According to new research from Britain, new generation anti-depressants could be for many patients a waste of time and money and offer little clinical benefit.
Scientists from Hull University carried out an analysis of a 47 clinical trials involving five thousand patients with depression and found that most widely-prescribed anti-depressants work little better than dummy pills.
The antidepressants - Selective Serotonin Reuptake Inhibitors (SSRI) - which were studied included Prozac, Seroxat and Efexor and were found to be little more effective than placebos in improving the mental health in the majority of cases.
The new research found that the most widely prescribed anti-depressant pills are ineffective and only in the most extreme cases of depression did the tablets prove substantially superior in improving mental health.
Lead researcher Professor Irving Kirsch says the difference in improvement between patients taking placebos and patients taking anti-depressants is not very great, which means that depressed people can improve without chemical treatments.
Antidepressants are taken by around two million Britons and have been linked to a host of side effects including suicide, suicidal thoughts, self harm, anxiety, insomnia, nausea, headaches and vomiting.
Some experts have described the results as "fantastically important".
The study has credibility because it has included data which drug companies had chosen not to publicise possibly because it was less favourable than they would like, and it suggests that millions are needlessly taking powerful and potentially dangerous drugs.
Seroxat alone has been linked to at least 50 suicides in both adults and children in the UK since 1990.
The research is timely as it comes as prescriptions for anti-depressants have reached record levels, with 31 million written in 2006 at a cost to Britain's NHS of almost £300million, around half of these were for Prozac, Seroxat, Efexor and other SSRIs.
Researcher Professor Irving Kirsch says the data suggests there is little evidence to support the prescription of anti-depressant medication to any but the most severely depressed patients, unless alternative treatments have failed to provide benefit.
The data on Prozac, Seroxat, Efexor and a fourth drug not used in the UK had been submitted to the U.S. drug watchdog ahead of the antidepressants being licensed for sale.
Of those taking part in the studies two-thirds were prescribed the SSRIs, while the remainder took placebo tablets.
A comparison of the two groups showed that in the majority of cases the mental health of those taking anti-depressants improved little more than those on the placebos, and only those who were severely depressed (a very small proportion of those studied) improved substantially on the medication.
The results suggest that those taking the tablets mainly benefit from the "placebo effect" - in which symptoms are eased not by medication but by relief in diagnosis and the simple expectation a treatment will work.
Professor Kirsch says patients should not change their treatment without speaking to their doctor, but said other approaches such as physical exercise, psychoanalysis and self-help books, have been found to help.
The Association of the British Pharmaceutical Industry says all medicines have to be proven to be more effective than a placebo before they are put on sale while the National Institute for Health and Clinical Excellence (NICE), which issues guidelines on the treatment of illnesses, says the routine use of anti-depressants is not recommended for mild depression.
NICE says Professor Kirsch's results would be taken into consideration when the existing guidelines are reviewed later this year. Professor Kirsch says the findings call into question the current system of reporting drug trials.
Prozac and Seroxat are two of the most widely used anti-depressants and GlaxoSmithKline, which makes Seroxat, said the study only looked at a "small subset of the total data available".
Experts say the findings undermined confidence in the ability to draw meaningful conclusions about the merit of drugs based on published data alone and call for drug companies to be forced to publish all their data.
The study is published in the journal PLoS Medicine.