New Research Shows Natural Progesterone Can Help Treat Breast Cancer

An Exclusive Feature Article from the October 2015 Issue of the Hormone Balance Hotline

Two months ago, a large team of scientists working on multiple continents published a research study that came to startling conclusions about breast cancer and natural progesterone. The team determined that unlike synthetic progestins, which increase breast cancer risks, natural progesterone has the potential to slow the growth of many breast cancer tumors or even shrink them.

While this finding is stunning, it is not new. It is one of several conclusions about progesterone that John R. Lee, M.D. and David Zava, Ph.D. made more than a decade ago when they co-wrote the book, What Your Doctor May Not Tell You About Breast Cancer. Now that their findings have been confirmed by other scientists, the medical community can no longer assume that natural progesterone promotes breast cancer like progestins do. Progestins are molecularly altered synthetic versions of progesterone.

It’s All About Receptors

For years, breast cancer researchers have known that women whose breast cancers contain both estrogen receptors and progesterone receptors (known as ER positive/PR positive tumors) have better treatment outcomes than women whose tumors do not have these receptors. What researchers have not understood is why this is the case. To find out, scientists at Cancer Research UK and the University of Adelaide in Australia studied the interactions between estrogen and progesterone receptors in breast cancer cells. They published their findings in the July 16, 2015 issue of the scientific journal Nature.

Before we discuss the study, let’s answer the question that many of you may be asking. What are estrogen and progesterone receptors, and what do they do? Estrogen and progesterone receptors are proteins found within many of the cells of our bodies, including cells in the breasts. They are the mechanism that allows estrogen and progesterone to change the behavior of our cells. In the process, they change how many tissues and organs in the body function. Estrogen receptors can only interact with estrogen molecules, while progesterone receptors can only interact with progesterone molecules.

When an estrogen or progesterone molecule comes in contact with its respective receptor, the molecule binds to the receptor and activates it. Once this happens, the receptor enters the nucleus of its cell and attaches to specific spots on the chromosomes that contain all of the cell’s genetic coding. When the receptor does this, it “turns on” and “turns off” specific genes that govern the behavior of the cell. So in a real sense, estrogen and progesterone receptors are constantly reprogramming our cells by turning selected genes on and off. However, these receptors can only do their work if the body provides them with estrogen and progesterone to activate them.

For years, scientists have known that when activated by most forms of estrogen, estrogen receptors turn on genes within cancerous cells that program those cells to multiply rapidly and stay alive rather than die off as normal, healthy cells do. This means that most forms of estrogen – especially estradiol and its metabolites – are potent fuels for breast cancer. That is why oncologists try so hard to reduce estrogen levels in breast cancer patients with drugs such as Tamoxifen, Femara, and Arimidex.

While scientists know how estrogen receptors fuel the growth of cancer cells, they know a lot less about what progesterone receptors do in those same cells. That lack of knowledge is what the latest research study was designed to correct. In the study, scientists took breast cancer cells that were ER positive/PR positive and exposed them to enough estrogen and progesterone to activate both the estrogen and progesterone receptors. They then used new, cutting-edge techniques to examine what the receptors did within the cancer cells. What they found amazed them. When activated by progesterone, the progesterone receptors attached themselves to the estrogen receptors. Once this happened, the estrogen receptors stopped turning on genes that promote the growth of the cancer cells. Instead, they turned on genes that promote the death of cancer cells (known as apoptosis) and the growth of healthy, normal cells!

Since these experiments were only performed on cancer cells in test tubes, the researchers took the next step and ran tests on breast cancer tumors in live mice. After embedding ER positive/PR positive breast tumors in a number of mice, they exposed some of the mice to estrogen only, others to both estrogen and progesterone, and others to no hormones at all. After 25 days, the team found that while the tumors in the mice that received only estrogen grew, the tumors in the mice that received both estrogen and progesterone decreased in size.

It should be noted that the research team gave the estrogen inhibitor Tamoxifen to some of the mice that had also been treated with natural progesterone. They then compared the tumors of these mice to the tumors of mice that received progesterone but not Tamoxifen. While tumor growth was reduced in both sets of mice, the tumors of the mice treated with both progesterone and Tamoxifen experienced the greatest growth reduction.

This led the research team to advise that doctors combine progesterone with estrogen inhibitors such as Tamoxifen in their patients’ treatment plans. While this advice deserves consideration and further research, Dr. Lee and Dr. Zava point out that Tamoxifen and other estrogen inhibitors have serious side effects that should play a role in any decision about their use.

