Mifepristone works by blocking progesterone, a hormone essential for maintaining pregnancy. It binds to progesterone receptors with higher affinity than progesterone itself, effectively locking the hormone out. Without progesterone signaling, the uterine lining breaks down, the cervix softens, and the uterus becomes more responsive to contractions. This is why mifepristone is the first step in a medication abortion, though it also has a separate use in managing excess cortisol.
Blocking Progesterone at the Receptor
Progesterone is sometimes called the “pregnancy hormone” because it keeps the uterine lining thick, nourished, and stable enough to support a developing embryo. Mifepristone works by sitting in the same receptor that progesterone normally occupies, but instead of activating it, it blocks the signal entirely. Think of it like a key that fits the lock but won’t turn: progesterone can no longer get in and do its job.
This blockade triggers a cascade of changes. The body releases its own prostaglandins, natural compounds that promote uterine contractions and help shed the lining. At the same time, mifepristone also reduces the number of both progesterone and estrogen receptors in the uterine lining, amplifying the effect. Without functioning receptors, the hormonal support system for a pregnancy collapses even if progesterone is still circulating in the bloodstream.
What Happens in the Uterus
Once progesterone is blocked, the uterine lining (called the decidua during pregnancy) begins to break down. The tissue undergoes programmed cell death, the same natural process the body uses to clear old or unneeded cells. Blood vessels in the lining destabilize, and bleeding begins as the tissue separates from the uterine wall.
Mifepristone also softens and thins the cervix. Progesterone normally keeps the cervix firm and closed during pregnancy. Blocking that signal allows the cervix to ripen, meaning it becomes softer and starts to open. Studies show that mifepristone significantly increases the likelihood of a favorable cervix within 48 to 96 hours. This cervical change is critical because it allows the uterus to expel its contents when contractions begin.
Sensitizing the Uterus to Contractions
Beyond breaking down the lining and softening the cervix, mifepristone makes the uterine muscle far more responsive to prostaglandins. Progesterone normally keeps the uterus relatively quiet during pregnancy by dampening its sensitivity to contraction signals. When mifepristone removes that dampening effect, the uterus becomes primed to contract.
This is why a second medication, misoprostol (a synthetic prostaglandin), is taken 24 to 48 hours after mifepristone. By that point, the uterus is already sensitized, so misoprostol triggers strong contractions that complete the process. Mifepristone alone can sometimes cause a pregnancy to end, but the combination is far more reliable. In a study of over 13,000 women, the two-drug regimen was 97.7% effective through 63 days of gestation. Effectiveness was highest earlier in pregnancy (98.8% at five to six weeks) and slightly lower closer to nine weeks (95.5%).
How Quickly It Works in the Body
Mifepristone is absorbed rapidly after swallowing. Blood levels peak about 90 minutes after taking the pill. The drug has an average half-life of 18 hours, meaning half of it is cleared from your system in that time. Your liver processes it primarily through a specific enzyme pathway (CYP3A4), producing metabolites that also bind to progesterone receptors and contribute to the drug’s effect.
The standard regimen uses a single 200 mg dose of mifepristone taken by mouth, followed by 800 mcg of misoprostol placed between the cheek and gum one to two days later. A follow-up appointment is typically scheduled seven to fourteen days afterward to confirm the process is complete.
Its Other Use: Blocking Cortisol
Mifepristone doesn’t only block progesterone. It also binds to the receptor for cortisol, the body’s primary stress hormone. It actually has a stronger grip on this receptor than cortisol itself, binding roughly 18 times more tightly. When it sits on the cortisol receptor, it prevents the receptor from moving into the cell nucleus where it would normally switch on genes. The result is that cortisol’s effects throughout the body are blunted.
This property makes mifepristone useful for people with Cushing’s syndrome, a condition where the body produces dangerously high levels of cortisol. Excess cortisol causes high blood sugar, high blood pressure, central weight gain, and depression. At higher daily doses (not the single dose used for pregnancy), mifepristone can counteract these effects by blocking cortisol at the tissue level. In this application, the half-life extends to roughly 85 hours because the drug accumulates with repeated dosing.
Who Should Not Take It
Mifepristone is not appropriate for everyone. It cannot treat an ectopic pregnancy (one growing outside the uterus), because it only affects the uterine lining. Taking it with an ectopic pregnancy would delay proper treatment for a potentially dangerous condition. It’s also contraindicated for people with chronic adrenal insufficiency, since blocking cortisol receptors in someone who already can’t produce enough cortisol could trigger a crisis.
Other situations where mifepristone should not be used include:
- Bleeding disorders or blood-thinning medications, due to the risk of heavy, uncontrolled bleeding
- Long-term corticosteroid therapy, because blocking cortisol signaling could destabilize the body’s stress response
- Inherited porphyria, a group of rare metabolic conditions that mifepristone can worsen
- An IUD in place, which must be removed before taking the medication
- Known allergy to mifepristone or prostaglandin medications
For pregnancy termination, the FDA-approved window extends through 70 days (10 weeks) of gestation. Beyond that point, the regimen’s effectiveness drops and the approach to ending a pregnancy changes.