An international team of researchers, led by John Gordon, professor of immunology at Birmingham University in the U.K., 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.

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,? leading to a more vigorous growth of the tumors. The scientists tested Paxil and Celexa, and found they, too, had the same effect in stimulating the growth of a type of tumor known as Burkitt’s lymphoma.

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 was first approved in the U.S. in 1987 and is widely used for the treatment of depression, obsessive-compulsive disorder and bulimia.

The researchers were investigating how the brain communicates with the immune system to induce ?positive thinking? with serotonin. Prozac works by preventing serotonin from being quickly reabsorbed by nerve cells in the brain. ?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,? says Gordon.

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.

Professor Gordon says it?s premature to suggest that the drugs are unsafe. ?We must stress the effects shown for the [drugs] on cancer cells is indirect and should cause no concern whatsoever to the many millions of people throughout the world who are prescribed this class of antidepressants,? he says??However, it?s important that we look again and again.?

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