Stimulant drugs like Ritalin should be used judiciously in hyperactive children because they can have unpleasant side effects, say experts.
Nearly a million ADHD prescriptions were handed out last year in England - a figure that has more than doubled in the last decade, NHS figures show.
The Cochrane Group looked at 185 drug trials involving more than 12,000 children or adolescents.
Common side effects included loss of appetite and sleeplessness.
The international researchers said this did not mean that the drugs are not useful - the Cochrane findings suggest they do help children concentrate at school - but any benefits must be closely weighed against the risk of side effects.
And better studies are needed to help doctors do this, they said.
The Cochrane research, intended to inform policymakers and doctors, looked at children aged three to 18 living in the UK, the US and Canada.
Among the children who received the active drug, on average, 526 in every 1,000 experienced unpleasant side-effects, compared with 408 per 1,000 of those in the control group given a dummy pill or no medication - meaning a 29% increased risk of side effects for the children on Ritalin (methylphenidate).
The risk of serious adverse events was extremely low.
But it was difficult to predict who might experience more common, unpleasant side effects.
"Despite more than 50 years of research in the field, we have no knowledge on how to identify patients that may obtain more benefits than harms," said the researchers.
They recommend more studies to better spot when side effects might occur.
Co-author, Dr Morris Zwi, consultant child & adolescent psychiatrist, added: "Our expectations of this treatment are probably greater than they should be, and whilst our review shows some evidence of benefit, we should bear in mind that this finding was based on very low-quality evidence.
"What we still need are large, well-conducted trials in order to clarify the risks versus the benefits for this widely used treatment."
Dr Daniel Hawcutt, from the British Pharmacological Society, advised parents with questions or concerns to get in contact with their doctor.
"But there is no reason to immediately discontinue this medicine," he said.
Dr Tony Lloyd, from the charity ADHD Foundation, said drug treatment should be used only as an adjunct to behavioural therapies, as recommended by NHS guidelines. But he said this was not happening.
"The fact of the matter is that in the UK medication is the first line of treatment and pretty much the only line of treatment. That needs to change."
He said ADHD was still under-recognised and under-treated in the UK.