You can prevent pregnancy by taking one emergency contraceptive pill up to 120 hours (5 days) after penis-in-vagina sex.

There are two forms of emergency contraception — over-the-counter and prescription pills you take orally and intrauterine devices (IUDs) that a healthcare professional inserts.

This article primarily focuses on emergency contraceptive pills.

Although experts understand that a person’s weight or body mass index (BMI) can affect the effectiveness of emergency contraceptive pills, clinical research hasn’t yet established a threshold for efficacy.

That means healthcare professionals currently don’t have a concrete answer as to when emergency contraceptive pills become less effective or ineffective.

Experts need more research to understand whether doubling the dose can increase efficacy in people with a higher weight or BMI.

The copper IUD can be the most effective form of emergency contraception, regardless of weight or BMI.

If you have the option, talking with a healthcare professional may be helpful. They can recommend the best emergency contraceptive for your individual needs.

“Ella is more effective when taken at any point within 120 hours (5 days) after sex than any other emergency contraceptive pill option,” says Sophia Yen, MD, co-founder and CEO of Pandia Health, an online birth control provider.

Ella uses a nonhormonal ingredient called ulipristal acetate to block the hormone progesterone, delaying ovulation for 120 hours (5 days). This gives sperm time to die off before your body releases an egg.

Blocking progesterone also thins the uterine lining, so implantation is unlikely to occur even if sperm can fertilize the egg.

According to Planned Parenthood, ella can be most effective for people with a BMI of 30 or less. It can be less effective — potentially ineffective — for people with a BMI of 35 or higher.

Ella is available by prescription only. If you have a primary care physician or gynecologist, they can prescribe ella for you.

You can get a prescription through your local health department, a community clinic like Planned Parenthood, or a telemedicine platform like Nurx.

Ella usually costs around $50, but some providers offer the medication for free or at a reduced cost for people who qualify. Insurances and Medicaid typically cover the medication.

The most ubiquitous emergency contraceptive on the market, Plan B, contains a synthetic version of progesterone called levonorgestrel. The synthetic hormone can help delay ovulation, reducing the risk of potential fertilization.

Other levonorgestrel-based emergency contraceptives include:

“Levonorgestrel pills have been shown to be 89% effective if taken up to 72 hours (3 days) after unprotected intercourse,” says Cristin Hackel, a nurse practitioner at Nurx, a telemedicine provider.

“They do continue to work 72–120 hours (5 days) after intercourse, but to a much lesser degree,” explains Hackel.

According to Planned Parenthood, levonorgestrel-based pills can be most effective for people with a BMI of 24 or less. They can be less effective — potentially ineffective — for people with a BMI of 25 or higher.

You can purchase Plan B and other levonorgestrel pills without a prescription at most drugstores, including CVS, Rite-Aid, and Walgreens. Some grocery stores and many online retailers also offer the medication.

You can also get the medication at your local health department, a community clinic like Planned Parenthood, or through a telemedicine platform like Nurx.

Plan B usually costs around $50. Generic levonorgestrel pills aren’t as widely available as Plan B — you may have better luck finding them online — but they’re usually less expensive.

Many providers offer financial assistance, and people who have insurance or Medicaid can usually get the medication at no cost.

You can take some birth control pills as emergency contraception when you take them in higher-than-usual amounts. The uptick in hormones can help delay ovulation and prevent implantation.

“The Yuzpe method involves taking one first dose within 72 hours and a second dose taken 12 hours later,” explains Hackel.

This approach only works with combination birth control pills containing estrogen and progestin. Progestin-only minipills can be ineffective.

While convenient, most experts don’t recommend it if other options for emergency contraception are available.

“The Yuzpe method is less effective than Plan B and ella and comes with more side effects,” says Hackel.

Older research found the Yuzpe method to be 56–86% effective, depending on how soon after sex people took the medication.

Is it possible to make a ‘homemade morning-after pill’?

Not exactly. But the Yuzpe method is sometimes called a ”homemade morning-after pill.”

“While it’s better than nothing, it’s the least effective method of emergency contraception,” says Yen.

Can you prevent pregnancy after 120 hours have passed?

Your best bet can be to get a copper IUD.

“IUDs have been approved for use as emergency contraception for up to 120 hours (5 days) after unprotected sex in the U.S.,” says Hackel. “But they’re approved and used for up to 168 hours (7 days) after unprotected sex in Canada.”

A small 2019 study suggested that copper IUD insertion up to 14 days (2 weeks) after sex may help prevent pregnancy.

If you have the choice, is it better to take emergency contraceptive pills or get an IUD?

“IUDs are the most effective methods of emergency contraception,” says Hackel.

Copper IUDs cause an inflammatory reaction that’s toxic to sperm and keeps sperm from ever meeting the egg, explains Yen.

Experts need to perform more studies to determine whether people can reliably use hormonal IUDs as emergency contraception, but a 2021 study seems promising.

“Hormonal IUDs contain levonorgestrel, which interferes with sperm and egg transport through the fallopian tubes, as well as making the uterine wall inhospitable for anything to implant,” says Hackel.

The sooner you use emergency contraception, the more it may be effective. That said, you have options if 72 hours (3 days) or more pass.

You can use whatever method you have available for up to 120 hours (5 days) after sex. The ulipristal acetate pill branded as ella can be the most effective emergency contraceptive pill after 72 hours (3 days).

The copper IUD may be the most effective form of emergency contraception overall.

Gabrielle Kassel (she/her) is a queer sex educator and wellness journalist who is committed to helping people feel the best they can in their bodies. In addition to Healthline, her work has appeared in publications such as Shape, Cosmopolitan, Well+Good, Health, Self, Women’s Health, Greatist, and more! In her free time, Gabrielle can be found coaching CrossFit, reviewing pleasure products, hiking with her border collie, or recording episodes of the podcast she co-hosts called Bad In Bed. Follow her on Instagram @Gabriellekassel.