Strattera, which is manufactured by Lilly and is used to treat attention deficit and hyperactive disorder (ADHD), was licensed in the UK in July 2004.

Yesterday's warning was issued by the Medicines and Healthcare Regulatory Agency (MHRA), which said it would re-examine the risks and benefits of the drug. It follows the finding of an increased risk of suicide in children and young people taking antidepressants called Selective Serotonin Re-uptake Inhibitors (SSRIs). All SSRIs except Prozac were banned in children by the MHRA in December 2003.

June Raine, the director of medicines post-licensing at the MHRA, said: "We are advising healthcare professionals that patients should be carefully monitored for signs of depression, suicidal thoughts or suicidal behaviour and referred for alternative treatment if necessary.

"Children who are doing well on the medication should continue their treatment. Those who experience any unusual symptoms, or are concerned, should speak to their doctor to discuss the best course of action."

Strattera is approved for use in children aged six and over. The heightened suicide risk emerged following a review of all clinical trial data in the US.

In a statement, Lilly said there had been six reports of suicidal thoughts and behaviour out of 1,357 patients - a rate of 0.44 per cent. There had been no suicides, the company said.

Strattera was the most extensively studied of all ADHD medicines with 5,000 patients in clinical trials and more than 3.5 million patients since its launch, the statement said.