The morning after pill is a form of emergency contraception you take by mouth after unprotected sex to prevent pregnancy. It works primarily by delaying or stopping ovulation, and it does not end an existing pregnancy. Two main types are available: one containing levonorgestrel (sold as Plan B One-Step and generics), which is available over the counter with no age restriction, and one containing ulipristal acetate (sold as ella), which requires a prescription.
How It Prevents Pregnancy
Both types of morning after pill work by interfering with ovulation, the release of an egg from the ovary. If no egg is released, sperm has nothing to fertilize, and pregnancy cannot occur. The FDA has reviewed the evidence for levonorgestrel-based pills specifically and concluded there is no direct effect on fertilization or implantation. The pill simply delays or blocks the hormonal surge that triggers egg release.
This is an important distinction: the morning after pill is not the same as an abortion pill. The American College of Obstetricians and Gynecologists states plainly that morning after pills do not cause an abortion and only work if you are not already pregnant. If a fertilized egg has already implanted in the uterus, the morning after pill will not affect it.
The Two Types and How They Differ
Levonorgestrel pills (Plan B One-Step, Take Action, and other generics) contain a single 1.5 mg dose of a synthetic hormone similar to progesterone. You can buy them at most pharmacies without a prescription, and there is no age requirement. They are most effective when taken as soon as possible after unprotected sex and are approved for use up to 72 hours (three days) afterward, though effectiveness drops as time passes.
Ulipristal acetate (ella) is a prescription-only pill that comes as a single 30 mg dose. It works somewhat differently by blocking progesterone receptors, which can delay ovulation even when the hormonal process is further along. Ella is approved for use up to 120 hours (five days) after unprotected sex and maintains its effectiveness better across that full window.
Effectiveness by Timing
Speed matters. Both pills are more effective the sooner you take them, but their performance over time is not identical. In a large clinical trial published in The Lancet comparing the two, the pregnancy rate within 72 hours was 1.8% for ulipristal acetate and 2.6% for levonorgestrel. That translates to preventing the vast majority of pregnancies that would otherwise have occurred.
The gap widens after 72 hours. In the same study, 203 women took emergency contraception between 72 and 120 hours after sex. All three pregnancies that occurred in that group were in women who took levonorgestrel, while none occurred in the ulipristal acetate group. If more than three days have passed, ella is the stronger option.
Body Weight Affects How Well It Works
This is one of the most underreported aspects of emergency contraception. Research shows that levonorgestrel-based pills become significantly less effective as body weight increases. Women with a BMI of 30 or higher had more than four times the risk of pregnancy compared to women with a BMI under 25. The data suggest that levonorgestrel pills begin losing effectiveness around 70 kg (about 154 pounds) and may have essentially no effect at 80 kg (about 176 pounds) and above.
The reason is pharmacological: at higher body weights, peak blood levels of the drug drop by roughly 50%, meaning the dose simply isn’t strong enough to reliably suppress ovulation. UK clinical guidelines now suggest that higher-weight individuals consider taking a double dose (3.0 mg instead of 1.5 mg) of levonorgestrel. Ulipristal acetate appears to maintain better effectiveness across a wider weight range, making it a more reliable choice for people who weigh more. A copper IUD, inserted by a healthcare provider within five days, is another highly effective option regardless of weight.
Common Side Effects
Side effects are generally mild and short-lived. The most frequently reported ones include nausea, headache, dizziness, fatigue, breast tenderness, and cramping or abdominal pain. Some people experience light spotting or bleeding between periods. Vomiting can also occur, and if you throw up within two hours of taking the pill, you may need another dose.
The most noticeable aftereffect for many people is a change in their next menstrual period. Your period might arrive earlier or later than expected, and the flow may be lighter or heavier than usual. If your period is more than seven days late, take a pregnancy test. A delay of up to one week is considered normal after taking emergency contraception.
What Can Make It Less Effective
Certain medications can interfere with how well the morning after pill works. Anti-seizure drugs, some HIV medications, and the herbal supplement St. John’s wort are among the most common culprits. These substances speed up the body’s processing of the pill’s active ingredient, lowering the amount available to do its job. If you take any of these regularly and need emergency contraception, talk to a pharmacist or provider about whether an alternative method like a copper IUD would be more reliable.
Smoking (tobacco use) is also flagged as a concern with levonorgestrel. More broadly, if you are taking any prescription medications, it is worth checking for interactions before relying on an emergency contraceptive pill.
What to Expect Afterward
After taking the morning after pill, there is nothing else you need to do immediately. You can eat, drink, and go about your day normally. The pill is a one-time dose, not an ongoing regimen. It does not protect you from pregnancy for the rest of your cycle, so if you have unprotected sex again afterward, you would need another dose or a different form of contraception.
The morning after pill is safe to use more than once, but it is not designed as a regular contraceptive method. It is less effective than daily birth control pills, IUDs, or implants at preventing pregnancy over time. If you find yourself reaching for emergency contraception frequently, a longer-term method will offer more reliable protection with fewer side effects.
One practical note about the two types: because ulipristal acetate works by blocking progesterone, starting or resuming hormonal birth control (pills, patches, rings) immediately after taking ella can reduce the effectiveness of both. If you take ella, wait at least five days before starting hormonal contraception and use a barrier method like condoms in the meantime. This delay does not apply to levonorgestrel pills, after which you can resume regular birth control right away.