Does Paxil cause breast cancer?

(This article was first printed in the March 2006 issue of the Harvard Women's Health Watch. For more information or to order, please go to www.health.harvard.edu/womens.)

Q. I heard there’s a relationship between Paxil and breast cancer risk. Is this true?

A. The short answer to your question is no: The available research does not show that Paxil (paroxetine) raises the risk of breast cancer. The story of how researchers have addressed this question illustrates how medical knowledge advances.

Paxil is one of a class of antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs), the best known of which is fluoxetine (Prozac). Some SSRIs have also been shown to reduce hot flashes, and are an alternative for postmenopausal women who would like to avoid hormone therapy, which increases breast cancer risk. But they can also increase levels of prolactin, a hormone that stimulates the growth of certain breast cells, so there’s been some concern they might increase breast cancer risk.

A 1992 laboratory study showed that Paxil accelerated the growth of mammary tumors in rodents. Later research did not replicate these findings in various human cells, including a human breast cancer cell line. But the rat study had spurred further research on the issue. In 2000, researchers reported a sevenfold increase in breast cancer risk among Paxil users compared with nonusers. And a 2003 study concluded that breast cancer risk doubled among women who had used SSRIs for three years or more. But both investigations were very small, and the results could have been due to chance.

In 2005, several large studies analyzing data from thousands rather than dozens of women all concluded that antidepressant use did not raise breast cancer risk. The largest study analyzed the medical records of 109,004 women who had used antidepressants for five years. It showed no increase in breast cancer risk overall, and no greater risk for women taking Paxil than for those taking other antidepressants.

The most recent study, conducted at the University of Seattle and published in December 2005 reported similar results.

There has been no randomized, controlled clinical trial — the scientific gold standard — to resolve this question definitively. But the latest group of studies is very reassuring. A woman who’s getting relief from depression by taking an SSRI should not be concerned that it will increase her risk of breast cancer.

— Celeste Robb-Nicholson, M.D.
Editor in Chief, Harvard Women’s Health Watch

(This article was first printed in the March 2006 issue of the Harvard Women's Health Watch. For more information or to order, please go to www.health.harvard.edu/womens.)

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