Myfembree is a once-daily pill that treats two conditions: heavy menstrual bleeding caused by uterine fibroids and moderate to severe pain caused by endometriosis. It works by lowering your body’s estrogen and progesterone levels enough to reduce bleeding and pain, while including small amounts of hormones to protect your bones and prevent menopause-like side effects. The FDA approved it in 2021.
How Myfembree Works
Myfembree contains three active ingredients working together. The primary one is a hormone blocker that attaches to receptors in the pituitary gland, a small structure at the base of your brain that controls your reproductive hormones. By blocking those receptors, it stops the signals that tell your ovaries to produce estrogen and progesterone. Lower estrogen levels shrink fibroid-related blood vessel growth and reduce the inflammation that drives endometriosis pain.
The problem with simply shutting down estrogen is that it mimics menopause. You’d get hot flashes, night sweats, and progressive bone loss. That’s why Myfembree also includes a low dose of estradiol (a form of estrogen) and norethindrone acetate (a synthetic progesterone). This “add-back therapy” keeps estrogen levels high enough to protect your bones and minimize vasomotor symptoms, but low enough that the therapeutic effect on fibroids or endometriosis is preserved. The progesterone component also protects the uterine lining from the effects of unopposed estrogen.
What It Treats
Myfembree has two distinct FDA-approved uses, both in premenopausal women:
- Uterine fibroids: Specifically for managing the heavy menstrual bleeding that fibroids cause. It doesn’t eliminate fibroids themselves, but it can dramatically reduce the blood loss that leads to anemia, fatigue, and disruption of daily life.
- Endometriosis: For managing moderate to severe pain, including period pain (dysmenorrhea) and the chronic pelvic pain that persists between periods.
How Well It Works for Pain
Clinical trials called SPIRIT 1 and SPIRIT 2, published in The Lancet, tested Myfembree against a placebo in women with endometriosis-related pain. The results were consistent across both studies. In SPIRIT 1, 75% of women taking Myfembree met the threshold for meaningful improvement in period pain, compared to 27% on placebo. For non-menstrual pelvic pain, 58% responded versus 40% on placebo.
SPIRIT 2 showed nearly identical numbers: 75% responded for period pain versus 30% on placebo, and 66% responded for non-menstrual pelvic pain versus 43% on placebo. The period pain results are especially striking, with response rates roughly two and a half times higher than placebo.
How to Take It
Myfembree is one tablet taken once daily, at roughly the same time each day, with or without food. If you miss a dose, take it as soon as you remember that same day, then go back to your regular schedule the next day. The recommended maximum duration of treatment is 24 months, a limit set because of concerns about bone density loss over longer periods.
Bone Density: The Key Trade-Off
Even with add-back therapy included, Myfembree still causes some bone loss. The degree is modest for most women, but it’s the main reason treatment is capped at two years.
In fibroid studies, women lost an average of 0.23% of lumbar spine bone density at six months. By 12 months, that increased to 0.80%. About 23% of women who continued treatment for a full year experienced bone density declines greater than 3%.
In endometriosis studies, bone density at the lumbar spine dropped an average of 0.72% at six months. Interestingly, the longer-term extension study showed the loss didn’t keep accelerating in a straight line: at 24 months, the average decline was 0.45%, suggesting some stabilization. Still, about 20% of women treated for two years had bone density declines exceeding 3%, and roughly 2.5% had declines greater than 7%.
These numbers matter most if you already have low bone density or other risk factors for osteoporosis. The FDA label is explicit: use should be limited to 24 months because continued bone loss may not be fully reversible.
Common Side Effects
Beyond bone density changes, the most frequently reported side effects mirror what you’d expect from a medication that alters hormone levels. Hot flashes are the most common, though the add-back therapy keeps them milder than they would be without it. Headaches, mood changes, and irregular bleeding patterns are also reported, particularly in the first few months as your body adjusts. Some women experience nausea, fatigue, or decreased sex drive.
Most of these side effects are manageable and tend to stabilize over time. That said, Myfembree carries a boxed warning related to the estrogen component. Women with a history of blood clots, stroke, or certain hormone-sensitive cancers are not candidates for this medication, because the estradiol in the formulation carries cardiovascular and clotting risks similar to other estrogen-containing therapies.
Who Should Not Take Myfembree
Myfembree is designed for premenopausal women. It is not appropriate if you are pregnant, trying to become pregnant, or breastfeeding. Women with a history of blood clots in the legs or lungs, stroke, heart attack, or breast cancer should not take it. Liver disease and undiagnosed abnormal uterine bleeding are also reasons to avoid it. If you smoke and are over 35, the estrogen component raises your cardiovascular risk significantly.
What the Treatment Experience Looks Like
For most women, the first few weeks involve an adjustment period. Bleeding patterns may be irregular initially before settling. Period pain and heavy bleeding typically start improving within the first one to two months. By the six-month mark in clinical trials, the full benefit was clearly measurable.
Because the treatment window is limited to two years, Myfembree is often used as a bridge: to control symptoms while you decide on longer-term management, to buy time before surgery, or to provide relief during peak symptom years. When you stop taking it, your natural hormone cycles resume, and symptoms may return. This is not a permanent fix for either fibroids or endometriosis, but for many women it offers meaningful relief during treatment.