Scientists have discovered that Prozac, the antidepressant taken by millions of people around the world, may stimulate the growth of brain tumors by blocking the body's natural ability to kill cancer cells.

An international team of researchers led by John Gordon, professor of immunology at Birmingham University, found evidence to suggest cancer cells can be killed by "positive thinking", which could be blocked when people take Prozac.

The study examined the effects of Prozac and other antidepressants on a group of tumor cells growing in a test tube. The researchers found that the drug prevented the cancer cells from committing "suicide", thereby leading to a more vigorous growth of the tumors.

Although an increased risk of cancer has not so far been detected in Prozac patients, the latest findings could lead to a global re-evaluation of the drug's long-term safety.

Prozac, a "happiness pill" that was first approved in the United States in 1987, is widely used for the treatment of depression, obsessive-compulsive disorder and bulimia nervosa.

The research work was designed to find new ways of treating lymphomas, a type of blood cancer, by investigating how the brain communicates with the immune system to induce "positive thinking" through a neuro-transmitter in the brain called serotonin.

Serotonin is a natural chemical that regulates people's moods, keeping them balanced. Too much serotonin affects appetite and sleep and too little affects the mood - often causing depression.

Prozac, along with other members of the class of antidepressants known as selective serotonin re-uptake inhibitors (SSRIs), works by preventing serotonin from being quickly reabsorbed by nerve cells in the brain.

The scientists tested other SSRIs such as Paxil and Celexa and found they, too, had the same effect in stimulating the growth of a type of tumour known as Burkitt's lymphoma.

An exciting property of serotonin is that it can tell some cells to self-destruct. We have found that serotonin can get inside the lymphoma cells and instruct them to commit suicide, thereby providing the potential for an effective therapy.

The researchers found that Prozac blocked the entry of serotonin into the test-tube tumor cells and therefore stopped them from committing suicide. That raised the question of whether Prozac can do the same in the brains of people taking the drug.

Further work is underway to test Prozac further in this field. In particular, the scientists want to develop drugs that will mimic the cancer-destroying feature of serotonin that is blocked by Prozac.

Blood, April 1, 2002, Vol. 99, No. 7, pp. 2545-2553