Plan B is not bad for you in any lasting way. It delivers a single large dose of a synthetic hormone your body already produces naturally, and its effects are temporary. You may feel rough for a day or two afterward, and your next period might show up at an unexpected time, but there is no evidence of long-term harm to your health or fertility. That said, there are some real nuances worth understanding, especially around side effects, how often you use it, and whether it will actually work for your body weight.
How Plan B Works
Plan B contains 1.5 mg of levonorgestrel, a synthetic version of progesterone. Its primary job is to delay or prevent ovulation, the release of an egg from the ovary. If no egg is available, sperm have nothing to fertilize, and pregnancy doesn’t happen. It does not interrupt an existing pregnancy or harm a developing embryo. The World Health Organization is explicit on this point: emergency contraception pills prevent pregnancy by preventing or delaying ovulation, and they do not cause an abortion.
A review published in the journal Contraception looked at ten studies examining whether levonorgestrel changes the uterine lining in a way that could block a fertilized egg from implanting. Nine of the ten found no difference in implantation receptivity compared to controls. When taken after ovulation has already occurred, levonorgestrel resulted in conception rates similar to placebo. In other words, the pill works by stopping ovulation, not by interfering with anything afterward.
Short-Term Side Effects
The most common complaint is nausea, reported by about 14% of users in FDA clinical data. Fatigue hits a similar percentage (13.3%), followed by headache (10.3%) and dizziness (9.6%). These side effects typically resolve within 24 to 48 hours. Some people feel nothing at all.
Your next period is the other thing likely to change. It may arrive earlier than expected, later than expected, or look different from your usual flow: heavier, lighter, or spottier. This is a normal response to the hormone surge and not a sign that something went wrong. If your period is more than a week late, a pregnancy test is a reasonable next step.
No Evidence of Long-Term Harm
The fear that drives most people to search “is Plan B bad for you” is whether it causes lasting damage, particularly to fertility. The clinical evidence is reassuring. Levonorgestrel delays ovulation temporarily, and normal ovulatory cycles resume afterward. The review in Contraception found that conception rates after levonorgestrel exposure were no different from placebo groups, meaning the drug does not reduce your ability to get pregnant in subsequent cycles.
Plan B does not accumulate in your body. It’s a single dose that your liver metabolizes and clears. There is no known mechanism by which occasional use would cause organ damage, hormonal disruption, or chronic health problems.
Using It More Than Once
The FDA states that Plan B “is not for routine contraceptive use,” but this is a recommendation about effectiveness strategy, not a safety warning. Using it multiple times is not dangerous. It simply isn’t a great ongoing plan because it is less reliable than daily birth control pills, IUDs, or implants, and it comes with that unpleasant day of nausea and fatigue each time. Repeated use also means repeated disruption to your menstrual cycle timing, which can be annoying and anxiety-inducing even when nothing is medically wrong.
If you find yourself reaching for Plan B frequently, that’s a signal to explore a regular contraceptive method that will protect you more consistently, not because the pill itself is harming you.
The Weight Factor
This is the dimension of Plan B that most people don’t know about, and it matters. Levonorgestrel’s effectiveness drops significantly at higher body weights. Research shows a clear ceiling: the standard 1.5 mg dose appears to lose efficacy starting around 70 kg (about 154 pounds) and may have essentially no effect for people weighing 80 kg (176 pounds) or more.
The reason is pharmacokinetic. People with a BMI of 30 or higher absorb the drug differently, reaching peak blood concentrations roughly 50% lower than those with a BMI under 25. One analysis found that people in the higher BMI group had more than four times the odds of becoming pregnant after using levonorgestrel-based emergency contraception compared to those in the lower BMI group. If you weigh more than about 155 pounds, a copper IUD inserted as emergency contraception is the most effective option, and an alternative emergency contraceptive pill (ella, which uses a different active ingredient) performs better at higher weights than Plan B does.
Drug Interactions and Contraindications
Plan B has very few medical contraindications. It is not recommended for use alongside certain antiviral medications (including some used for hepatitis C treatment) or tranexamic acid, a drug used to control heavy bleeding. Tobacco use is also flagged as a concern when combined with hormonal contraceptives, consistent with the well-established link between smoking and blood clot risk in people taking synthetic hormones.
Plan B is safe to use while breastfeeding, though the general guidance is to weigh benefits and risks. It can be used by teenagers who have started menstruating. There is no age-based restriction on its use for people of reproductive age.
One Rare Risk Worth Knowing
If Plan B fails and pregnancy occurs anyway, there is a slightly elevated concern about ectopic pregnancy, where the embryo implants in a fallopian tube instead of the uterus. Progesterone-type hormones can slow the movement of an egg through the fallopian tubes, and this mechanism could theoretically increase ectopic risk when the drug fails to prevent pregnancy altogether. New Zealand’s medicines safety authority has flagged this pattern based on case reports. Ectopic pregnancy is a medical emergency that causes sharp, one-sided abdominal pain and sometimes abnormal bleeding. If you took Plan B, it didn’t work, and you experience those symptoms, seek care immediately. This is a risk from the rare failure of the drug, not from the drug itself working as intended.