A weight loss pill which was initially hailed as some kind of wonder drug is now thought to more than double the risk of depression, anxiety and suicidal thoughts.
The drug Acomplia has already been denied approval in the U.S. because of the increased suicide risk.
In the 18 months it has been available in Britain there have already been reports to the nation's drug regulator, the Medicines and Healthcare Products Regulatory Agency, of negative psychiatric reactions associated with the drug.
It is estimated that as many as 40,000 Britons use Acomplia and trials have shown that two out of five patients lose 10 per cent of their weight while taking it.
But new evidence suggests one in ten of those using Acomplia may develop side effects such as low mood, anxiety, irritability, nervousness and sleep disorders.
In July the European Medicines Agency concluded that, except for patients with major depression or those taking antidepressants who should not be prescribed it, the benefits of Acomplia outweighed the risks.
Other research however has found that anti- obesity drugs including Acomplia offer poor results in terms of weight loss.
While some lost less than five per cent of their body weight it was found that many remained significantly overweight or obese.
A scientific review of four randomised controlled trials involving 4,105 patients found those using Acomplia were three times more likely to stop treatment because of anxiety than those on a dummy drug and there was a 40 per cent higher risk of being harmed by "adverse events or serious adverse events" among those using the drug.
Acomplia (rimonabant) costs about £2 a day.
In the review, Professor Arne Astrup, of the University of Copenhagen found while patients on Acomplia on average lost 10lb or more after a year, than those on the placebo, they also had an increased risk of depressed mood disorders and anxiety.