Drugs given to children to reduce symptoms of attention deficit hyperactivity disorder (ADHD) have no effect in the long term, a new study claims.
Researchers in the US found that while drugs such as Ritalin and Concerta work in the short term, there is no improvement in behaviour after three years of treatment.
The findings also suggest prolonged use of the drugs may stunt a child's growth.
The study looked at the treatment of 600 children across the US since the 1990s.
Seven years ago, the experts concluded that after one year, medication for ADHD worked better than behavioural therapy.
This influenced medical practice on both sides of the Atlantic and prescription rates in the UK have since tripled.
But the report's co-author, Professor William Pelham, from the University at Buffalo, said: "I think that we exaggerated the beneficial impact of medication in the first study.
"We had thought that children medicated longer would have better outcomes. That didn't happen to be the case.
"The children had a substantial decrease in their rate of growth so they weren't growing as much as other kids both in terms of their height and in terms of their weight. And the second was that there were no beneficial effects - none.
"In the short run [medication] will help the child behave better, in the long run it won't. And that information should be made very clear to parents."