Plan B is an emergency contraceptive pill used to prevent pregnancy after unprotected sex or contraceptive failure, such as a broken condom or a missed birth control pill. It contains a single dose of levonorgestrel, a synthetic hormone, and is available over the counter without a prescription or age restriction. It is not an abortion pill and will not end an existing pregnancy.
How Plan B Prevents Pregnancy
Plan B works primarily by blocking or delaying ovulation, the release of an egg from the ovary. Levonorgestrel suppresses the hormonal surge that triggers ovulation, so the egg never reaches a point where it can be fertilized by sperm. A study published in Human Reproduction Update analyzing the available evidence concluded that levonorgestrel in emergency contraception doses “acts mainly to inhibit or delay ovulation but does not prevent fertilization or implantation.” In other words, it stops pregnancy from starting rather than disrupting one that has already begun.
This distinction matters because emergency contraception is frequently confused with medication abortion. The two are completely different. Abortion pills use a different drug to end an established pregnancy. Plan B has no effect on a fertilized egg that has already implanted in the uterus.
When to Take It
Plan B should be taken as soon as possible after unprotected sex. It can be used up to 72 hours (three days) afterward, but its effectiveness drops significantly with each passing day. The sooner you take it, the better it works.
There is one critical limitation: Plan B will not work if your body has already started ovulating. Because the pill’s entire mechanism depends on preventing or delaying that egg release, taking it after ovulation has begun offers little to no protection. If you’re in the second half of your menstrual cycle and think ovulation may have already occurred, another type of emergency contraception called ella works closer to the time of ovulation and may be a better option, though it requires a prescription.
Effectiveness by Body Weight
Plan B’s effectiveness decreases as body weight increases, and this is one of the most under-discussed aspects of the pill. Research shows that people with a BMI of 30 or higher who use levonorgestrel-based emergency contraception have roughly 4.4 times the risk of pregnancy compared to those with a BMI under 25. The decline appears tied to absolute weight rather than BMI alone: Plan B’s effectiveness begins to drop around 70 kg (about 154 pounds) and shows essentially no efficacy for people weighing 80 kg (about 176 pounds) or more.
The reason is pharmacological. At the standard 1.5 mg dose, people with a BMI of 30 or higher reach peak blood concentrations of levonorgestrel that are roughly 50% lower than those in people with a BMI under 25. The drug simply doesn’t reach high enough levels in the body to reliably block ovulation. If you weigh more than 155 pounds, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraceptive option, or you can ask a provider about ella, which maintains better efficacy at higher weights.
Common Side Effects
Side effects are mild for most people and typically last only a few days. The most common include:
- Nausea or vomiting
- Headache or dizziness
- Fatigue
- Breast tenderness
- Abdominal pain or cramping
- Spotting or changes in your next period
Your next period may arrive earlier or later than expected. Light bleeding between periods is also normal. If your period is more than a week late after taking Plan B, it’s worth taking a pregnancy test.
Using Plan B More Than Once
Taking Plan B multiple times does not reduce its effectiveness or cause long-term side effects. You can use it whenever you need it, even more than once in the same menstrual cycle. That said, it’s not designed to be a regular contraceptive method. It’s less effective at preventing pregnancy over time than consistent use of birth control like an IUD, the pill, or condoms, and repeated use means repeated exposure to a large hormone dose that can make your cycle unpredictable.
What Can Reduce Its Effectiveness
Certain medications can interfere with how your body processes levonorgestrel, lowering Plan B’s effectiveness. St. John’s wort, an herbal supplement commonly used for mood support, is one well-known interaction. Some medications used to treat seizures and certain infections can also speed up the breakdown of levonorgestrel in your body, reducing how much of the drug is available to block ovulation. If you take any prescription medications regularly, it’s worth checking whether they interact with levonorgestrel before relying on Plan B as a backup.