The U.S. Food and Drug Administration has warned that people with heart problems should not take the hyperactivity drug Adderall, but 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 than it is in the general population, said Dr. Robert Temple, director of the FDA's Office of Medical Policy.