US authorities are warning that a stimulant used in treatments for the childhood condition attention deficit-hyperactivity disorder (ADHD) can trigger painful, long-lasting erections in rare cases.

The Food and Drug Administration (FDA) is updating drug labels to include information about priapism, a condition that can permanently damage a patient’s penis.

The stimulant, methylphenidate, is found in treatments including Ritalin, Concerta and Daytrana.

ADHD is a common childhood disorder that hampers a child’s ability to pay attention and control behaviour.

The FDA said the median age of patients taking methylphenidate who had priapism was 12-and-a-half.

The FDA added people should talk to their doctors before halting the medication, while parents should talk to boys taking the drug to make them aware of the warning signs.