An antidepressant used by about 3,000 adolescents will no longer be prescribed to children after a study found it could cause hostility, suicidal ideas and self-harm.

Efexor was licensed eight years ago and doctors wrote two million prescriptions last year, almost all for adults. But although the drug - chemical name venlafaxine - is not licensed for under 18s, GPs can prescribe it to children they think could benefit if they take personal responsibility.

The Committee on Safety of Medicines (CSM) which set up an expert working group to look at the safety of antidepressants in April, warned doctors yesterday no longer to prescribe Efexor to children. Children already on the drug will not be taken off it immediately because of concerns of the impact that would cause.

The warning is the second to be issued by the CSM on the newer anti-depressants and is certain to increase concerns that the drugs may be unsafe. In June the committee announced that Seroxat, one of the biggest selling drugs in the world, should not be prescribed to under 18s after evidence showed it could provoke suicidal thoughts and self-harm.

The Department of Health said the new advice on Efexor was based on results from two clinical trials submitted by the manufacturers, Wyeth, two weeks ago, although the company said it was submitted "earlier this year". The trials found cases of hostility and suicidal thoughts were twice as common among those taking Efexor compared with a placebo.

Young people on the drug were also found to be more likely to complain of abdominal pain, chest pain, anorexia and mood swings.

The Department of Health emphasised that patients should not stop taking Efexor suddenly but should consult their doctor for advice. The sudden discontinuation of an antidepressant can cause withdrawal symptoms.

Professor Ian Weller, chairman of the expert group, said: "We will be examining urgently what implications, if any, these new findings have for other antidepressants. Patients who are experiencing any side-effects or are concerned about their treatment should discuss these with their doctor."

Wyeth said it informed the Medicines and Healthcare Products Regulatory Agency, which oversees the CSM, when data on the use of Efexor in children became available. "The data was submitted earlier this year and has been included as part of the CSM's ongoing review of antidepressants," it said.