Taken together, the experiments conducted by the research team led them to a powerful conclusion. When activated by progesterone, progesterone receptors bind to and “reprogram” estrogen receptors, transforming them from agents that turn on cancer-promoting genes to ones that turn on genes which slow down or even reverse the growth of cancer cells. The researchers also pointed out that their conclusions apply to natural, bioidentical progesterone. They rightly observed that many progestins – the synthetic, molecularly altered forms of progesterone found in pharmaceutical drugs – have been clearly shown to increase rather than decrease breast cancer risks.

These findings are incredibly good news for women diagnosed with estrogen receptor positive/progesterone receptor positive breast cancers. If such women have healthy progesterone levels or raise them to those levels through natural progesterone supplementation, they could dramatically improve their treatment outcomes. According to the American Cancer Society, around two out of three of all breast cancers are hormone receptor-positive. This means that the majority of women suffering from breast cancer may benefit from adding natural progesterone to their treatment plans.

How the Latest Study Vindicates Dr. Lee and Dr. Zava

While the scientists behind the Nature study may not know it, Dr. Lee and Dr. Zava came to the same conclusions about natural progesterone and breast cancer 13 years ago. In 2002, the two men reviewed all of the available research on breast cancer in their groundbreaking book, What Your Doctor May Not Tell You About Breast Cancer, and came to the following conclusions:

  • Women with progesterone levels that are low relative to estrogen levels are more likely to get breast cancer and have poorer treatment outcomes. Dr. Lee coined the term estrogen dominance to identify the hormonal condition of progesterone being low relative to estrogen. Based on available research, he and Dr. Zava concluded that estrogen dominance causes estrogen receptors to activate genes such as Bcl-2 that are known to promote the rapid growth of cancer cells.

  • When progesterone is raised to healthy levels relative to estrogen, it turns on genes that can prevent breast cancer from occurring and reduce the size of existing tumors. Dr. Lee and Dr. Zava cited research studies which show that progesterone receptors activate genes such as p53 that promote apoptosis. Apoptosis enables the body to “kill off” many cancer cells before they develop into tumors.

    Because progesterone promotes the healthy growth and death of cells, the hormone does two things. First, it can prevent healthy cells in breast tissue from mutating into tumors. Second, it can limit the growth of existing breast tumors or even reduce them in size. Dr. Lee said many times that women with hormone receptor positive breast cancers could especially benefit from natural progesterone supplements because their tumors had progesterone receptors to which the progesterone could bind.

In short, Dr. Lee and Dr. Zava anticipated the conclusions of the latest research study 13 years ago. At the time, many doctors dismissed their statements about the importance of progesterone. To this day, many oncologists refuse to let their breast cancer patients use natural, bioidentical progesterone out of fear that it could fuel tumor growth. Now, thanks to the new study in Nature, the medical community must rethink its position. It turns out that Dr. Lee and Dr. Zava were right all along.

A Recipe for Beating (and Preventing) Breast Cancer

The latest research on natural progesterone and breast cancer clearly indicates how important it is for women to maintain healthy, normal levels of progesterone that are in proper balance with estrogen. Doing so could not only increase many womens’ chances of recovering from breast cancer – as the latest research indicates – but could also help them to avoid getting breast cancer in the first place.

Sadly, as Dr. Lee and Dr. Zava point out in their book, hormonal imbalances have reached epidemic proportions in most developed countries over the last several decades. Due to poor diets, lack of exercise, a rise in obesity levels, the widespread use of hormone-altering chemicals, and other factors, many women suffer from chronically higher than normal estrogen levels and much lower than normal progesterone levels. In other words, many women are in chronic states of estrogen dominance. This is one of the key reasons why breast cancer rates are as high as they are.

Considering the epidemic levels of hormonal imbalance we are experiencing, how can a woman know if her progesterone and estrogen levels are in proper balance? If they are out of balance, how can she return them to proper balance and maintain them in that all-important state? Dr. Lee and Dr. Zava used What Your Doctor May Not Tell You About Breast Cancer to answer these questions. While it is not possible here to cover everything they wrote, here is a short summary of their recommendations.

  • Check yourself for symptoms of estrogen dominance. While being estrogen dominant is bad news, the good news is that it usually leaves a clear trail of symptoms. To find out if you may be estrogen dominant, read Dr. Lee’s list of estrogen dominance symptoms. If you find that you have a number of the symptoms on this list, chances are good that you are suffering from this syndrome. You can learn more about hormonal imbalances you may have by taking Dr. Lee’s free Hormone Balance Test.

