Tamoxifen for Breast Cancer Treatment

Frequently given after breast cancer treatment, but could bioidentical progesterone help as well?


One of the questions I am most often asked is whether bioidentical progesterone can be used by women who have either a history, or risk, of breast cancer.

Certainly many women do use bioidentical progesterone alongside Tamoxifen and hopefully I can answer some of the most common questions about its use here.

Tamoxifen is a weak oestrogen medication that blocks the oestrogen in the body. It has been prescribed for more than 30 years to fight breast cancer tumours that depend on oestrogen to grow.

How does it work?

Scientists aren’t sure exactly how the drug works. But they do know that some breast cancer cells are sensitive to oestrogen and they need it to grow and spread. In order to fuel that growth, oestrogen must attach itself to those breast cancer cells.

Doctors think that Tamoxifen stops oestrogen from attaching to the cell and no oestrogen means no growth for these types of breast cancer cells.

Bioidentical doctors however believe that tackling the excess oestrogen (oestrogen dominance) which is linked to such hormonal cancers by using bioidentical progesterone can also be effective.

Progesterone is the hormone in the body that balances and opposes excess oestrogen but that hormone is often in much lower amounts in the body than oestrogen.

Why is it prescribed?

There are different reasons, but the most common are:

  • To treat breast cancer after surgery or radiation
  • To treat cancer in one breast and reduce the risk of the disease in the other breast
  • To prevent invasive breast cancer in women at high risk
  • To treat the earliest stage of breast cancer, called ductal carcinoma in situ (DCIS), after surgery and radiation
  • To treat other cancers such as ovarian cancer, uterine cancer, melanoma, and brain tumors

It can also be used to prompt ovulation in women with fertility problems, and In higher than normal doses, Tamoxifen may even kill some breast cancer cells that don’t depend on oestrogen.

It is an oestrogen blocking drug and can be used in premenopausal and post-menopausal women.

What are the side effects?

Despite the evidence that Tamoxifen reduces the risk of developing breast cancer, the drug is not widely used for breast cancer prevention because it has not been found to improve survival when taken as a preventive drug.

It also comes with a small but serious risk of side effects, including:

  • Blood clots
  • Strokes
  • Heart disease
  • Uterine cancer
  • Cataracts

Milder side effects include symptoms that are similar to menopause, such as:

  • Vaginal dryness
  • Hot flushes
  • Leg cramps
  • Joint pain

Tamoxifen may also cause irregular periods and sexual problems.

If you’ve been diagnosed with breast cancer, or you’re at high risk for the disease, talk with your doctor to see if Tamoxifen is right for you to weigh the benefits and risks of this drug to make sure you get the healthiest result.

The role of bioidentical progesterone

As it is excess oestrogen (oestrogen dominance) that is associated with hormonal cancers, it is the natural balancing hormone of progesterone that can be most helpful for women with a history, or risk of breast cancer for which Tamoxifen, or similar drugs, are most commonly prescribed.

There are no clinical trials that I’m aware of, but there certainly has been clinical research into the benefit of progesterone in breast cancer. Researchers from the United Kingdom and Australia found that when the cancer cells were exposed to oestrogen alone vs oestrogen plus progesterone it was linked to better clinical outcomes.

Adding progesterone to Tamoxifen, the researchers found the use of progesterone with tamoxifen slowed tumour growth. The results were published in Oncology, the Journal of the Cancer Network, but it is important to distinguish between natural progesterone and synthetic progestins as these are not the not the same.

Dame Dr Shirley Bond – a private GP who has been prescribing bioidentical hormones for many years – has recommended using bioidentical progesterone alongside Tamoxifen to offset the side effects of the drug and this would probably be the same for all drugs of that class.

Rebalancing with bioidentical progesterone has helped many women, but as with any hormone cream, it should not be applied to the chest or breast area and vaginal application can be used instead or alongside this.

Helpful information: 

Keeping oestrogen and progesterone in balance is essential in order not only for overall hormonal health, but to reduce the risks associated with excess oestrogen such as hormonal cancers, heart disease and strokes.

What is not often realised is that women can be oestrogen dominant even with low levels of oestrogen, if their progesterone levels are even lower, so it is helpful to keep an eye on your symptoms to see whether or not you maybe oestrogen dominance or not.Frequently given after breast cancer treatment, but could bioidentical progesterone help as well?