A drug used to treat behaviour problems in children has been linked to liver damage, say experts.
Children with ADHD find it hard to concentrate
Strattera was licensed for the treatment of attention deficit hyperactivity disorder (ADHD) last year.
The Medicines and Healthcare products Regulatory Agency (MHRA) has warned doctors to look out for signs of liver problems in children taking the drug.
However, they stress that the chances of liver damage are very slim.
The Committee on Safety of Medicines, which advises the MHRA, met at the end of January 2005 to review new evidence on the use of the drug, known technically as atomexetine.
This evidence suggests that the drug may very rarely be associated with liver reactions.
Over 2m patients have been treated with the drug in the US, and around 10,000 patients in the UK.
It is estimated that serious liver problems occur in less than one in 50,000 patients.
Professor Gordon Duff, chairman of the Committee on Safety of Medicines, said: "This is an important drug in the treatment of ADHD in children, and it has been widely and effectively used in the US and increasingly in the UK.
"It is therefore important that we take a balanced approach to this new advice.
"Parents should not be dissuaded from having their children treated with this medicine, but it is right that they should be aware of possible, but rare, side effects."
The MHRA has issued new advice to doctors, and a question and answer sheet for parents and patients.
Professor Duff said parents who are concerned should discuss the subject with their doctor.
He stressed that they should not stop treatment with the drug before doing so.
"We have advised doctors that if they suspect that liver problems are occurring, treatment should be stopped and an alternative treatment initiated."
The Committee on Safety of Medicines has advised that further work should be undertaken to establish what may be causing these side effects in a small minority of patients.
ADHD affects up to 7% of school-age children in the UK and the condition can be difficult to diagnose.
Children with ADHD have extreme difficulty sitting still, learning or concentrating. Looking after affected children can be exhausting for parents.
Guidelines from the NHS watchdog the National Institute of Clinical Excellence say only the most severely affected should be given drug therapy.
Ritalin, an alternative drug treatment for ADHD, has also been linked to rare cases of liver problems.
Signs and symptoms of potential liver problems include; itchy skin, jaundice (yellowing of the eyes or skin), dark urine, upper right-side abdominal tenderness and unexplained flu-like symptoms.
A spokesman for the makers, Lilly said: "Patient safety is our top priority.
"Some 2.3 million patients have been prescribed Strattera and less than 50 instances of hepatic events have been reported worldwide."