Plan B is considered very safe. The FDA approved it for over-the-counter sale without age restrictions in 2013, and the World Health Organization states there are no absolute medical contraindications to using emergency contraceptive pills. Out of the millions of people who have taken it, no serious health problems have been reported.
How Plan B Works
Plan B contains a single dose of a synthetic hormone that prevents or delays ovulation, stopping the release of an egg so sperm has nothing to fertilize. It works before implantation, not after. If you’re already pregnant, Plan B will not affect the pregnancy. The FDA has confirmed that evidence does not support the idea that it disrupts implantation or terminates a pregnancy.
This distinction matters because it means Plan B is not an abortion pill. It simply prevents pregnancy from occurring in the first place by pausing your body’s ovulation process.
Common Side Effects
The most frequently reported side effect is a change in your next menstrual period. Your period may come earlier or later than expected, and the flow may be heavier or lighter than usual. Nausea occurs in fewer than 20% of users and is rarely accompanied by vomiting. Some people experience headaches, fatigue, or breast tenderness, but these typically resolve within a day or two.
None of these side effects are dangerous. They’re a temporary response to the hormone dose and don’t signal any lasting change in your body.
No Evidence of Long-Term Harm
A 2022 review of 33 studies found no evidence that Plan B affects future fertility. Women who took it had similar conception rates compared to those who took a placebo. The review also found no increased risk of miscarriage, no harm to fetal development, and no lasting disruption to menstrual cycles. In short, taking Plan B now does not make it harder to get pregnant later.
Using It More Than Once
Taking Plan B multiple times is not harmful. There’s no medical limit on how many times you can use it. However, using it frequently can make your periods irregular and harder to predict. It’s also less effective than ongoing birth control methods like an IUD or the pill, which is the practical reason doctors recommend those instead. If you find yourself reaching for emergency contraception regularly, switching to a routine method will give you both better protection and more predictable cycles.
One important note: if you need emergency contraception again within five days of taking Plan B, use another levonorgestrel pill (Plan B or a generic), not ella. Taking both types close together can reduce their effectiveness.
Weight and Effectiveness
Plan B may be less effective for people who weigh more than 165 pounds or have a BMI over 25. Health Canada is the only regulator that currently includes this guidance on its labeling, recommending that people over 165 pounds consider alternative emergency contraception and noting it may be ineffective above 176 pounds. Other regulatory bodies, including the WHO, take a more cautious stance, saying that while effectiveness may decrease at higher weights, it doesn’t drop to zero and people should not be discouraged from using it.
If your BMI is over 30, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraceptive option. Another emergency contraceptive pill (sold under the brand ella) also appears to work better than Plan B at higher body weights, though it requires a prescription.
Medications That Can Reduce Effectiveness
Certain medications speed up how quickly your body breaks down the active ingredient in Plan B, potentially making it less effective. The main culprits are drugs that ramp up a specific liver enzyme involved in processing hormones. These include some anti-seizure medications (like carbamazepine, phenytoin, and phenobarbital), the antibiotic rifampicin (used for tuberculosis), and certain HIV medications like efavirenz. One study found that carbamazepine cut the body’s exposure to Plan B’s active ingredient roughly in half.
The herbal supplement St. John’s wort activates the same liver pathway and could also reduce effectiveness. If you take any of these medications regularly, a copper IUD is a more reliable emergency option.
Who Can Take It
Since 2013, Plan B has been available over the counter in the United States with no age restrictions and no prescription required. The WHO confirms there are no medical conditions that prohibit its use. You don’t need to take a pregnancy test first, though it won’t work if you’re already pregnant. There’s no need for a doctor’s visit or approval.
Plan B is most effective when taken as soon as possible after unprotected sex and can be used up to 72 hours afterward, with effectiveness declining over that window. It does not protect against sexually transmitted infections.