The research, which analysed 47 clinical trials using data released under US freedom of information laws, prompted questions over whether people with mild or moderate depression should be prescribed drugs at all.
The team, led by Professor Irving Kirsch from Hull University in north-east England, said their findings constituted one of the most thorough probes into the effectiveness of so-called new generation anti-depressants.
"The difference in improvement between patients taking placebos and patients taking anti-depressants is not very great," said Kirsch. "This means that depressed people can improve without chemical treatments.
Antidepressants don't work for everyone
"Given these results, there seems little reason to prescribe antidepressant medication to any but the most severely depressed patients unless alternative treatments have failed to provide a benefit."
Alison Cobb of British mental health charity Mind hailed the findings as "a serious challenge" to the predominance of drugs in treating depression. "Anti-depressants do help many people but by no means all and some people experience severe side-effects with them," she said.
"Nine out of 10 GPs say they've been forced to dish out drugs because they don't have proper access to 'talking treatments' such as cognitive behavioural therapy, which are recommended as the first-line treatment for mild to moderate depression."
A spokesman for GlaxoSmithKline, which makes the anti-depressant Seroxat, said the study had not acknowledged the "very positive benefits" the treatments have provided. "Their conclusions are at odds with what has been seen in actual clinical practice," he added.
"This one study should not be used to cause unnecessary alarm and concern for patients."
The research was published in the journal PLoS (Public Library of Science) Medicine. – (Sapa) - February 2008