Dear Science,

WTF is the difference between hormone replacement therapy and birth control pills? Why is hormone replacement therapy considered so horrible and cancer-causing while birth control pills are not?

Waiting For My Annual Exam

Your question is an astute one. The hormones in birth control pills and in hormone replacement therapy are (roughly) the same: a mix of progesterone and estrogen. Here’s the truth: There is actually almost no difference in absolute risk between being on the pill and being on hormone replacement therapy. What differs is the perception of the risk.

Ovulation, objectively, is an unwise idea for your health. Unmolested, the female reproductive cycle involves a crescendo and decrescendo of estrogen followed by a peak and crash of progesterone each month. With the rise and fall of estrogen, tissues in the breast, the endometrium, and the ovaries are stimulated to grow, divide, and then die off. All of this brisk growth and destruction—as you’d expect—adds up to a risk of cancer later in life. The more cycles, the bigger the risk of cancer in the reproductive system—generally the big cancers for women.

Stopping this cycling is the key to reducing the risk—having lots of pregnancies or staying on the oral contraceptive pill (which hormonally mimics pregnancy). Menopause stops all this cycling—resulting in low levels of both progesterone and estrogen—and ends the accumulation of risk.

Hormone replacement therapy started as a means of smoothing out the chaotic estrogen and progesterone levels that cause the symptoms at the start of menopause: About half of women experience sleep disturbances, one-third experience mood disturbances, and up to 80 percent experience hot flashes. A huge study was undertaken to learn about the effects of hormone replacement therapy: the Women’s Health Initiative. With hormone therapy, in the WHI study, came increased risk of heart attack, stroke, and invasive breast cancer. For example, 37 out of 10,000 women receiving the hormone had a heart attack, as compared to 30 out of 10,000 not receiving the hormone.

There are two ways to think about this risk. As a relative increase, this represents a 23 percent increase (sounds scary). In absolute terms, seven out of 10,000 women had heart attacks they wouldn’t have had if not started on hormones. That’s a much less scary way of looking at the same results. On hormones or not, the risk is small in absolute terms—higher on hormones, but still small. Similarly, being on the pill slightly increases your chances of a heart attack, stroke, or blood clot (particularly if you are a smoker). The chances are similarly tiny—on the contraceptive pill or not. We don’t worry about the pill, because we’re more rationally considering the absolute risk—increasing the chance of a very unlikely bad thing really isn’t that big of a deal overall.

Riskily yours,

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