How safe and effective is mifepristone when used for emergency contraception?

Key messages

• Compared to levonorgestrel (a common emergency contraception pill), mifepristone probably results in fewer pregnancies. It is also probably more effective at preventing pregnancy than the Yuzpe method (a combination of emergency pills).

• Mifepristone's main unwanted effect is that it delays a woman's next period; this is more likely to happen with higher doses.

• Future studies should investigate whether the time interval between unprotected sex and taking mifepristone (e.g. one day versus five days or more) reduces its effectiveness, and explore how acceptable women find different emergency contraception treatments.

What is emergency contraception?

Emergency contraception is used to prevent pregnancy after unprotected sex, the failure of another birth control method (e.g. broken condom), or in cases of sexual assault. Common methods of emergency contraception include copper intrauterine devices (IUDs) and pills (known as 'morning-after pills').

IUDs are small, T-shaped frames inserted into the uterus. They prevent pregnancy by interfering with sperm movement and egg fertilization. They are very effective at preventing pregnancy, but timely access can be difficult because a healthcare professional needs to insert the device.

Emergency contraception pills work by stopping or delaying the release of an egg from the ovary. They include:

• mifepristone, which blocks the hormone progesterone needed for pregnancy;

• levonorgestrel (brand name 'Plan B One-Step'), which tells the body to pause releasing the egg;

• the Yuzpe method, which involves taking two large doses of regular daily birth control pills, about 12 hours apart.

Some emergency contraception pills are expensive and not widely available.

What did we want to find out?

We wanted to find out how mifepristone compares to other emergency contraception methods at preventing pregnancy. We also compared these methods to see if they differed in terms of:

• unwanted effects (such as nausea and vomiting);

• the timing of women's next menstrual period.

What did we do?

We searched for studies that compared giving mifepristone versus other emergency contraception pills or IUDs to reproductive-aged women using emergency contraception to prevent pregnancy (not to terminate an early pregnancy).

We summarized the results of the studies and rated our confidence in the evidence, based on factors such as the studies' methods and number of women involved.

What did we find?

We included 87 studies with around 36,000 women. Most were conducted in China, between 1992 and 2015. The smallest study included 16 women and the largest, 4000. Nearly half (42 studies) compared two different doses of mifepristone, while 41 studies compared mifepristone to levonorgestrel. Five studies compared mifepristone to the Yuzpe method or IUDs.

Main results

Compared to levonorgestrel, a medium dose of mifepristone (25 mg to 50 mg) probably:

• results in fewer pregnancies: if 35 out of 1000 women given levonorgestrel became pregnant, about 23 out of 1000 women would with mifepristone (27 studies, 6052 women);

• reduces the risk of any unwanted effects (17 studies, 4350 women);

• increases the risk of having a late period (17 studies, 3615 women).

Compared to levonorgestrel, a low dose of mifepristone (under 25 mg):

• results in fewer pregnancies: if 20 out of 1000 women given levonorgestrel became pregnant, about 15 out of 1000 given mifepristone would (14 studies, 8752 women);

• probably substantially reduces the risk of any unwanted effects (3 studies, 609 women);

• slightly increases the risk of having a late period (10 studies, 7618 women).

Compared to the Yuzpe method, mifepristone (any dose):

• probably results in fewer pregnancies: if 25 out of 1000 women given the Yuzpe pill combination became pregnant, about 3 out of 1000 women would with mifepristone (3 studies, 2144 women);

• probably substantially reduces the risk of any unwanted effects (2 studies, 1693 women);

• increases the risk of having a late period (3 studies, 1912 women).

Compared to IUDs, we are unsure about the effects of mifepristone on the number of pregnancies, unwanted effects, and late periods (2 studies).

Compared to a low dose of mifepristone (under 25 mg), a medium dose of mifepristone (25 mg to 50 mg):

• results in slightly fewer pregnancies: if 16 out of 1000 women given low-dose mifepristone became pregnant, about 13 out of 1000 would with mid-dose mifepristone (26 studies, 12,358 women);

• may make little to no difference to the risk of experiencing unwanted effects (12 studies, 2908 women);

• slightly increases the risk of having a late period (22 studies, 11,733 women).

What are the limitations of the evidence?

We are moderately or very confident in most of the results. However, many studies did not clearly report their methods, which reduced our confidence in some findings. Most studies were conducted in China, so the results may not apply to women in other parts of the world. We found no studies that compared mifepristone to ulipristal acetate (brand name 'Ella'), a highly effective emergency contraception pill.

How up to date is this evidence?

The evidence is current to February 2025.