New Study: Paxil Neutralizes Breast Cancer Drug Tamoxifen

A new study has determined the antidepressant Paxil, taken while being treated for estrogen sensitive breast cancer with tamoxifen, increased the risk of death by blocking the beneficial effects of tamoxifen.

The study conducted by the Institute for Clinical Evaluative Sciences (ICES) in Toronto, and published in the February 8, 2010 edition of the British Medical Journal, has found the antidepressant, Paxil, or paroxetine, a member of the SSRI class of antidepressants, can block the effects of the breast cancer drug tamoxifen and so indirectly increase the risk of death. Tamoxifen, a drug commonly used as a treatment in estrogen sensitive breast cancer, is said to decrease the risk of death by one half. It works by being converted to an active metabolite (endoxifen) that blocks estrogen. Tamoxifen is also said to lower cholesterol and decrease the risk of osteoporosis. Dr. David Juurlink, one of the scientists and authors of the study, urges women not to stop using amoxifen, but if taking Paxil at the same time to treat depression, they should consult with their doctor. Stopping Paxil should take place gradually, not abruptly. According to ICES,

"Tamoxifen is a ‘prodrug’, meaning it must be converted into an active metabolite (endoxifen) by the liver in order to work. However, some drugs can interfere with this process. Antidepressants are of particular importance because they are commonly used by women with breast cancer, often for long periods of time. Although many antidepressants have little or no impact on tamoxifen’s metabolism, paroxetine, a member of the so-called SSRI class, is a potent inhibitor of the metabolic step that converts tamoxifen to endoxifen. "

While this particular study indicates no other antidepressants were found to block the effects of tamoxifen, other sources, such as The Toronto Star, report that there have been suspicions by scientists about similar effects in other antidepressants, such as Wellbutrin and Prozac. (For related DJ article, click here.) According to BreastCancer.Org, other medications that can block the effects of tamoxifen, are Cardioquin (chemical name: quinidine), Benadryl (chemical name: diphenhydramine), and Tagamet (chemical name: cimetidine). Hormone sensitive breast cancers account for 75 per cent of all breast cancers, and are more common in post menopausal women whose hormones decrease and become unbalanced. (For more details on estrogen imbalance and hormone replacement therapy according to the late Dr. John R. Lee, whose studies published a decade ago revolutionized thinking in this field, click here.)