Emergency Contraception After Unprotected Sex: How It Works and How Quickly to Act

How emergency contraception works — and what it does not do

Emergency contraception prevents pregnancy — it does not end an existing pregnancy 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive. The pill-based options work primarily by delaying or preventing ovulation. If an egg has already been released and fertilization has occurred, a levonorgestrel pill is unlikely to prevent pregnancy 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed.

The copper IUD, placed by a clinician, creates a uterine environment hostile to fertilization and can work even after ovulation has occurred, making it effective regardless of where in the menstrual cycle it is placed. It is the most effective emergency contraceptive option across the entire five-day window 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive.

Emergency contraception does not protect against sexually transmitted infections. If the encounter involved STI risk, testing is a separate consideration.

Your options compared

Levonorgestrel pills (Plan B and generics) Available over the counter at most pharmacies without a prescription or age restriction. Most effective within 72 hours of unprotected sex; can be used up to 120 hours with reduced effectiveness. Common side effects include nausea, headache, and irregular spotting. Effectiveness may be reduced at higher body weights — a clinician or pharmacist can advise 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed.

Ulipristal acetate (ella) Requires a prescription in the United States. Effective up to 120 hours and maintains effectiveness more consistently across the five-day window than levonorgestrel, particularly when more than 72 hours have passed or when body weight is a factor 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed. Some telehealth services can provide a prescription quickly.

Copper IUD Placed by a clinician within five days of unprotected sex. The most effective emergency contraceptive option overall — more than 99% effective — and not affected by body weight 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive. Continues as highly effective ongoing contraception afterward. Involves a brief in-office procedure. For those who want both immediate and long-term contraception, this is the most effective single choice.

Does body weight affect emergency contraception?

Levonorgestrel pills may be less effective at higher body weights 1Ref 1American College of Obstetricians and Gynecologists (2015).Practice Bulletin No. 152: Emergency Contraception.Mechanism of action for levonorgestrel, ulipristal acetate, and copper IUD; efficacy comparison across the five-day window; body weight considerations; copper IUD as most effective emergency contraceptive2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed. A study published in the New England Journal of Medicine found that levonorgestrel's efficacy was substantially reduced in people with higher body mass index compared with ulipristal acetate 2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed. If this is a concern, ulipristal acetate (ella) or the copper IUD may be more appropriate options. A clinician or pharmacist can help you choose based on your specific situation and how much time has passed since unprotected sex.

What to expect afterward

After taking a levonorgestrel or ulipristal pill, your next period may arrive a few days earlier or later than expected. This is normal. Some people experience nausea, breast tenderness, headache, or spotting in the days after taking emergency contraception.

If your next period is more than a week late, or if you have symptoms consistent with pregnancy, take a home pregnancy test 2Ref 2American College of Obstetricians and Gynecologists (2019).ACOG Practice Bulletin No. 206: Use of Hormonal Contraception in Women With Coexisting Medical Conditions.Body weight and BMI affecting levonorgestrel efficacy; ulipristal acetate preferred when >72 hours have passed or body weight is a concern; follow-up pregnancy testing if period is delayed. Emergency contraception is not designed for regular use — if you find yourself needing it frequently, a clinician can discuss more reliable ongoing contraception options.

Should you also get tested for STIs?

Emergency contraception addresses pregnancy risk only. If the sexual encounter involved a partner whose STI status is unknown to you, STI testing is worth considering — either now for infections detectable immediately, or after the window period for infections such as HIV and syphilis that require time to appear on testing 3Ref 3Workowski KA, Bachmann LH, Chan PA, Johnston CM, Muzny CA, Park I, Reno H, Zenilman JM, Bolan GA (2021).Sexually Transmitted Infections Treatment Guidelines, 2021.STI testing considerations after potential exposure; HIV PEP protocol requiring initiation within 72 hours of exposure.

HIV post-exposure prophylaxis (PEP) is a different medication from emergency contraception and must be started within 72 hours of a potential HIV exposure. If you are concerned about HIV, contact a clinician or emergency room promptly — PEP and emergency contraception can be addressed at the same visit.