A new study suggests that a pill used for emergency contraception could be repurposed at a higher dose as an abortion drug, providing a possible alternative to mifepristone, one of the two drugs used in the most common type of abortion in the United States.

Mifepristone has been under attack by abortion opponents, with several states seeking in federal court to restrict its use.

Now used in two-thirds of U.S. abortions, mifepristone blocks a hormone needed to sustain a pregnancy. It's typically used with misoprostol, which causes contractions and bleeding.

In the study, 133 women who were up to nine weeks' pregnant took a 60 milligram dose of ulipristal acetate, the active ingredient in the prescription contraceptive Ella, followed by misoprostol 24 hours later.

For 97% of them, that drug combo was effective at inducing an abortion, an effectiveness equal to the mifepristone-misoprostol combination. Four women needed a procedure or an additional medication to complete the abortion.

The 60 milligram dose of ulipristal used in the study is twice the dose of Ella, a prescription drug used for emergency contraception.

The company that makes Ella says on its website that it won't end an existing pregnancy. It can be taken up to five days after unprotected sex to prevent pregnancy.

The findings, published Thursday in the journal NEJM Evidence, may make emergency contraception a target of abortion opponents.

“I’m really worried that these results could be misapplied by anti-abortion activists to try to further their assault on contraception,” said Dr. Daniel Grossman of the University of California, San Francisco, who wrote an accompanying editorial in the journal. Grossman praised the study but said more research is needed on ulipristal as an abortion drug before doctors would prescribe it routinely for that use.

Lead author Dr. Beverly Winikoff, president of Gynuity Health Projects, a not-for-profit research group, said women need information about ulipristal, especially with mifepristone challenged in court.

“At least now we would have an alternative,” Winikoff said. “I think it’s better to have more things that you could use.”