Children who take the anti-depressant Prozac are at greater risk of attempting suicide, say US experts.
There are concerns about side effects
The drug is currently the only anti-depressant which doctors can prescribe to under-18s in the UK.
Other similar drugs are considered too dangerous because previous studies have
linked them to an increase in suicidal tendencies.
However, an analysis by the US Food and Drug Administration has found that Prozac too may pose a risk.
The analysis was overseen by Dr Robert Temple, director of the FDA office of drug evaluation, who gave evidence on Tuesday at a hearing to determine whether tougher warning labels were needed for anti-depressants.
He said: "I think we now all believe there is an increase in suicidal thinking and action that is consistent across all the drugs."
The FDA decided that all antidepressant drugs should carry the strongest possible warnings that they could cause children to harm themselves.
In future, the drugs will have to black boxes spelling out the potential risks.
On average, the analysis, carried out by experts at Columbia University, New York, found anti-depressants taken by children will cause an extra 2% to 3% to have increased suicidal thoughts.
Relative risks of suicidal behavior were highest among young people taking Luvox, Effexor and Paxil and lower - but still heightened - among those taking Celexa, Zoloft and Prozac.
However, the FDA also heard that for many children the benefits of taking the drugs may outweigh the risks.
Dinah Morley, of the charity Young Minds, told BBC News Online: "Children who are put on these heavy duty drugs are very ill, and sadly more likely to commit suicide anyway.
"We would urge the greatest care possible to be taken in using these medications, but there are times when a young person is so deeply depressed that anything that may help must be considered because the alternative is death."
A Department of Health spokesperson said: "The Medicines and Healthcare products Regulatory Agency is reviewing these new data and will seek the advice of the Committee on the Safety of Medicines on what if any implications there are for the current advice that the balance of benefits and risks of Prozac (fluoxetine) is favourable in the treatment of depression in children and adolescents.
"As with all medicines, the MHRA/CSM keep the safety of fluoxetine under critical review."
Eli Lilly, the makers of Prozac, issued a statement stressing that while the use of anti-depressants increased the risk of suicidal behaviour in children, none who took part in the study actually killed themselves.
"However, when people with depression are left untreated, 15 percent will actually commit suicide.
"Lilly believes medication should be prescribed only after a careful diagnosis is made, and continued only if it clearly benefits the child. Because suicidal thinking is an inherent part of depression, we believe careful monitoring of patients is very important. "
The company stressed it had never marketed Prozac for use in paediatric patients.