People taking popular Prozac-type antidepressants may be twice as likely to attempt suicide than those on placebo and the risks are particularly high early in treatment, according to research released on Friday.

The British Medical Journal, which published results from three studies on selective serotonin re-uptake inhibitors (SSRIs), said the findings underlined the need for caution in using the medicines.

“How many people who turned to ‘happy pills’ would not have done so if they had been fully aware of the potential harm?” acting editor Kamran Abbasi wrote in an editorial.

One study, pooling results from 702 trials involving 87,000 patients, found a doubling of suicide attempts, although there was no increase in actual suicides.

A second analysis of 477 trials submitted by drug companies to regulators found weak evidence of an increased risk of self-harm but not of suicide, while a third study highlighted a raised risk among under-18s.

Concerns about the safety of SSRIs are not new and drugs including GlaxoSmithKline Plc’s Seroxat - the most widely prescribed among the drug class in Britain - were banned from use in children in 2003.

The UK’s Medicines and Healthcare Products Regulatory Agency said the latest studies in the BMJ had been taken into account when the regulator issued new guidelines on the use of SSRIs last December.

Those called for doctors to use antidepressants sparingly and, in most cases, to only prescribe the lowest recommended dose.

Drug companies argue that millions of people have been prescribed SSRIs without suffering major adverse events and that suicidal thoughts are more likely to be the result of their Depression rather than the treatment.