Mifepristone blocks the hormone progesterone from working in the body. This single action sets off a chain of biological changes that make it useful for two very different medical purposes: ending an early pregnancy and managing a hormonal disorder called Cushing’s syndrome. Here’s how it works and what to expect.
How Mifepristone Works in the Body
Mifepristone binds to progesterone receptors inside cells, locking into the same spot progesterone would normally occupy. By sitting in that receptor without activating it, the drug effectively blocks progesterone’s effects. Think of it like a key that fits the lock but won’t turn: it prevents the real key from getting in.
In pregnancy, progesterone plays a critical role in maintaining the uterine lining and keeping the uterus relaxed. When mifepristone blocks progesterone, two things happen. First, the uterine lining begins to break down, cutting off the support a pregnancy needs. Second, the body releases natural compounds called prostaglandins, which cause the uterus to contract. At the same time, the uterus becomes more sensitive to those contractions. The result is cramping and bleeding similar to a heavy period, which leads to the pregnancy ending.
Mifepristone also blocks cortisol receptors at higher doses. Cortisol is the body’s main stress hormone, and in Cushing’s syndrome, the body produces far too much of it. Rather than lowering cortisol levels directly, mifepristone prevents cortisol from acting on its receptors, reducing the downstream effects of excess cortisol like dangerously high blood sugar.
Medication Abortion
The most common use of mifepristone is as the first step in medication abortion, which is FDA-approved through 10 weeks of pregnancy. The process uses two medications taken at different times. On day one, you take 200 mg of mifepristone by mouth. Then, 24 to 48 hours later, you take a second medication (misoprostol) by placing it between your cheek and gum, where it dissolves. Misoprostol amplifies the uterine contractions that mifepristone has already set in motion.
This two-drug regimen is highly effective. In a study of more than 13,000 women, the overall success rate was 97.7%. Effectiveness was highest between 4 and 6 weeks of pregnancy, where it reached 98.8%, and slightly lower at 8 to 9 weeks, where it was still 95.5%. The earlier the gestational age, the more reliably the regimen works.
Bleeding and cramping are expected parts of the process, not side effects. Bleeding or spotting typically lasts 9 to 16 days on average and can continue for up to 30 days. The heaviest bleeding usually occurs in the hours after taking misoprostol.
Managing Miscarriage
Mifepristone is also used to help the body complete an early miscarriage that hasn’t resolved on its own. The protocol is similar: 200 mg of mifepristone followed about 24 hours later by misoprostol. In a clinical trial, this combination led to complete passage of pregnancy tissue in 83.8% of women after one dose of misoprostol, compared to 67.1% when misoprostol was used alone. That 25% improvement in success rate means fewer people need a follow-up procedure.
Treating Cushing’s Syndrome
At much higher doses, mifepristone is sold under the brand name Korlym for a completely different purpose. It’s approved for adults with Cushing’s syndrome who have type 2 diabetes or glucose intolerance and who either can’t have surgery or whose surgery didn’t work. In these patients, the body produces excess cortisol, which drives blood sugar dangerously high. Mifepristone blocks cortisol at the receptor level, helping bring blood sugar under control even though cortisol levels in the blood remain elevated.
Safety Profile
Roughly 5.9 million women in the U.S. used mifepristone for pregnancy termination between its approval and the end of 2022. Serious complications are rare but not zero. Over that entire period, about 600 people needed blood transfusions due to heavy bleeding, and roughly 400 experienced pelvic infections. Of those infections, 75 were classified as severe based on factors like hospitalization lasting several days or requiring intravenous antibiotics. These numbers span millions of uses over more than two decades.
The FDA notes that not all reported adverse events can be definitively attributed to mifepristone itself, since gaps in patient health records, other medications, and pre-existing conditions make it difficult to confirm causation in every case.
How It’s Prescribed and Dispensed
Mifepristone for pregnancy termination is regulated under a special FDA safety program. It can only be prescribed by healthcare providers who have completed a certification process, and it can only be dispensed by certified prescribers or certified pharmacies. Certified pharmacies are permitted to dispense mifepristone by mail, provided they use a shipping service with tracking and ensure timely delivery. You cannot pick it up at a standard pharmacy counter without the pharmacy being specifically certified for this medication.