This is one of the most common questions I am asked by women who have received a breast cancer diagnosis.  This is encouraging to me, because now informed women are questioning the side effects of this pill, which is the most widely prescribed oral medication for breast cancer.

Tamoxifen has an interesting history. It is not a new drug — it was synthesized back in the 70’s as a birth control pill. But though it prevented pregnancy in mice, it instead increased fertility in women. Other scientists then decided to try it on advanced breast cancer patients and it did help — extending survival by many months or years. Tamoxifen is now seen as an estrogen blocker to the breasts — it inhibits estrogen from binding to cells. However, in other parts of the body – the uterus, bones, ovaries – it acts like an estrogen.

More information is now available about negative side effects of Tamoxifen, including increasing hot flashes, mood swings, and most significantly, increasing blood clots and uterine cancer. The last two are pretty rare, but significant.

When Tamoxifen is used in women with more advanced breast cancer, positive nodes or metastases, the benefits exceed these risks. However, for women with earlier stages, like DCIS or very small tumors, one does need to balance the benefits with the risks. A woman should be able to have an objective discussion with her provider about the true benefits of Tamoxifen (Is there a 3% or 10% improvement?) and then make an informed decision. Some women want to try everything, even if the benefit is small; others do not see it that way. For some women Tamoxifen is a safety net; for others it is a poison they cannot swallow.

Tamoxifen is now FDA approved for prevention of breast cancer in women with strong family histories or with atypical biopsies or LCIS. I have found that Tamoxifen is actually best tolerated by the younger women, from the age 35 to 45, who are still having their periods. It is recommended that they take it for 5 years (using contraception, because of potential birth defects) and is said that the benefits last a lifetime.

In my experience, perimenopausal women have the most problems with Tamoxifen, as far as aggravating the symptoms of menopause and “just not feeling right.” Some women tough it out, others stop. The recommendation is to take it for 5 years only, but to get the most benefit you have to take it for at least 2 years. No one has tested it for in between lengths, like 3 or 4 years. I often suggest that women give Tamoxifen a try for at least 3 months and keep a diary of how they feel taking it. Belief is important here and if you don’t think it is going to work, I doubt that it will.

For post menopausal women, newer drugs called aromatase inhibitors are on the market, and appear to be superior to Tamoxifen. They block the conversion of estrogen from one’s fat, cholesterol or adrenal sources. Women seem to have fewer side effects, except for joint pain, with these drugs. However, these have been studied for a relatively short period of time, so we don’t have long term data on them.

Another option to ask your doctor about is ovarian suppression by surgery or pills. The benefits may be equal to chemotherapy, but also have other side effects, such as going into sudden menopause.

A 2014 study  found that a tamoxifen gel applied topically was as effective at the pill in reducing the growth of breast cancer cells in women with non-invasive cancer, without as many side effects.  This finding was a promising new step towards other alternatives for women.

While tamoxifen might be the best treatment for some women with breast cancer (or who want to prevent it), my hope is that soon everyone will have a better understanding of which drugs will benefit which women most — and when alternative options might be a better route to take.