Does Progesterone Cause Cancer?

Many people, including regulatory agencies and members of the medical community, make the mistake of thinking that progesterone and synthetic progestins have the same potential for adverse effects. This erroneous thinking is the root of women’s concerns that progesterone causes cancer. It’s also why some states require labeling on progesterone that suggests that natural progesterone products may increase the risk of cancer.

What Does Progesterone Do?

One of progesterone’s most important functions in the body is to help women maintain pregnancy. But it’s not just about baby-making. There are many research studies that show the protective role progesterone plays in the body. It protects against hypertension, lowers blood fats, protects nerves, and prevents coronary hyperactivity (a sign of coronary artery disease) and helps with hot flashes, night sweats, insomnia, and anxiety. There is also evidence that is may be protective against cancer.

There is NO evidence that it increases the risk of breast cancer.

What Does the Data Show?

If you look at the evidence, it’s no wonder there’s confusion.

We know that estrogen replacement (HRT) given without progesterone to women who still have a uterus causes endometrial cancer. Studies published in 1995 found that women who had been exposed to HRT for longer than 5 years had a 32% increased risk of getting breast cancer. If the estrogen was combined with a progestin, it went up to 41%, and for those women who were post-menopausal the risk went up to 71%, while the risk of getting ovarian cancer went up by 72%.

Then in 2002, all hell broke loose. The huge Women’s Health Initiative study was stopped early when the data showed an increase in breast cancer, heart disease, blood clots, and strokes in women who were given traditional HRT. As a result, millions of women were thrown into hormone hell when they were abruptly taken off the drugs by their doctors.

In this study, women were given synthetic estrogen (conjugated equine estrogen) PLUS synthetic progestin (medroxyprogesterone), synthetic estrogen alone (for women who had hysterectomies), or placebo (sugar pill).  Bio-identical progesterone was NOT studied.

The results showed that women in the two-drug group were 24% more likely to develop breast cancer than women in the placebo group. However, women in the estrogen-alone group did NOT have an increased risk (and in fact the, the trend was toward reduced risk). Further sub-analysis showed that the SYNTHETIC PROGESTIN was actually the culprit causing the increased breast cancer risk.

Progestin ≠ Progesterone

The medical community has lumped PROGESTERONE into the category of PROGESTINS. Progesterone is chemically identical to the hormone made by your body. PROGESTIN is not chemically identical and may have many side effects such as acne, breast pain, nausea, weight gain, headaches, and irregular periods. Synthetic progestins are found in IUDs, implants, and oral contraceptives.

Bio-identical progesterone is made from a compound found in wild yams (diosgenin). This compound is altered in the lab to be identical to what your body produces. Your body cannot make this conversion on its own, so wild yam cream is not effective. Bio-identical products are available in creams, oils and oral capsules. Creams and oils are available over-the-counter, but oral preparations require a prescription.

What If I’ve had Breast Cancer?

In his book “What Your Doctor May Not Tell You about Menopause”, Dr. John Lee (a pioneer in the study of progesterone) discusses the cancer-protective benefits of progesterone. He refers to various studies; one over a 20-year period which reported a 5.4 times likelihood of breast cancer developing in women with low progesterone levels. He concludes that “the evidence is strong that unopposed estradiol and estrone are carcinogenic for breasts, and both progesterone and estriol, the two major hormones throughout pregnancy, are protective against breast cancer.”

I am not aware of any large-scale clinical trials looking specifically at progesterone use in patients who have had hormone-sensitive cancers. As with many therapies, it’s important to assess risk vs. benefit (including quality of life issues). Any woman who has a history of cancer should discuss the use of any hormonal therapy with her care team.

Dr. Anna Garrett is a menopause expert and Doctor of Pharmacy. She helps women who are struggling with symptoms of perimenopause and menopause find natural hormone balancing solutions so they can rock their mojo through midlife and beyond. Dr. Anna is the author of Perimenopause: The Savvy Sister’s Guide to Hormone Harmony. Order your copy at

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