Plan B delivers a large, concentrated dose of a synthetic hormone called levonorgestrel (1.5 mg in a single tablet) that primarily works by delaying or preventing ovulation. That one dose contains roughly the same amount of hormone found in about 12 daily birth control pills, which is why it can produce noticeable short-term effects on your body. Here’s what actually happens after you take it.
How Plan B Prevents Pregnancy
The primary way Plan B works is straightforward: it stops your ovary from releasing an egg. Before ovulation, your brain sends a hormonal signal (called the LH surge) that triggers the egg’s release. The burst of levonorgestrel from Plan B suppresses that signal, essentially hitting pause on ovulation for long enough that any sperm in your reproductive tract die before they can meet an egg. Sperm typically survive three to five days, so delaying ovulation by even a few days can be the difference between pregnancy and no pregnancy.
Plan B may also thicken cervical mucus, making it harder for sperm to reach an egg in the first place. There’s been longstanding debate about whether it can also prevent a fertilized egg from implanting in the uterus, but a 2022 review in the journal Contraception found overwhelming evidence that levonorgestrel taken after ovulation does not affect implantation. In studies, conception rates after taking Plan B post-ovulation were similar to placebo. This matters because if you’ve already ovulated, Plan B is unlikely to work at all, which is why timing is so critical.
Why Timing Changes Everything
Plan B is most effective the sooner you take it. Within the first 24 hours after unprotected sex, it prevents about 95% of expected pregnancies. That number drops to roughly 89% overall when taken within 72 hours. Effectiveness continues to decline after that point, and the pill is generally not recommended beyond 72 hours. If ovulation has already happened, Plan B has little to no ability to prevent pregnancy regardless of when you take it.
What You’ll Feel in the First Few Days
The most common side effect is nausea, which can range from mild queasiness to vomiting. If you vomit within two hours of taking the pill, it may not have been fully absorbed and you may need another dose. Other effects that many women experience include headaches, fatigue, dizziness, breast tenderness, and lower abdominal cramping. These are all responses to the sudden spike of synthetic hormone in your system.
Most of these side effects resolve within a day or two. Levonorgestrel has an elimination half-life of about 27.5 hours, meaning your body clears half the dose in roughly a day and most of it within a few days. By the end of a week, the hormone levels from the pill are negligible.
Changes to Your Next Period
Plan B commonly disrupts your menstrual cycle for that month. Your next period may come earlier than expected, later than expected, or arrive on time but look different. According to the Mayo Clinic, the pill can delay your period by up to one week. You might also experience spotting or irregular bleeding between your period and when you took the pill.
If your period is more than a week late, that’s worth noting, not because Plan B caused lasting harm, but because it could indicate the pill didn’t work and you may be pregnant. A pregnancy test at that point will give a reliable answer. After one full cycle, your period should return to its normal pattern.
Weight and Body Size Affect How Well It Works
This is one of the most underreported aspects of Plan B. Research shows that the pill becomes significantly less effective as body weight increases. In women with a BMI of 30 or higher, peak blood levels of levonorgestrel are roughly 50% lower than in women with a BMI under 25. That translates to real differences in pregnancy risk: women with a BMI over 30 had more than four times the odds of becoming pregnant compared to women under a BMI of 25 when using levonorgestrel-based emergency contraception.
The decline in effectiveness appears tied to absolute weight rather than BMI alone. Studies suggest levonorgestrel-based emergency contraception hits a ceiling of effectiveness around 70 kg (about 154 pounds) and shows essentially no efficacy at 80 kg (about 176 pounds) and above. If you weigh more than 155 pounds, a copper IUD inserted within five days is the most effective emergency contraception option. Another pill-based alternative, which uses a different active ingredient, maintains better effectiveness at higher weights.
No Lasting Effects on Fertility or Health
Plan B does not affect your ability to get pregnant in the future. The hormone clears your body within days, and there is no evidence that taking it, even multiple times, causes lasting changes to your reproductive system. A comprehensive review of levonorgestrel’s effects on the uterine lining found that while it can temporarily alter certain receptivity markers if taken before ovulation, these changes don’t translate into any clinical impact on future embryo implantation.
Plan B is also not an abortion pill. It cannot end an existing pregnancy. If a fertilized egg has already implanted in the uterine wall, levonorgestrel will not disrupt that process. The pill works entirely in the window before pregnancy begins.
Taking Plan B more than once in a cycle or multiple times over months won’t cause cumulative damage, though it can make your periods less predictable for a while. It’s not designed as a regular contraceptive method simply because it’s less effective than daily birth control and more likely to cause side effects like nausea, not because repeated use is dangerous.