A new study suggests a possible alternative to the abortion pill mifepristone, a drug that continues to be a target of lawsuits and legislation from abortion opponents.

But the potential substitute could further complicate the politics of reproductive health because it is also the key ingredient in a contraceptive morning-after pill.

The new study, published Thursday in the journal NEJM Evidence, involved a drug called ulipristal acetate, the active ingredient in the prescription contraceptive Ella, one of two types of morning-after pills approved in the United States. (The other, Plan B One-Step, which does not require a prescription, contains a different drug and does not work in a way that would terminate a pregnancy, according to scientific evidence.)

In the study, 133 women who were up to nine weeks’ pregnant took twice the dose of the ulipristal acetate contained in Ella, followed by misoprostol, the second drug used in the typical medication abortion regimen. All but four of the women completed the termination of their pregnancies without further intervention, a 97 percent completion rate that is similar to the regimen using mifepristone. (The others finished the process with additional medication or a procedure.)

There were no serious complications, and the study concluded that using ulipristal acetate for abortion was safe.

Dr. Beverly Winikoff, the lead author of the study and the president of Gynuity Health Projects, a reproductive health research organization, said that after the Supreme Court overturned the national right to abortion in 2022, she began wondering about a possible role for ulipristal acetate, which has a similar chemical structure to mifepristone.

“I was thinking, there’s maybe something else we can do,” she said. “Another option. And this one is already on the market.”

The political implications of the study are complex because of their potential to blur the line between the public perception of emergency contraception and abortion. For years, abortion opponents have opposed morning-after pills by saying they can cause abortions, and reproductive health experts have countered by pointing to scientific evidence that the pills do not terminate pregnancies but instead act to prevent pregnancy after sex.

Some reproductive health experts are concerned that research showing that a morning-after pill ingredient can be used for abortion could stoke attempts to crack down on emergency contraception and sow confusion that could bolster the larger anti-abortion strategy.

Kristi Hamrick, a spokeswoman for Students for Life of America, said her organization would “absolutely” consider litigation over Ella.

“The pro-life movement should be vindicated,” Ms. Hamrick said. “We’ve been arguing for years that Ella acts as an abortifacient.”

Mary Ziegler, a law professor and abortion expert at the University of California, Davis, said the study could present challenges for both sides of the abortion debate.

“It’s going to put wind in the sails of abortion opponents who have been saying things like contraceptives can be abortifacients,” she said, adding, “This study being released will be difficult, I think, for abortion rights supporters to manage.”

But Ms. Ziegler said the study’s findings could also be “politically risky” for abortion opponents because public support for contraception is high and many voters in conservative states endorsed ballot measures protecting abortion rights. “I think it’s one of those things that’s going to tempt social conservatives to push probably faster than politics would currently permit in the direction of regulating contraception,” she said. “And I think that could backfire.”

Reproductive health experts said the new study did not refute the science showing that morning-after pills don’t induce abortions, because it involved a different dose of the drug.

Abortion opponents said they were not surprised by the study’s findings.

“After years of denying ulipristal acetate’s potential to end the life of an embryo, abortion advocates are now starting to use it as a substitute for the abortion drug mifepristone,” Dr. Donna Harrison, director of research for the American Association of Pro-Life Obstetricians and Gynecologists, said in a statement. “The reason for this is simple. Ulipristal and mifepristone function in the same way.”

Ella was already a target for some conservatives. Project 2025, a right-wing policy blueprint that has been strongly tied to the new Trump administration, said that Ella should be removed from required insurance coverage of contraception under the Affordable Care Act because it is “a potential abortifacient.”

Mifepristone, the first pill in the standard two-drug medication abortion regimen, is the only drug specifically approved for abortion in the United States. Typically used through 12 weeks’ gestation, mifepristone stops the development of a pregnancy by blocking the hormone progesterone. The second drug, misoprostol, is taken 24 to 48 hours later and causes contractions similar to a miscarriage.

Ulipristal acetate is in the same class of medications as mifepristone and also blocks the activity of progesterone, a hormone that prepares the uterus to receive and hold an embryo, said Dr. Daniel Grossman, a reproductive health physician and researcher at the University of California, San Francisco, who was not involved in the new study.

In the study, a 60 milligram dose of ulipristal acetate (double the 30 milligrams in Ella) was substituted for mifepristone and followed by misoprostol, which has various medical uses and has not been targeted as much by abortion opponents. (Misoprostol can also facilitate an abortion on its own, but is considered more effective in a combination regimen.)

Reproductive health experts said they welcomed the search for alternatives to mifepristone because abortion opponents have been waging efforts to sharply restrict the medication across the country, most notably with a federal lawsuit against the Food and Drug Administration. The Supreme Court rejected that lawsuit last year, ruling that the initial plaintiffs lacked standing to sue, but the suit has since been revived with three states as plaintiffs.

Dr. Grossman, who wrote an editorial about the study, said the prospect of a substitute for mifepristone was “certainly a promising finding.” But, he added, “if because of this new evidence that at higher doses, ulipristal acetate could cause an abortion, that were to lead to ulipristal acetate being taken off the market for emergency contraception, that would be really, really bad.”

The company that manufactures Ella, Perrigo, issued a statement saying that Ella “was not tested in this study.” It added, “Ella is an F.D.A.-approved emergency contraception pill that acts before pregnancy can occur,” she continued. “There continues to be no evidence to show that, on its own, Ella causes an abortion.”

The study was conducted in Mexico City and was co-led by researchers there.

Several experts said that because it was a relatively small first study with no comparison group of patients, more research was needed before ulipristal acetate should be used for abortion. “We can’t change clinical practice based on this study,” said Kelly Cleland, a researcher who is the executive director of the American Society for Emergency Contraception.

Scientists have long understood that hormone-based drugs may be able to play different functions at different doses along the spectrum of a woman’s reproductive health cycle.

In Europe, researchers including Dr. Rebecca Gomperts, a Dutch physician and founder of telemedicine organizations that provide abortion pills globally, are studying low doses of mifepristone as a weekly birth control pill. Dr. Gomperts said she considered the new ulipristal acetate study sufficient to prescribe the drug off-label for medication abortion.

“The more uses we have for these medications, the harder it will be for people to take them away,” said Dr. Paul Blumenthal, an emeritus professor of obstetrics and gynecology at Stanford University who was in an advisory group for the study.

Plan B is a much more widely used morning-after pill in the United States, but Ella is considered more effective for some women, including people who are overweight. Plan B is intended to be taken within three days after unprotected sex, while Ella can be taken within five days.

Both pills prevent pregnancy by blocking ovulation, the release of eggs from the ovaries that occurs before eggs can be fertilized, scientific studies have shown.

The claim by some abortion opponents that morning-after pills are abortion drugs is based on a theory that they might also prevent a fertilized egg from implanting in the womb. Most scientific research has not found that to be the case.

For years, despite scientific evidence to the contrary detailed in an investigation by The New York Times, the F.D.A.-approved label and packaging for Plan B One-Step said that while the pill worked by blocking ovulation, there was a possibility it might prevent implantation. In 2022, the agency changed the language to make it clear that Plan B acts only before fertilization, “will not work if you’re already pregnant, and will not affect an existing pregnancy.”

The F.D.A. label for Ella says that its “likely primary mechanism of action” is to stop or delay ovulation. The label adds that the medication may also affect implantation. Studies in recent years, however, suggest that Ella does not operate by blocking a fertilized egg from implanting in the womb.