The morning after pill works primarily by delaying or preventing ovulation, stopping your ovary from releasing an egg so sperm have nothing to fertilize. It does not end an existing pregnancy. In 2022, the FDA updated Plan B labeling to remove an older, unsupported claim that it might prevent a fertilized egg from implanting in the uterus. Emergency contraception is most effective the sooner you take it after unprotected sex, with a window of up to five days depending on the type.
How It Prevents Pregnancy
Your body goes through a predictable sequence each menstrual cycle: an egg matures, hormones surge to trigger its release, and the egg travels down the fallopian tube where it can meet sperm. The morning after pill interrupts this process at the ovulation stage. By flooding your system with a high dose of synthetic hormone, it suppresses or delays the hormonal surge that would release the egg. No egg, no fertilization, no pregnancy.
The timing relative to your cycle matters enormously. If you take the pill before your body has begun the ovulation process, it can effectively halt it. If ovulation has already happened, the pill is far less reliable. This is the main reason effectiveness drops the longer you wait to take it.
Two Types of Pills, Two Different Drugs
There are two main morning after pills available, and they use different active ingredients with slightly different strengths.
The first and most widely known contains levonorgestrel (sold as Plan B One-Step and several generics). It’s approved for use within 72 hours (three days) of unprotected sex, though it can still offer some protection up to five days. It’s available over the counter without a prescription or age restriction.
The second type contains ulipristal acetate (sold as ella). It’s approved for use within 120 hours (five days) and holds its effectiveness better across that full window. Between 72 and 120 hours after unprotected sex, ulipristal is notably more effective than levonorgestrel. It does require a prescription in the United States.
Both work best when taken as early as possible. The difference is that ulipristal doesn’t lose as much potency on days four and five.
How Effective It Is
Neither pill is as reliable as regular contraception. Levonorgestrel reduces the risk of pregnancy by about 85% when taken within 72 hours, with effectiveness declining after that. Ulipristal reduces the risk by about 85% across the full five-day window. For comparison, a copper IUD inserted as emergency contraception within five days is over 99% effective, making it the most reliable option.
These percentages describe risk reduction, not absolute protection. If 100 women had unprotected sex at a fertile point in their cycle and none used emergency contraception, roughly eight would become pregnant. With levonorgestrel taken promptly, that number drops to about one or two.
Body Weight Can Affect How Well It Works
Research has shown that levonorgestrel becomes less effective in women who weigh more than 165 pounds or have a BMI above 25. A 2011 meta-analysis found that women with obesity had a fourfold greater risk of pregnancy compared to women of normal weight when using levonorgestrel. Health Canada went so far as to state the drug may be ineffective in women over 176 pounds.
That said, major medical organizations emphasize that reduced effectiveness is not the same as zero effectiveness. The Society of Obstetricians and Gynaecologists of Canada recommends that women with a BMI over 30 should not be discouraged from using levonorgestrel if they can’t access a copper IUD. Ulipristal may maintain better effectiveness at higher body weights, and a copper IUD remains the most reliable option regardless of weight.
Common Side Effects
Side effects are typically mild and short-lived, lasting only a few days. The most common ones include nausea, headache, fatigue, dizziness, breast tenderness, and cramping or abdominal pain. Some people experience light spotting or bleeding between periods. Vomiting is possible. If you throw up within two hours of taking the pill, you need to take another dose.
The most noticeable effect for many people is a change in their next period. The morning after pill can shift your cycle by up to a week in either direction. Your period might come early, late, or be heavier or lighter than usual. If your period is more than a week late, take a pregnancy test.
Safety With Repeated Use
There are no known health risks from using the morning after pill more than once. The World Health Organization states that repeated use poses no known dangers, though frequent use can increase menstrual irregularities. It is not designed to replace regular contraception, simply because it’s less effective and more likely to cause side effects than daily birth control methods.
There is no evidence that using emergency contraception affects your future ability to get pregnant. It does not cause long-term changes to your fertility. It also does not protect against sexually transmitted infections.
Breastfeeding and Medical Conditions
Levonorgestrel passes into breast milk in very small quantities. Studies comparing breastfeeding outcomes between women who took levonorgestrel and those who didn’t found no differences, making it generally considered safe during breastfeeding. Ulipristal is handled differently: guidelines typically recommend expressing and discarding breast milk for about a week after taking it, since less safety data is available.
According to the WHO, there are no absolute medical conditions that prevent someone from using emergency contraception. There are no age limits either. However, certain medications can reduce its effectiveness, particularly drugs used to treat epilepsy, HIV, and tuberculosis, as well as some herbal supplements like St. John’s wort. These medications speed up how quickly your liver breaks down the pill, potentially lowering hormone levels before they can do their job.
What It Does Not Do
The morning after pill does not terminate a pregnancy that has already implanted. This is the key distinction between emergency contraception and medication abortion, which uses a completely different drug at a much higher dose to end an established pregnancy. The FDA has stated plainly that morning after pills are not abortion pills. If you are already pregnant, taking emergency contraception will not harm the pregnancy or affect the embryo.
It also does not provide ongoing protection. If you have unprotected sex again after taking the pill, even the next day, you are not covered. Each act of unprotected sex requires its own consideration for contraception.