point, there is no telling how the millions of kids on ADHD medicines will fare down the road.

"We will need to wait another decade to understand whether even slightly increased blood pressure and heart rate over several years during childhood results in increased cardiovascular risk in later life," she said in an email.

The U.S. Food and Drug Administration is currently awaiting data from a large safety study on stimulants.

Hennessy said he was looking forward to seeing the outcome of that work.

"Studying cardiovascular events using insurance data in kids in somewhat complicated," he noted.

"I would like to see the results of the FDA study before the matter is closed."