Prozac was launched in 1987 - it has been around for long enough for any cancer risk of this kind to become clear

Professor John Gordon, University of Birmingham

However, although millions of patients worldwide take Prozac or similar drugs, there is no evidence that they are more vulnerable to cancer.

The drug's maker insists that there is "conclusive evidence" that it does not cause cancer, or speed its growth.

Professor Gordon, told BBC News Online: "I have looked at a number of large-scale studies looking specifically at these drugs in relation to cancer, and there is nothing to suggest that they increase cancer risk.

"Prozac was launched in 1987 - it has been around for long enough for any cancer risk of this kind to become clear."

The risk of stopping drugs you need is much higher than any risk suggested by this study

Dr John Cleare, Institute of Psychiatry

There are fears that patients alarmed by a newspaper report suggesting the link between antidepressants and cancer might suddenly choose to stop taking their drugs.

Patients can fall back into depression when they stop taking medication, and in a few cases, this has contributed to their suicide.

Professor Gordon said that patients should definitely carry on taking the drugs.

He said: "The benefits of these drugs are huge."

Dr John Cleare, from the Institute of Psychiatry in London, told BBC News Online: "The risk of suicide is an extreme risk - but the risk of stopping drugs you need is much higher than any risk suggested by this study.

"It would be very sensible for any person worried by this to make an appointment with their GP to talk about the issue."

Professor Gordon's work, funded by the Medical Research Council, aimed to investigate the potential of positive thinking to influence the outcome for cancer patients.

Fighting spirit

Previous studies on this issue have been mixed - some have shown that a "fighting spirit" improves outcome, others that it had no bearing, or that feelings of hopelessness made recurrence or death more likely.

However, the unexpected qualities of seratonin were potentially far more exciting, he said.

The brain chemical serotonin is important in regulating the mood of the patient - people with depression often have less of the chemical than average.

Prozac works by stopping brain cells absorbing seratonin too quickly.

The Birmingham team put serotonin in a test tube with cells from a type of cancer called Burkitt's lymphoma.

They found that serotonin was able to get inside the tumour cells - and "instruct" them to commit suicide.

This suggests that the chemical - and its target within the cancer cell - has potential as a source of cancer drugs in the future.

Professor Gordon said: "Now we have to find out if the same thing actually happens in the human body itself."

Big earner

Prozac is the best known of the SSRIs, and doctors in the UK issue approximately 3m prescriptions a year for it.

Worldwide sales topped �1.8bn in 1999.

A spokesman for Eli Lilly, the pharmaceutical firm which makes it, said: "There is no medical or scientific evidence of a connection between fluoxetine (Prozac) and cancer.

"Results of Eli Lilly and Company's long-term studies have been reviewed extensively by various regulatory agencies and these studies have provided conclusive evidence that fluoxetine is not a cancer initiator or promoter.

"It is important that patients already receiving treatment with fluoxetine or other treatments continue to take their medication as advised by their doctor."