About the Premarin Study and Breast Cancer

This week a paper came out in JAMA showing a surprising reduction in breast cancer cases among women who had hysterectomies and then took Premarin, an estrogen-only remedy compounded from steroids in horses’ urine.

drawing of a horse (Wiki-Commons)

The research merits attention because it’s part of the Women’s Health Initiative and is well-done by several measures: The study is large, placebo-controlled and randomized. The investigators followed 10,739 post-menopausal women ages 50-79, all who previously had surgical removal of the uterus. The women took CEE (conjugated equine estrogens) at a dose of 0.625 mg by mouth per day, or a placebo. The trial was stopped a year early, after an average follow-up of seven years, after the analysts noted that more women taking Premarin had strokes. But I digress…

The main findings were two: 1) Premarin, at the dose prescribed, had no clear effect on cardiovascular disease, overall, either way; and 2) there was a significant reduction in invasive breast cancer among women taking estrogen in this study. More precisely: the incidence of BC was 0.27% in women who took Premarin and 0.35% in the placebo group. For the stats-guys: the relative risk (RR) was 0.77, with 95% confidence intervals of 0.62-0.95.

The findings were covered by major news organizations and other women’s health bloggers. Still you may wonder, what’s ML’s take on this subject?

According to the Times as many as a third of women in the U.S. have had hysterectomies. The CDC says that approximately 600,000 hysterectomies are performed each year in the U.S. No matter what the numbers – which seem to me discordant – my first reaction is to wonder why so many women have their uterus removed.

About the apparently lower incidence of breast cancer, it may be that this particular dose of estrogen in the form of Premarin does tend to reduce or somehow suppress BC in post-menopausal women who’ve had hysterectomies.

But it could just as well be a statistical fluke-

If you examine a sufficient number of outcomes in a research group of thousands of women, you’re likely to find one aberration or another. What we do know is that most breast cancer cells do express receptors for estrogen, which can stimulate those cells’ growth.

Presumably, time and the scientific process will yield the truth about Premarin and other hormones some people take so that they might feel better. Then again

Personally, I wouldn’t go near the stuff.

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  • Nice post, Elaine. The JAMA article is indeed well done. That said, after seeing the mothers of several friends develop breast or ovarian cancer, virtually all after long term use of Premarin, I am personally inclined to agree with your conclusion!

  • I was put on estrogen at age 40 after a complete hysterectomy and removal of ovaries. I took premarin 0.625 until now, age 73. I was recently diagnosed with estrogen type breast cancer and told to STOP the premarin immediately. I had a lumpectomy and facing more surgery to stage the cancer, chemo therapy at the very least, and possibly more. There is absolutely no history of breast cancer in my family, therefore I believe it is because of long term use of PREMARIN!

  • I am also 73, and took 0.625 Premarin until about one month ago. There is also no history of breast cancer in my family. I had a total hysterectomy 34 years ago due to endometriosis. In the past month I have had a modified radical mastectomy and am currently undergoing tests to determine the appropriate treatment(s) to follow. I had mamograms yearly for over 25 years, and the mamogram taken in May 2013 was clear. Since there was a definite lump, an ultrasound was done immediately after, and on the same day as the mamogram. The tumor was over 6 cm.

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