I have very dense breast tissue, a family history of breast cancer, and therefore a greater-than-average lifetime risk for breast cancer. During my annual gynecologist appointment, my doctor recommended that I consider taking a low dose of tamoxifen daily for breast cancer prevention. I had not heard of this and hope you can share any data available on the efficacy and side effects of this treatment.
—Anonymous in PA
Tamoxifen is a selective estrogen receptor modulator. This means it acts on some estrogen receptors to block the effects of estrogen. In women who have had estrogen-receptor-positive breast cancer, tamoxifen has been shown to reduce the risk of recurrence.
It is not a new drug. It was approved in 1998 for use to prevent breast cancer recurrence. As is often the case with older drugs, researchers have studied tamoxifen in a variety of different scenarios and doses.
Studies exploring the use of tamoxifen to prevent breast cancer in women at high risk began in the early ’90s and have found that 20 milligrams of tamoxifen does reduce the risk of breast cancer in these women.
Based on an observation that tamoxifen decreases breast density, a more recent randomized controlled trial looked at the effect of low-dose tamoxifen on breast density. In this case, breast density is a marker of how well tamoxifen is working in the breast. The researchers found that doses as small as 2.5 mg of tamoxifen showed a reduction in breast density with minimal side effects. However, doses this low have not yet been shown to prevent breast cancer in women at high risk.
The takeaway: Tamoxifen has been shown to reduce the risk of breast cancer in high-risk women. Lower doses may also be effective but have yet to be studied.
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