Plan B is not unhealthy for most people. It delivers a single dose of a synthetic hormone that your body clears quickly, and research shows no lasting effects on fertility, menstrual cycles, or future pregnancies. That said, it does cause short-term side effects, works less reliably above certain body weights, and interacts with some medications. Here’s what actually matters.
What Plan B Does in Your Body
Plan B contains levonorgestrel, a synthetic version of progesterone that has been used in birth control pills for decades. When you take it after unprotected sex, it prevents or delays ovulation so that sperm and egg never meet. It does not terminate a pregnancy that has already started, and it cannot harm a developing embryo. The World Health Organization states this clearly: emergency contraceptive pills prevent pregnancy by preventing or delaying ovulation, and they do not induce an abortion.
The hormone leaves your system within days. It’s essentially a concentrated burst of the same type of hormone found in many daily birth control pills, just delivered all at once rather than spread across a month.
Common Side Effects
Most people who take Plan B experience at least mild side effects. These are temporary and resolve on their own, usually within a day or two. The most common ones include nausea, fatigue, headache, breast tenderness, and lower abdominal cramping. Some people experience light spotting or bleeding in the days after taking it.
Your next period may come early or late, typically within a week of when you’d normally expect it. If your period is more than a week late, pregnancy is possible and worth checking for. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose.
No Evidence of Long-Term Health Risks
One of the biggest concerns people have is whether Plan B causes lasting damage, especially to fertility. A 2022 review published in the journal Contraception examined the available evidence and found no indication that levonorgestrel emergency contraception disrupts normal menstrual cycles, impairs future fertility, or increases the risk of miscarriage or fetal abnormalities in later pregnancies.
While no studies have tracked long-term fertility specifically after emergency contraception use, there is substantial evidence from other levonorgestrel-based contraceptives (like hormonal IUDs and mini-pills) showing no impact on future fertility, even after years of use. Three separate systematic reviews found that conception rates after stopping levonorgestrel contraceptives are the same as those in the general population, regardless of how long the person used them. Based on this, researchers concluded that repeated use of levonorgestrel emergency contraception is unlikely to affect future fertility.
Using It More Than Once
There’s a persistent belief that Plan B is only safe to take once or twice in your life. This isn’t supported by medical evidence. It’s safe to use more than once, even within the same menstrual cycle if needed. The reason doctors don’t recommend it as a primary method isn’t that it’s dangerous with repeated use. It’s that it’s simply less effective than regular contraception, and the side effects (nausea, cycle disruption) are more unpleasant than what you’d get from a daily pill or an IUD.
Weight Affects How Well It Works
This is a real and underreported limitation. Plan B’s effectiveness drops significantly as body weight increases. Research shows that people with a BMI of 30 or higher had more than four times the risk of pregnancy compared to those with a BMI under 25 when using levonorgestrel-based emergency contraception. The decline tracks with absolute weight: effectiveness begins to drop at around 70 kg (about 155 pounds) and appears to offer essentially no protection at 80 kg (about 176 pounds).
The reason is straightforward. At higher body weights, blood levels of the hormone peak at roughly 50% of what lighter individuals achieve from the same dose. The drug is simply too diluted to reliably block ovulation. A copper IUD, which can be placed by a provider within five days of unprotected sex, remains highly effective regardless of weight and is worth considering as an alternative.
Medications That Reduce Effectiveness
Certain drugs speed up your liver’s processing of levonorgestrel, breaking it down before it can do its job. These include several common anti-seizure medications (such as carbamazepine, phenytoin, and topiramate), the antibiotic rifampin, the antifungal griseofulvin, and the herbal supplement St. John’s wort. Some HIV medications can also raise or lower hormone levels unpredictably. If you take any of these regularly, Plan B may be less reliable for you.
Who Should Avoid It
The list of true contraindications is short. You should not take Plan B if you’ve had an allergic reaction to levonorgestrel before. It’s also not indicated during a confirmed pregnancy, though not because it’s harmful. It simply won’t do anything. There are no cardiovascular restrictions, no age limits, and no conditions like blood clotting disorders that would make it dangerous, which distinguishes it from combined-hormone contraceptives that contain estrogen.
For most people, Plan B is a safe, well-studied backup option. The short-term side effects are real but minor, and the long-term health risks are, based on current evidence, nonexistent. The more practical concern is whether it will actually work for you, which depends on timing, weight, and what other medications you’re taking.