A new study suggests that anti-depressant drugs, such as Prozac, could be largely ineffective.

The Public Library of Science Medicine journal examined all available data on the drugs, which are prescribed to millions of people around the world.

Researchers from Britain, the US and Canada obtained the data under US freedom of information laws, including the results of clinical trials the manufacturers chose not to publish.

Anti-depressant medications appear to help only very severely depressed people and work no better than placebos in many patients, British researchers said.

Researchers reviewed a series of studies on four anti-depressants examining the question of whether a person's response to these drugs hinged on how depressed they were before getting treatment.

The drugs are all so-called selective serotonin reuptake inhibitors, or SSRIs.

The researchers found that compared with a placebo, these new-generation anti-depressant medications did not yield clinically significant improvements in depression in patients who initially had moderate or even very severe depression.

The study found that significant benefits occurred only in the most severely depressed patients.

'Although patients get better when they take anti-depressants, they also get better when they take a placebo, and the difference in improvement is not very great,' the lead researcher said.

However, drugs manufacturers say their products have been highly effective since their introduction and that the new study's findings fly in the face of clinical evidence.