The Biden administration and a manufacturer of the drug had asked the justices to reverse a federal appeals court decision that, if allowed to go into effect, would restrict access to the drug. At the same time, abortion opponents want the court to hold that the initial approval of the drug was unlawful. The new case could be decided by July.
Mifepristone was approved by the US Food and Drug Administration decades ago and has been shown to be safe and effective. But anti-abortion advocates have claimed that the drug is not safe and that the FDA didn't study it enough to approve it.
Earlier this year, a federal judge in Texas suspended the approval of the drug, and the US Supreme Court stayed a ruling - which means mifepristone remains available for now.
Along with misoprostol, mifepristone is one of the drugs used for an abortion via medication, as opposed to surgery.
Mifepristone is marketed under the brand names Mifeprex and Korlym, and it's sometimes known as RU 486.
Mifepristone blocks a hormone called progesterone, which helps the body maintain the inside of the uterus so a pregnancy can continue. A healthy uterine lining is what supports a fertilized egg, embryo and fetus.
Without progesterone, the uterus will expel its contents.
Someone having a medication abortion takes mifepristone and then, after 24 to 48 hours, takes misoprostol. That drug helps empty the uterus through heavy bleeding and muscle contractions.
The medications can be taken as soon as someone learns that they are pregnant and up to 70 days or less since the first day of their last period.
This method is effective 99.6% of the time when used to end a pregnancy, studies show.
Data from hundreds of studies and 23 years of approved use has shown that mifepristone is highly safe and effective, according to 12 of the country's most respected medical associations, including the American College of Obstetricians and Gynecologists and the American Medical Association, which signed an amicus brief in the Texas case.
This medicine combination for abortion is also available in more than 60 other countries.
Since its approval in the US in 2000, there have been 5 deaths associated with mifepristone for every 1 million people who used it, according to the FDA. That means the death rate is 0.0005%.
Mifepristone's safety is on par with those of common over-the-counter pain relievers like ibuprofen and acetaminophen, studies show.
Data analyzed by CNN shows that mifepristone is even safer than some of the most common prescription medications. The risk of death from penicillin, an antibiotic used to treat bacterial infections like pneumonia, for example, is four times greater than it is for mifepristone. The risk of death after taking Viagra - used to treat erectile dysfunction - is nearly 10 times higher.
Mifepristone usually doesn't have many side effects, doctors say, but as with any drug, there can be short-lived ones.
Side effects of mifepristone may include dizziness, weakness, vomiting, headache, diarrhea, nausea, and fever or chills, according to the FDA.
Major adverse events like blood loss, hospitalization or a significant infection are "exceedingly rare," happening in less than 0.3% of patients, according to the medical associations' amicus brief.
The mifepristone-misoprostol combination is the most common abortion method in the US.
Data from the Guttmacher Institute, a research and policy organization focused on sexual and reproductive health that supports abortion rights, showed that medication abortion accounted for 53% of all abortions in the US.
No matter the status of mifepristone, misoprostol would still be available and could be used for a medication abortion on its own.
The only use of misoprostol that's approved by the FDA is for the prevention and treatment of gastric ulcers. But some doctors use it "off-label" by itself for an abortion; it can also have other off-label obstetric or gynecologic uses such as inducing labor.
Studies show that misoprostol alone is effective and safe for an abortion and is a "reasonable" option for people who want an abortion in the first trimester, according to a 2019 review of multiple studies, but the two-drug regimen is considered more effective.
This review said that across all studies of people who took only misoprostol, about 78% had complete abortions, and a viable pregnancy was terminated in more than 93% of the cases. At most, 0.2% of cases had serious complications that required a transfusion or hospitalization.
People who take misoprostol on its own for an abortion typically need to take more for it to work, and the side effects can be more intense, Dr. Melissa L. Wong, an obstetrician/gynecologist and a fellow with Physicians for Reproductive Health, previously told CNN.
"Misoprostol still works very well and is very safe," Wong said. "Mifeprestone typically does not cause any side effects as it stops the pregnancy hormone. With misoprostol, some people may tolerate it a bit less because of some side effects like nausea, vomiting, sometimes things like diarrhea or a transient fever. Those are still safe and expected side effects, but they are still uncomfortable for anyone."
Devan Cole and Ariane de Vogue contributed to this report.
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