The U.S. Food and Drug Administration has warned that people with heart problems should not take the hyperactivity drug Adderall, and studies may be done to see if it raises risk in others, a top official at the agency said on Thursday.

The FDA is aware that children have died after taking the drug, but cannot tell whether the rate is higher among these children than it is in the general population, said Dr. Robert Temple, director of the FDA’s Office of Medical Policy.

“There is a background rate of sudden death in children,” he said.

Canadian regulators on Thursday suspended sales of Adderall XR, a drug for attention deficit hyperactivity disorder (ADHD) made by Britain’s Shire Pharmaceuticals Group Plc, after the company said 20 people taking the drug, including 12 children, had suddenly died between 1999 and 2003.

The U.S. FDA last September ordered Shire to add a warning to the drug’s label indicating that it should not be given to people with structural heart defects, but has not taken further action.

“We were aware of these reports of sudden deaths…what we concluded was that five of the cases involved people with evidence of heart disease,” Temple said.

Another case involved a child with type 1 diabetes, a couple of the children had toxic levels of the drug in their blood and one child was exercised in the heat “until he was half dead from that,” the FDA official said.

Over the same time period, the FDA’s reporting system logged seven sudden deaths of children taking other ADHD stimulants - Concerta, sold by Johnson & Johnson and Ritalin, made by Novartis AG.

All three drugs are amphetamines.

“The benefits of ADHD medicines are clear and the risks of undertreating patients are also clear,” said Dr. Lenard Adler, director of the ADHD program at New York University School of Medicine. Untreated patients are more prone to be involved in automobile accidents, become divorced, to be unemployed, to smoke and to have substance abuse problems, he said.

Many patients who have previously had electrocardiograms will know whether they have cardiac problems. “For other patients, you have to discuss with them whether they should get an electrocardiogram,” he said.

Adderall has been prescribed for 700,000 Americans, according to Shire. The rates of death are too low for a randomized clinical trial to detect, but the FDA is considering conducting epidemiological studies to determine risk, Temple said.

In addition, the agency may compare results from its reporting system for patients on Adderall, Concerta or Ritalin to results for patients taking Strattera, the first non-stimulant approved to treat hyperactivity.

Strattera is made by Eli Lilly.