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Put simply, an antiprogestin is a substance that blocks the body from producing or using a hormone called progesterone.

Progesterone is one of the main female sex hormones. It prepares the body for getting and staying pregnant, and it regulates the menstrual cycle.

By stopping progesterone action, antiprogestins can affect the uterine lining and cause uterine contractions, which can lead to the ending of a pregnancy.

Antiprogestins work in a few different ways. They mainly block progesterone action.

When antiprogestins bind to progestin receptors in the body and stop progesterone production or use, the endometrium breaks down, resulting in menstruation.

Antiprogestins boost the concentration of prostaglandin, which helps stimulate the uterine muscle wall to contract, leading to menstrual bleeding. Antiprogestins also stop cortisol from binding to certain receptors, which boosts circulating cortisol levels and alleviates high blood sugar levels.

Research shows that antiprogestins may kill and stop the growth of cancer cells in the breasts, ovaries, and cervix, too.

If antiprogestins block progesterone, what do progestins do?

Well, these are just synthetic substances that bind to receptors in the body and produce similar results as natural progesterone.

You may have heard of progestins regarding things like hormonal contraceptives and hormone replacement therapy.

Basically, antiprogestins and progestins play opposite roles. One stops progesterone, and the other is a manufactured version of the hormone.

People first used antiprogestins for emergency contraception and to end pregnancies.

Nowadays, people still use them in these areas —particularly to end pregnancies of up to 10 weeks.

But people also use them to treat other gynecological conditions, such as endometriosis and high blood sugar levels in people with Cushing disease.

Antiprogestins’ ability to block progesterone can be useful in treating some menopausal symptoms, like hot flashes.

People can use antiprogestins alone, in some cases, or along with other therapies.

Experts are still studying the treatment of breast cancer using antiprogestins.

First developed in 1980, mifepristone is the most widely known antiprogestin. You may also see that people refer to it as RU 846 or Mifeprex.

People commonly use it with another medication called misoprostol to end an early pregnancy.

A medication abortion, as it’s known, involves taking mifepristone first. This binds to progesterone receptors inside the body, blocking the production and use of progesterone, which your body needs to help a pregnancy grow.

It can also cause the uterus to contract.

Misoprostol then softens the cervix, eventually leading to the body expelling the pregnancy from the womb.

Korlym is another name for mifepristone used to treat symptoms of Cushing disease.

And emergency contraceptives that act as antiprogestins contain ulipristal acetate.

Antiprogestin medications aren’t suitable for everyone, including those with an ectopic pregnancy, intrauterine device, or severe anemia.

If you have a bleeding disorder or sensitivity to compounds like prostaglandins, you may also have a difficult time taking an antiprogestin.

Common side effects of taking an antiprogestin to end a pregnancy include:

  • cramps
  • vaginal bleeding
  • nausea
  • vomiting
  • fever
  • chills
  • diarrhea
  • headache

And if a doctor prescribes an antiprogestin for other reasons, you may also experience:

  • nausea
  • vomiting
  • stomach or pelvic pain
  • fatigue
  • headache
  • diarrhea
  • menstrual irregularities

Rarer side effects of mifepristone range from skin reactions to uterine rupture and toxic shock syndrome.

If you feel unwell after taking an antiprogestin medication, it’s a good idea to speak with a clinician as soon as possible.

They can give you advice on relieving side effects and can see you in person for an examination if necessary.

Get immediate medical attention if you experience serious side effects, such as:

  • swelling
  • difficulty breathing
  • severe pain
  • weakness
  • extremely heavy bleeding, soaking through one or more menstrual pad per hour

Antiprogestins mainly block the hormone progesterone, which makes them useful for early-stage abortions and emergency contraceptives.

But they can also relieve symptoms of conditions like endometriosis and Cushing disease, and they may even assist in treating certain cancers.

Side effects like headaches, diarrhea, cramps, and nausea can occur. But if you’re worried about any symptoms you’re experiencing, get in touch with a healthcare professional for some personalized advice.

Lauren Sharkey is a U.K.-based journalist and author specializing in women’s issues. When she isn’t trying to discover a way to banish migraines, she can be found uncovering the answers to your lurking health questions. She has also written a book profiling young female activists across the globe and is currently building a community of such resisters. Catch her on Twitter.