Plan B does not harm your fertility. Multiple systematic reviews covering dozens of studies have found no evidence that levonorgestrel, the hormone in Plan B, affects your ability to get pregnant in the future. This holds true whether you’ve taken it once or multiple times. The hormone leaves your body within days, and your normal ovulation cycle resumes without lasting disruption.
How Plan B Actually Works
Plan B contains a single dose of levonorgestrel, a synthetic version of progesterone that your body already produces naturally. It prevents pregnancy by delaying or blocking ovulation, meaning the egg never releases from the ovary in the first place. If ovulation has already happened, Plan B has limited ability to prevent pregnancy.
This is an important distinction: Plan B does not end or interfere with an existing pregnancy. It cannot harm a developing embryo. It works entirely by hitting pause on ovulation for long enough that sperm (which survive about five days) die before an egg becomes available. The American College of Obstetricians and Gynecologists confirms that morning-after pills only work if you are not already pregnant.
What the Research Says About Long-Term Fertility
A 2022 systematic review analyzed 33 studies on levonorgestrel emergency contraception and found it did not affect fallopian tube function, rates of ectopic pregnancy, miscarriage, stillbirth, or subsequent menstrual cycles. Three additional systematic reviews looking at fertility after levonorgestrel use all reached the same conclusion: conception rates after use are similar to those expected in the general population, regardless of how many times someone used it or how long they used it.
A review published in the journal Contraception found no indication that Plan B disrupts the return of a normal menstrual cycle in the cycle following use. After you take the pill, the hormone’s half-life is roughly 42 hours. Only trace amounts remain after about 96 hours, or four days. Your body clears it quickly and completely.
Taking It More Than Once
One of the most common concerns is whether repeated use causes cumulative damage. It doesn’t. Planned Parenthood states directly that using emergency contraception more than once does not affect fertility and will not prevent future pregnancy. You can even take it more than once in the same menstrual cycle if needed.
That said, frequent use does come with practical downsides. A 2014 systematic review of 22 studies found that the most common side effect of repeated use was menstrual irregularities, meaning your period may become harder to predict. This is a temporary inconvenience, not a sign of reproductive harm. Plan B is also less effective as an ongoing contraceptive strategy compared to methods like an IUD, the implant, or daily birth control pills, so relying on it regularly increases your chances of an unintended pregnancy simply because it doesn’t work as well over time.
Short-Term Side Effects to Expect
Plan B can cause noticeable but temporary side effects in the days after you take it. The most common ones are nausea, stomach pain, headache, dizziness, breast tenderness, fatigue, and vomiting. These typically resolve on their own as the hormone clears your system.
Your next period may arrive up to a week earlier or later than expected. Light spotting or breakthrough bleeding a few days after taking Plan B is also normal, though not everyone experiences it. If your period is more than a week late, a pregnancy test is a reasonable next step, since the delay could indicate that the pill didn’t work rather than a side effect.
When Plan B Is Less Effective
While Plan B doesn’t affect fertility, it’s worth knowing when it may not work well enough to prevent pregnancy in the first place. Timing matters most: the sooner you take it after unprotected sex, the better. It’s most effective within the first 24 hours and becomes progressively less reliable over 72 hours.
Body weight also plays a role. If you weigh more than 165 pounds, levonorgestrel-based pills like Plan B are less effective. In that case, ella (a different type of emergency contraception that requires a prescription) or a copper IUD inserted within five days are more reliable options. The copper IUD is the most effective form of emergency contraception available and has the added benefit of providing ongoing birth control for up to 10 years.
Plan B Is Not an Abortion Pill
Confusion between Plan B and medication abortion drugs like mifepristone fuels a lot of the fear around fertility effects. These are entirely different medications that work through different mechanisms. Mifepristone ends an established pregnancy by blocking the hormone progesterone. Plan B prevents pregnancy from occurring by delaying ovulation. It has no effect on an embryo that has already implanted. The World Health Organization states plainly that emergency contraceptive pills do not induce an abortion and cannot interrupt an established pregnancy.
This distinction matters because some of the anxiety about Plan B harming fertility comes from conflating it with abortion, which itself does not generally impair future fertility. But the two aren’t even in the same category of medication. Plan B is a higher dose of the same hormone found in many daily birth control pills, delivered as a one-time event rather than a daily regimen.