Plan B’s most common side effects are nausea, headache, fatigue, abdominal cramping, and changes to your next period. These effects are temporary, typically resolving within a day or two, and are caused by the large dose of synthetic hormone in the pill. Plan B contains 1.5 mg of levonorgestrel, a progestin that works by delaying ovulation.
The Most Common Side Effects
Nausea is the side effect people notice first, and it makes sense: you’re taking a concentrated burst of hormone all at once. Some people also experience headaches, dizziness, breast tenderness, and fatigue. Lower abdominal cramping is common too, similar to what you might feel before a period. Most of these symptoms are mild and pass within 24 to 48 hours.
Vomiting is less common but matters practically. If you throw up within three hours of taking the pill, your body may not have absorbed enough of the medication for it to work. In that case, you may need to take another dose.
How It Affects Your Period
Plan B frequently shifts the timing of your next period. Your cycle might come a few days early or a few days late, and the flow can be heavier or lighter than usual. Spotting or light bleeding between periods is also normal in the days or weeks after taking it. These changes happen because the hormone disrupts your body’s usual ovulation schedule, and your cycle needs time to reset.
If your period is more than a week late, take a pregnancy test. A late period after Plan B is often just a hormonal delay, but it can also mean the pill didn’t work.
What Counts as a Warning Sign
Normal cramping after Plan B feels like mild to moderate period pain. What isn’t normal: severe abdominal pain, especially if it’s sharp and concentrated on one side, or heavy vaginal bleeding that soaks through pads quickly. These can be signs of an ectopic pregnancy (a pregnancy that implants outside the uterus), which requires immediate medical attention. The side effects of Plan B are mild for most people, so anything intense or persistent is worth getting checked.
No Long-Term Effects on Fertility
Plan B does not affect your ability to get pregnant in the future. The World Health Organization states clearly that emergency contraception pills do not harm future fertility, and there is no delay in the return to fertility after taking them. The hormone leaves your system quickly. It’s also safe to use Plan B more than once, though it’s not intended as a regular birth control method since it’s less effective than ongoing contraception.
Effectiveness Drops at Higher Body Weights
This isn’t a side effect, but it’s something many people searching about Plan B need to know. The pill may become less effective if you weigh more than 165 pounds and may not work well at all above 176 pounds. For people with a BMI over 30, the failure rate climbs to roughly 6%, compared to about 2.5% with an alternative emergency contraceptive called ella (ulipristal acetate), which requires a prescription. If your weight is in this range, ella or a copper IUD inserted within five days are more reliable options.
Medications That Can Reduce Effectiveness
Certain drugs interfere with how your body processes levonorgestrel, potentially making Plan B less effective. The most notable are enzyme-inducing medications, a category that includes some epilepsy drugs (like carbamazepine), certain HIV medications, and the herbal supplement St. John’s Wort. If you take any prescription medication regularly, it’s worth checking whether it interacts with levonorgestrel before relying on Plan B as your emergency contraception strategy.
How Plan B Differs From Abortion Pills
Plan B prevents pregnancy; it does not end one. The pill works by delaying or preventing ovulation so that sperm and egg never meet. If a fertilized egg has already implanted in the uterus, Plan B will not affect it. This is a fundamentally different mechanism from medication abortion, which uses different drugs entirely. Understanding this distinction matters because confusion between the two is widespread and can influence decisions about whether to use emergency contraception.