The risk of suicidal behaviour with newer antidepressants like Prozac is the same as the risk with old-style antidepressants, research shows.
SSRIs posed the same risk of suicidal behaviours as tricyclics
There have been concerns about reports of suicidal thoughts and behaviours among people, particularly under 18s, taking newer SSRI antidepressants.
But Boston University research suggests all antidepressants take time to work.
The findings appear in the Journal of the American Medical Association, published this week.
Prozac (fluoxetine) and Seroxat (paroxetine) both belong to the newer form of antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs).
SSRIs are prescribed as an alternative to older antidepressants called tricyclics.
Tricyclics are thought to be more dangerous in overdose, which makes SSRIs a good option for patients about whom the doctor is concerned may be suicidal.
Back in 2000, researchers questioned the safety of these newer alternatives.
They were concerned some SSRIs were addictive and could make people feel suicidal.
In response, the government commissioned a review into the safety of SSRIs in 2003.
The review looked at drug trials and reports of adverse drug reactions made to the Committee on Safety of Medicines and the Medicines Control Agency (now the Medicines and Healthcare products Regulatory Agency).
The CSM concluded there was no, or insufficient, evidence from clinical trials that benefits outweigh the risks of side effects for certain SSRIs in children younger than 18.
These were sertraline (trade name Lustral), paroxetine (Seroxat), citalopram (Cipramil), escitalopram (Cipralex), venlafaxine (Efexor) and fluvoxamine (Faverin).
Prozac (fluoxetine) was the only drug that had clearly demonstrated that the benefits outweighed the risks, said the CSM.
It said doctors could continue prescribe Prozac off licence for people under 18.
The CSM is still looking at the evidence on SSRIs in adults.
Dr Hershel Jick and colleagues at Boston University conducted their own research to estimate the risk of non-fatal suicidal behaviour in more than 2,500 people in the UK who had started taking one of four antidepressants.
Two were the commonly prescribed SSRIs fluoxetine (Prozac) and paroxetine (Seroxat) and the other two were tricyclic antidepressants - amitriptyline and dothiepin.
Overall, the patients had an increased risk of suicidal thoughts and behaviours during the first month of taking their antidepressant, especially during the first nine days.
But this increased risk was the same for each of the drugs.
Dr Jick said: "The most likely explanation for this finding is that antidepressant treatment may not be immediately effective, so there is a higher risk of suicidal behaviour in patients newly diagnosed and treated than in those who have been treated for a long time."
"It is also possible that this observation reflects patients starting to take an antidepressant drug when their depression, which naturally fluctuates over time, is at its worst," he said.
Dr Jick said it was less likely that SSRIs worsen depression, leading to suicidal thoughts and behaviour, but could not exclude it.
Dr Simon Wessely from the Institute of Psychiatry in London said the results contradicted the concern that SSRIs posed a higher risk of suicidal thoughts and behaviours than tricyclic antidepressants.
"Most doctors would be modestly reassured by that," he said.
"What it also shows, which is a worry, is that when people start treatment on antidepressants they are at particular risk of suicidal behaviour. We already knew that.
"It reinforces the need for patients to be monitored at the early stages of treatment irrespective of the drug that they are being given.
"Doctors need to remember that that is a dangerous period. But it doesn't suggest that anything specific has to be done for people taking for example Seroxat as opposed to an older drug," he said.
He said most psychiatrists still thought SSRIs were safer to use than tricyclic antidepressants in people who might be suicidal.
"They are much safer in overdose and most patients prefer them and I think most psychiatrists will continue to use them," he said.
A spokeswoman from the Department of Health said: "The results of this study are in line with existing advice from the CSM that the risk of suicidal behaviour may increase in the early stages of treatment with any antidepressant, including SSRIs.
"The study will also be looked at by the CSM's Expert Working group on the Safety of SSRIs as part of their assessment of all available evidence."
The National Institute for Clinical Excellence is due to produce guidelines on the treatment of depression and anxiety in adults in October of this year and will consider the evidence from the CSM.
It will also produce guidelines on the treatment of childhood depression in April 2005.