Ulipristal acetate works by blocking progesterone, one of the key hormones that drives ovulation and pregnancy. It sits on the body’s progesterone receptors and prevents progesterone from activating them, which delays or stops the release of an egg from the ovary. This makes it effective as emergency contraception (sold as ella) and, at different doses, as a treatment for uterine fibroids.
How It Blocks Progesterone
Ulipristal belongs to a class of drugs called selective progesterone receptor modulators. It was designed as a chemical derivative of progesterone itself, which gives it a strong affinity for the same receptors progesterone normally binds to. Once ulipristal occupies those receptors, progesterone can’t get in. The gene activity that progesterone would normally switch on stays off.
The drug also has some weak binding to glucocorticoid and androgen receptors, but this only becomes relevant at doses roughly 50 times higher than what’s used clinically. At its standard doses, it acts almost exclusively on progesterone signaling.
Delaying Ovulation for Emergency Contraception
For emergency contraception, the critical action is stopping or postponing ovulation. A single 30 mg dose, taken within five days (120 hours) of unprotected sex, can reliably prevent the follicle from rupturing and releasing an egg. It also thins the uterine lining, making implantation less favorable.
What sets ulipristal apart from levonorgestrel (Plan B) is its ability to work later in the menstrual cycle. When a follicle has already grown to 18 to 20 mm and ovulation would normally happen within 48 hours, ulipristal still delays follicle rupture for at least five days in about 59% of cycles. Levonorgestrel manages this in only 12% of cycles at that same stage. This matters because the window right before ovulation is when the probability of conception is highest and when people are most likely to have had intercourse.
In clinical trials comparing the two drugs over a five-day window after unprotected sex, the pregnancy rate was 1.3% with ulipristal versus 2.2% with levonorgestrel. The advantage grows the longer you wait to take it. After 72 hours, ulipristal is clearly the more effective option.
How It Shrinks Uterine Fibroids
Progesterone plays a direct role in stimulating fibroid cell growth. When ulipristal blocks progesterone at the receptor level in fibroid tissue, several things happen: the cells stop proliferating, programmed cell death (apoptosis) increases, and the growth factors that fibroids rely on get dialed down. The drug also has a direct effect on uterine blood vessels, which helps reduce the heavy menstrual bleeding that fibroids commonly cause.
For fibroid treatment, the dose is much smaller but taken daily: 5 mg per day rather than a single 30 mg dose. At this regimen, about 80% of patients experience suppressed ovulation, and 81% to 90% stop having periods altogether while on the medication. This combination of fibroid shrinkage and bleeding control is what made the drug appealing for pre-surgical management of fibroids in Europe and Canada, though it was never approved for this use in the United States.
Liver Safety Concerns With Daily Use
Daily ulipristal for fibroids has been linked to serious liver injury. An FDA analysis of postmarketing reports identified nine cases of significant liver damage in patients taking the 5 mg daily dose for fibroids. Five of those cases resulted in liver transplantation or death, and all five were judged to have a probable causal link to the drug. The remaining four cases resolved after the drug was stopped. These risks led to major regulatory restrictions on daily ulipristal use in Europe.
This concern applies specifically to the daily fibroid regimen. The single-dose emergency contraception use involves far less drug exposure and has not raised the same liver safety signals.
How the Body Processes Ulipristal
After swallowing the 30 mg tablet on an empty stomach, ulipristal reaches peak blood levels within about an hour. The liver breaks it down through an enzyme pathway called CYP3A4, producing an active metabolite that also contributes to the drug’s effect. The average half-life is about 32 hours, meaning it takes roughly a day and a half for your body to clear half the dose. This relatively long duration helps explain why a single pill can suppress ovulation for several days.
Because ulipristal relies on CYP3A4 for metabolism, drugs that strongly activate or inhibit that enzyme pathway can change how much ulipristal stays in your system. Certain medications for seizures, HIV, and fungal infections fall into this category.
Does Body Weight Affect How Well It Works?
Earlier research suggested that higher body weight might reduce ulipristal’s effectiveness as emergency contraception, which raised concerns for people with a BMI over 30. However, more recent data paints a reassuring picture. A study looking specifically at whether the standard 30 mg dose needed to be doubled for individuals with obesity found that the single dose produced a consistent therapeutic effect, delaying ovulation by at least five days regardless of the person’s weight or BMI. The proportion of cycles where ovulation was successfully delayed was similar between standard and double doses in the higher-BMI group.