How does the morning-after pill work?

Morning-after pills — perhaps best known by the brand name Plan B — are emergency contraceptives that can help prevent pregnancy after unprotected sex. However, myths and misinformation abound about how these medications exert their effects in the body.

So, how do morning-after pills actually work?

These medicines work by either inhibiting or delaying ovulation, the process during which an ovary releases an egg that could then be fertilized, if it encounters sperm.

Two types of morning-after pills are available in the United States: one containing progestin, a synthetic form of the hormone progesterone that regulates the menstrual cycle, and the other containing anti-progestin, which blocks progesterone receptors. Plan B is an example of the former type, while ella is an example of the latter. Both mechanisms keep the egg in the ovary longer than it would normally stay there.

"Even though there's sperm in the vagina and in the uterus after sex, it doesn't actually meet with the egg because the egg is safe inside the ovary," explained Dr. Colleen Denny, an obstetrician-gynecologist and director of the family planning service at NYU Langone Hospital in Brooklyn.

Because the pills delay ovulation, they aren't effective if an egg has already been released. That's why doctors advise users to ingest the pills as soon as possible after unprotected sex — you want to try to catch the window before ovulation begins.

"Although the morning-after pill is kind of a catchy name for emergency contraception, it’s actually misleading since emergency contraception works best if you take it as soon as possible after unprotected sex," Dr. Alison Edelman, an obstetrician-gynecologist and professor of obstetrics and gynecology at Oregon Health & Science University, told Live Science in an email.

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Is one pill safer or more effective than the other?

Most morning-after pills are of the synthetic progestin variety, which are available at pharmacies with no age restrictions and without a prescription. The only anti-progestin pill in the U.S. — ella — is also available to all ages but requires a prescription.

Side effects common to both pills include abdominal and menstrual pain, dizziness, headache and nausea, which emerge due to how the pills temporarily alter hormonal signaling. It is "extremely difficult to hurt yourself" with medications that affect progesterone signaling, Denny told Live Science. "They don't cause blood clots. They don't cause dangerous side effects."

Both types of pills may delay menstruation, but that's no cause for concern, Denny said. The morning-after pill can even be taken multiple times in the same cycle without causing harm because the mechanism it uses has no long-term side effects, studies have found.

Progestin pills, like Plan B, can reduce the chance of pregnancy by 81% to 90% when taken within three days of unprotected sex. The effectiveness of the pill falls the further out from sex it's taken, meaning it's more effective if taken within one day than it is two or three days out.

By comparison, anti-progestin pills, like ella, can lower the change of pregnancy by about 85% if taken within five days. "It can be taken up to 5 days after unprotected sex with no decrease in effectiveness," and works better than progestin pills "when taken as directed," according to the American College of Obstetricians and Gynecologists (ACOG).

Anti-progestin pills can still be effective five days after sex because they can prevent ovulation both before and after luteinizing hormone — which signals the start of ovulation to the body — is released. As long the pills are ingested before the hormone peaks and an egg is released, anti-progestin pills can be effective.

Can BMI change the efficacy of morning-after pills?

It's important to know that the efficacy of morning-after pills can vary depending on the body mass index (BMI) of the person ingesting the medicine. Numerous studies have shown that emergency contraceptive pills are less effective as the user's BMI increases.

Regarding progestin pills, studies have generally shown that the pills' effectiveness dips in people with BMIs over 30, which is in the "obese" range, though select studies show dips at BMIs of 25, in the "overweight" range. Some studies have found that people with a BMI over 30 have four times the chance of getting pregnant after taking progestin-based emergency contraceptives, compared to people with BMIs under 25, in the "healthy" or "underweight" range.

Anti-progestin pills have been shown to be more effective than progestin pills for people with higher BMIs. Recent research suggests ella's effectiveness starts to fall at BMIs of 35 or higher.

The exact reason that the pills work differently at higher BMIs is not yet clear.

"We don't really know why" the efficacy is BMI dependent, Denny said. Some experts think that the amount of fat issue in the body may impact the concentrations of the medicines in the blood, while others suggest that irregular ovulation rates associated with higher BMIs could complicate research data. However, neither of these explanations is conclusive.

Additionally, in 2022, Edelman led a clinical trial testing whether a higher dose could help counter this phenomenon, but didn't find that higher doses improved the efficacy.

Despite these possible efficacy differences, doctors still suggest that anyone worried about pregnancy after unprotected sex take the morning-after pill regardless of BMI.

"If that is what someone has available and they have unprotected sex, then they should take it," Edelman said. "It won't hurt but it might help."

Related: BMI should be replaced, experts argue — here's what the alternative could be

Is Plan B an abortion pill?

The most common misunderstanding about the morning-after pill, Edelman said, "is that emergency contraception is the same thing as a medical abortion." In short, they are not the same.

"Emergency contraception is preventing a pregnancy — you take it before you are pregnant," Edelman emphasized.

Denny said she has known patients and even other doctors to mix up emergency contraceptives with other pills related to reproductive health care. "They are totally different medicines, but they're all pills, and so sometimes people get a little confused in the conversation because they're all sort of swirled together in the news stories," she said.

Morning-after pills are taken after sex but before pregnancy occurs. By comparison, birth control pills are taken routinely to regulate the menstrual cycle on an ongoing basis and lower pregnancy risk, and abortion pills are taken after a pregnancy has begun. If you are already pregnant and you take Plan B, nothing would happen to the pregnancy, Denny said.

Edelman recommends that anyone who could become pregnant should keep emergency contraception like the morning-after pill at hand. They usually last several years unopened at room temperature, and exact expiration dates can be found listed on the packaging.

An alternative form of emergency contraceptive is more invasive than a pill but also more effective. A copper intrauterine device (IUD) requires a procedure to be placed inside the uterus, but is more than 99.9% effective at preventing pregnancy if placed within five days of unprotected sex.

The effectiveness of IUDs are also not affected by BMI. "They work better than the pill for everybody, but they especially work better for people who have a higher BMI," Denny said. "And many people keep it for years after they use it for emergency contraception," as a form of ongoing birth control.