Millions of people taking commonly prescribed antidepressants such as Prozac and Seroxat might as well be taking a placebo, according to the first study to include unpublished evidence.

The new generation of antidepressant drugs work no better than a placebo for the majority of patients with mild or even severe depression, comprehensive research of clinical trials has found.

The researchers said that the drug was more effective than a placebo in severely depressed patients but that this was because of a decreased placebo effect.

The study, described as “fantastically important” by British experts, comes as the Government publishes plans to help people to manage depression without popping pills.

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More than £291 million was spent on antidepressants in 2006, including nearly £120 million on SSRIs. As many as one in five people suffers depression at some point. With that in mind, ministers will today publish plans to train 3,600 therapists to treat depression. Spending on counselling and other psychological therapies will rise to at least £30 million a year.

The study, by Irving Kirsch, from the Department of Psychology at the University of Hull, is the first to examine both published and unpublished evidence of the effectiveness of selective serotonin reuptake inhibitors (SSRIs), which account for 16 million NHS prescriptions a year. It suggests that the effectiveness of the drugs may have been exaggerated in the past by drugs companies cherry-picking the best results for publication.

The National Institute for Health and Clinical Excellence (NICE), which is due to review its guidance on treating depression, said that it would consider the study.

Mental health charities say that most GPs admit that they are still overprescribing SSRIs, which are considered as effective as older drugs but with fewer side-effects. SSRIs account for more than half of all antidrepressant prescriptions, despite guidelines from NICE in 2004 that they should not be used as a first-stop remedy.

American and British experts led by Professor Kirsch examined the clinical trials submitted to gain licences for four commonly used SSRIs, including fluoxetine (better known as Prozac), venlafaxine (Efexor) and paroxetine (Seroxat).

The study is published today in the journal PLoS (Public Library of Science) Medicine. Analysing both the unpublished and published data from the trials, the team found little evidence that the drugs were much better than a placebo.

“Given these results there seems little reason to prescribe antide-pressant medication to any but the most severely depressed patients, unless alternative treatments have failed,” Professor Kirsch said. “The difference in improvement between patients taking placebos and patients taking antidepressants is not very great. This means that depressed people can improve without chemical treatments.” He added that the study “raises serious issues that need to be addressed surrounding drug licensing and how drug trial data is reported”.

The data for all 47 clinical trials for the drugs were released by the US Food and Drug Administration under freedom of information rules. They included unpublished trials that were not made available to NICE when it recommended the drugs for use on the NHS. “Had NICE seen all the relevant unpublished studies, it might have come to a different conclusion,” Professor Kirsch said.

Tim Kendall, a deputy director of the Royal College of Psychiatrists Research Unit, who helped to formulate the NICE guidance, said that the findings were “fantastically important” and that it was “dangerous” for drug companies not to have to publish their full data. He added: “Three of these drugs are some of the most commonly used antidepressants in this country. It’s not mandatory for drug companies to publish all this research. I think it should be.”

SSRIs are not prescribed to patients under 18 because of the risk of suicide.Drugs watchdogs in Europe are considering tighter controls on the development of new medicines, The Times reported this month, and may soon require regulators to monitor psychiatric effects and the risk of suicide more closely during clinical trials.

A spokesman for GlaxoSmithKline, which makes Seroxat, said: “The authors have failed to acknowledge the very positive benefits these treatments have provided to patients and their families dealing with depression and their conclusions are at odds with what has been seen in actual clinical practice. This one study should not be used to cause unnecessary alarm and concern for patients.”

A spokesman for Eli Lilly, which makes Prozac, said: “Extensive scientific and medical experience has demonstrated that fluoxetine is an effective antidepressant.”

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