How Does Mifepristone Work? Mechanism of Action Explained in Plain English

Mifepristone is a drug with a fascinating mechanism — at low doses, it blocks one hormone receptor; at high doses, it blocks a completely different one. This dual mechanism is what makes it useful for two seemingly unrelated conditions: ending a pregnancy and treating Cushing's syndrome. This guide explains how mifepristone works in plain, accessible terms, without unnecessary medical jargon.

What Type of Drug Is Mifepristone?

Mifepristone is a synthetic steroid — a lab-made compound that mimics the shape of naturally occurring steroid hormones but acts as a receptor blocker rather than an activator. Specifically, it is classified as a progesterone receptor modulator and, at higher doses, a glucocorticoid (cortisol) receptor antagonist. Think of it as a key that fits perfectly into a lock (a hormone receptor) but doesn't open it — and also prevents the real key (the hormone) from entering.

How Mifepristone Works for Medical Abortion

To understand how mifepristone ends a pregnancy, you first need to understand the role of progesterone. Progesterone is a hormone produced by the ovary (and later the placenta) that is absolutely essential for maintaining a pregnancy. It thickens the uterine lining so the embryo can implant, suppresses uterine contractions to keep the uterus calm, and softens the cervix. Without progesterone, a pregnancy cannot continue.

Mifepristone binds tightly to the progesterone receptor — more tightly, in fact, than progesterone itself. But unlike progesterone, mifepristone does not activate the receptor. Instead, it blocks it, preventing progesterone from doing its job. The result: the uterine lining begins to break down, the embryo detaches from the uterine wall, and the cervix starts to soften. Mifepristone also increases sensitivity to prostaglandins, which are hormone-like compounds that cause uterine contractions.

This is why mifepristone is paired with misoprostol 24-48 hours later. Misoprostol is a prostaglandin analog that directly causes the uterus to contract and the cervix to open further — expelling the pregnancy. Together, the two drugs are approximately 97-98% effective at ending a pregnancy within the first 10 weeks. Mifepristone alone is only about 54-92% effective.

Why Mifepristone Must Be Used Within 10 Weeks

In the early weeks of pregnancy, the embryo and developing fetus depend entirely on progesterone produced by the corpus luteum (a temporary gland in the ovary) to maintain the pregnancy. By around 10-12 weeks, the placenta takes over progesterone production. Once the placenta is producing enough progesterone independently, blocking the progesterone receptor has less effect on the pregnancy. This is why the mifepristone-misoprostol regimen is FDA-approved only through 70 days (10 weeks) of pregnancy.

How Mifepristone Works for Cushing's Syndrome

Cushing's syndrome is a condition in which the body produces too much cortisol — the primary stress hormone. Excess cortisol causes a wide range of problems including high blood sugar, weight gain (especially around the belly and face), muscle weakness, high blood pressure, and mood disorders. In endogenous Cushing's syndrome, the excess cortisol is produced by the body itself (as opposed to being caused by taking steroid medications).

At higher doses (300-1200 mg/day), mifepristone also blocks the glucocorticoid receptor — the receptor that cortisol binds to in order to have its effects on the body. By sitting in the glucocorticoid receptor without activating it, mifepristone prevents cortisol from raising blood sugar, even though cortisol levels in the blood may actually rise (because the body senses the blocked receptor and tries to compensate by making more cortisol).

This is an important point: mifepristone does NOT lower cortisol levels in the blood. It blocks the receptor that cortisol acts on. This means blood cortisol tests will show very HIGH cortisol during Korlym therapy — which is expected and does not indicate treatment failure. The drug is working by blocking the effect of cortisol, not by reducing its production.

How Quickly Does Mifepristone Work?

Mifepristone is rapidly absorbed after oral administration. Peak blood levels are reached within about 90 minutes. The drug binds tightly to plasma proteins and has an initial half-life of about 18 hours, with the terminal elimination half-life taking 18+ hours. The drug is primarily metabolized by the CYP3A4 enzyme system in the liver and eliminated mainly through the feces.

For medical abortion: effects begin within hours of taking mifepristone, but the bulk of the pregnancy-ending process happens with misoprostol 24-48 hours later. For Cushing's syndrome: it may take 6 weeks or longer to see the full reduction in blood sugar levels, requiring patience during the dose titration phase.

Why Grapefruit Juice Can Be a Problem With Korlym

Since mifepristone (especially at Korlym doses) is metabolized by the CYP3A4 enzyme, anything that blocks this enzyme can cause mifepristone levels to rise in the blood. Grapefruit and grapefruit juice are well-known CYP3A4 inhibitors. Patients taking Korlym are advised to avoid grapefruit entirely, as it can raise mifepristone blood levels and increase the risk of side effects.

The Bottom Line on Mifepristone's Mechanism

Mifepristone works by blocking hormone receptors — progesterone receptors at low doses for abortion, and cortisol receptors at high doses for Cushing's syndrome. Its mechanism is well understood and its effects are predictable when used as directed. If you have questions about how mifepristone might interact with other medications you take, see our mifepristone drug interactions guide. If you're trying to find a pharmacy that can fill your mifepristone prescription, medfinder can call pharmacies near you on your behalf.