Prozac linked to increased cancer growth

Experiments by John Gordon and colleagues at Birmingham University show that serotonin can act as a cancer suppressor in Burkitt's lymphoma by encouraging cancerous cells to self-destruct. Serotonin is one of the brain's main "happiness chemicals". The team also found that a group of anti-depressants known as the selective serotonin re-uptake inhibitors (SSRIs), which include Prozac, may block this effect.

However, Gordon agrees with psychiatrists that the findings should not prompt patients to stop taking their antidepressants to avoid the theoretical risk of increased cancer.

John Cleare, of the Institute of Psychiatry in London, said: "The risk of stopping the drugs you need is much higher than any risk suggested by this study."

A spokesman for Eli Lilly, the pharmaceutical firm that makes hundreds of millions of pounds from the drug, was also quick to reassure patients. A spokeswoman said: "There is no medical or scientific evidence of a connection between fluoxetine (Prozac) and cancer."

Positive thinking

Gordon's work focused on trying to elucidate the neurochemical basis for "the power of positive thinking" over disease. He told New Scientist: "Positive thinking can have a major impact on a variety of diseases including cancer. We wanted to know why."

"Our work shows that serotonin can get inside the lymphoma cells and instruct them to commit suicide, thereby providing the potential for an effective therapy," he says.

The body has 14 different serotonin receptors and the researchers found the key one was a serotonin transporter protein blocked by SSRIs. When it was blocked by Prozac, the experiments showed, serotonin ceased to have its suppressor effect. So, rather than commit suicide the cancerous cells continued to divide.

 Although the test tube experiments link Prozac with cancer, no epidemiological evidence has been found to support them.

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

Gordon's team now hopes to develop a range of anti-cancer drugs based on serotonin.

Journal reference: Blood (vol 99, p 2545)