Plan B is safe for the vast majority of people. The FDA approved it for over-the-counter use without age restrictions in 2013, meaning no prescription or ID is needed. It contains a single dose of a synthetic hormone that has been used in birth control pills for decades, and it carries no known risk of serious adverse events in healthy individuals. That said, there are some nuances worth understanding, especially around body weight, repeated use, and how the pill actually works.
How Plan B Works
Plan B contains levonorgestrel, a synthetic version of progesterone. It works by stopping or delaying the release of an egg from the ovary. If ovulation hasn’t happened yet, the pill prevents it from occurring during the window when sperm could still be viable. The FDA has confirmed that Plan B has no direct effect on fertilization or implantation. It is not the same as an abortion pill, and it will not end an existing pregnancy. If you’re already pregnant when you take it, the pill simply won’t do anything.
This distinction matters because of persistent confusion between Plan B and medication abortion. Medication abortion involves two different drugs that block pregnancy hormones and cause the uterus to contract. Plan B does neither of those things. It is a preventive measure, not an intervention on an established pregnancy.
Common Side Effects
The most frequently reported side effects are mild and temporary. You may experience nausea, headache, fatigue, or breast tenderness. Some people notice spotting or light bleeding in the days after taking the pill, and your next period may arrive earlier or later than expected. These effects result from the burst of synthetic hormone and typically resolve within a few days. There are no known long-term side effects from a single dose.
No Impact on Future Fertility
One of the most common concerns is whether Plan B could make it harder to get pregnant later. The evidence is clear that it does not. A review published in the journal Contraception found no indication that levonorgestrel emergency contraception disrupts normal menstrual cycles in the month following use. The hormone clears the body quickly, with only trace amounts remaining after about 96 hours.
While no studies have tracked long-term fertility specifically after emergency contraception use, there is substantial data on other levonorgestrel-based contraceptives like implants and hormonal IUDs. Three separate systematic reviews have concluded that levonorgestrel-containing contraceptives, regardless of how long someone uses them, have no detrimental effect on time to pregnancy. Conception rates after stopping are similar to those expected in the general population. Given that a single emergency dose is far less hormone exposure than years on an implant, repeated Plan B use is unlikely to affect future fertility.
Body Weight and Effectiveness
Plan B’s safety profile doesn’t change with body weight, but its effectiveness does. Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced contraceptive failure four times as often as those with a BMI under 25. The reason is pharmacological: blood levels of levonorgestrel were about 50% lower in individuals with a BMI of 30 after a standard dose, meaning the hormone never reaches the concentration needed to reliably prevent ovulation.
Doubling the dose doesn’t solve the problem. The same research group tested a double dose in people weighing at least 176 pounds with BMIs above 30 and found it was not effective at preventing pregnancy. If your BMI is 30 or higher, a copper IUD inserted within five days of unprotected sex is the most reliable emergency contraception option, and it can then serve as ongoing birth control.
Taking Plan B More Than Once
Using Plan B multiple times, even within the same menstrual cycle, does not appear to cause serious harm. A systematic review in BMJ Sexual & Reproductive Health examined the available evidence and found no increased risk of adverse events with repeated use. One study tracked women who used levonorgestrel pills four to seven times per month over more than six months. Of 102 reported adverse events, only three were classified as severe, and none were judged to be related to the medication. Bleeding patterns actually became lighter over time.
That said, there is one signal worth noting. Among women who became pregnant despite using Plan B, those who had used it multiple times in the same cycle had roughly 2.5 times higher odds of an ectopic pregnancy (a pregnancy that implants outside the uterus, usually in a fallopian tube) compared to those who used it once. This may be because progestins can slow the movement of eggs through the fallopian tubes. Ectopic pregnancies are rare but can be dangerous if untreated, so if you’ve taken Plan B and later experience sharp pelvic pain or unusual bleeding, seek medical attention.
Plan B is designed as a backup, not a primary method. It’s less effective than regular contraception, and repeated purchases add up financially. But the safety data does not support the idea that using it more than once is physically harmful.
Drug Interactions That Reduce Effectiveness
Certain medications can lower Plan B’s effectiveness by speeding up how quickly your liver breaks down levonorgestrel. These include some seizure medications (like carbamazepine), certain sedatives, the steroid dexamethasone, and the herbal supplement St. John’s Wort. If you take any of these regularly, Plan B may not work as well for you, and a copper IUD is a more reliable emergency option.
Safety for Teens
The FDA’s decision to remove all age restrictions was based on evidence that levonorgestrel is safe for adolescents. The pill works the same way regardless of age: it delays ovulation using a hormone already present in many forms of birth control prescribed to teens. There are no additional risks for younger users, and no medical screening or parental consent is required to purchase it.
Ectopic Pregnancy Risk if Plan B Fails
When Plan B works, it prevents pregnancy entirely, and there is no risk. But when it fails, the resulting pregnancy has a slightly higher chance of being ectopic. One large study found that among 73 pregnancies that occurred despite levonorgestrel use, three were ectopic, a rate of about 4.1%. In the general population, ectopic pregnancies account for roughly 1 to 2% of all pregnancies. The elevated rate likely relates to the way progestins can slow the transport of a fertilized egg through the fallopian tube.
This doesn’t mean Plan B causes ectopic pregnancies. It means that if the pill fails to prevent pregnancy, especially when taken close to the time of ovulation, the pregnancy that results is somewhat more likely to implant in the wrong place. Symptoms to watch for include sharp or stabbing pain on one side of the lower abdomen, vaginal bleeding that differs from your normal period, and dizziness or shoulder pain. These symptoms typically appear a few weeks after conception.