  • Get your hormone levels tested. While symptoms are good indicators of hormonal imbalances, the most decisive tool for identifying imbalances is a hormone test. As a general rule, Dr. Lee and Dr. Zava recommended that women who are concerned about breast cancer test at least five hormones. These are estradiol (the most potent estrogen in the human body and the one most frequently linked to breast cancer), progesterone, testosterone, cortisol, and DHEA-S. For more information, click here to read our article about how to test your hormones when breast cancer is a concern. You can also order these tests in saliva (as Dr. Lee and Dr. Zava recommended) from our website.

  • Work with doctors who understand natural hormones. Beating breast cancer is a team effort, so build a team that will support rather than thwart your quest for hormone balance. While growing numbers of doctors are becoming aware of the value of natural hormones, many have not kept up with the latest research and may resist your suggestions. To help you build your team, we offer a page on our website where you can find doctors who work with natural hormones. The page also offers a number of tips for finding such doctors in your community.

  • When needed, take physiological doses of bioidentical progesterone and other bioidentical hormones to restore proper balance. When it comes to taking natural hormone supplements, it is critical to remember that more is not better. The goal is to return hormone levels to what would be considered normal for a healthy person. In most cases, this means taking relatively small amounts of bioidentical hormones and regularly reevaluating hormone levels through saliva testing. Many women find after testing their hormones that all they need is some bioidentical progesterone to establish proper balances between the major hormones. Others, however, find that they may need to add other natural hormone supplements to achieve balance and get adequate symptom relief. A good doctor who understands natural hormones can advise you on your supplement strategy and help you consider your options.

  • Eliminate hormone-altering chemicals and xenohormones from your life. Every day, our bodies are exposed to toxic chemicals that did not exist just a decade or two ago. There are synthetic hormones in the foods we eat, pesticides in our air and water, and estrogen-like compounds in many of the products we use every day. Many of these chemicals and xenohormones are known cancer-causing agents. Fortunately, we can sharply reduce our exposure to these substances and dramatically reduce their presence in our bodies. What Your Doctor May Not Tell You About Breast Cancer identifies the sources of these chemicals and offers concrete advice for avoiding them.

  • Use diet and exercise to support hormone balance. Our modern diets are heavily tilted towards foods that promote obesity and estrogen dominance. Our sedentary lifestyles only reinforce this problem. Both women and men can benefit from reducing their intake of sugars, refined carbohydrates, and foods that are high in trans-fatty acids while increasing their intake of organic vegetables, fruits, and fiber. They can also benefit from regular, moderate exercise, which helps metabolize and eliminate excess estrogens. There are entire chapters about these subjects in Dr. Lee’s books for you to read.

  • Keep educating yourself, for you are your best health advocate. When it comes to preventing or fighting breast cancer in your body, you have every right to be the leading decision maker. Dr. Lee and Dr. Zava firmly believed this and wrote What Your Doctor May Not Tell You About Breast Cancer for patients as well as their doctors. The book contains a wealth of information that can help you make important decisions with your doctor. For instance, if your doctor is recommending you take an estrogen inhibitor such as Tamoxifen, the book can help you weigh the pros and cons of using such drugs as well as chemotherapy, radiation, and other treatment options. So we encourage you to buy this book, read it carefully, and discuss it with your doctor. If you prefer to educate yourself by listening to lectures, we also offer a two-CD lecture on hormones and breast cancer by Dr. Lee that contains excellent information from the book. In addition, we encourage you to read the free articles about breast cancer on The Official Website of John R. Lee, M.D. as well as the references listed at the end of this article.

Thanks to the latest research, we have further proof that Dr. Lee and Dr. Zava were ahead of their time when they said that natural hormone balance could help prevent and treat breast cancer. We support you in learning from them, putting what you learn into practice, and sharing what you learn with your family, friends, and doctors.


Mohammed, Hisham, et al “Progesterone receptor modulates ER-a action in breast cancer,” Nature 2015; 523; 313-317. Click here for abstract.

Perks, Bea “Progesterone receptor could slow breast cancer growth,” Pharmaceutical Journal, PJ 17 Jul 2015. Click here to read.


How Hormone Balance Can Help Save Your Life
by John R. Lee, M.D., David Zava, Ph.D. and Virginia Hopkins.
Warner Books 2002 (410 Pages)

Another pioneering book by John R. Lee, M.D. that really gets to the bottom of why women get breast cancer and how to prevent it. It covers a wide array of topics including how HRT may trigger breast cancer, why doctors use chemo and radiation even though they don't work very well, what causes breast cancer, how to prevent it, and the remarkably preventive benefits of natural hormones-- when used properly.