Plan B is not bad for your body. It delivers a single dose of a synthetic hormone that your body processes within days, and no evidence links it to long-term health consequences, fertility problems, or pregnancy complications down the road. That said, it does cause short-term side effects for many people, and there are practical limitations worth knowing about, especially around body weight and medication interactions.
What Plan B Actually Does
Plan B contains levonorgestrel, a synthetic version of progesterone that your body already produces naturally. When you take it, the hormone primarily stops or delays your ovary from releasing an egg. No egg release means no fertilization. This is why timing matters so much: Plan B works best within 72 hours of unprotected sex, though it retains some effectiveness up to 120 hours (five days). If ovulation has already happened, the pill is far less likely to work.
The dose you’re getting is higher than what’s in a daily birth control pill, but it’s a one-time spike that clears your system relatively quickly. It’s not accumulating in your tissues or altering your hormonal baseline in any lasting way.
Common Side Effects
Most people who take Plan B experience at least one side effect, but they’re generally mild to moderate and resolve on their own within a day or two. The most frequently reported ones are nausea, headache, fatigue, dizziness, lower abdominal pain, and breast tenderness. Some people vomit after taking it. If you throw up within two hours, the dose may not have been fully absorbed.
These side effects are a direct result of the hormone surge. They’re uncomfortable but not dangerous, and they don’t signal any kind of damage to your body.
How It Affects Your Next Period
Plan B commonly disrupts your menstrual cycle for the month you take it. About 30% of users experience unexpected bleeding or spotting within seven days of taking the pill. Your next period may arrive earlier or later than expected. Up to 13% of users see their period delayed by more than a week.
If your period is five or more days late, a pregnancy test is worth taking. The delay alone doesn’t mean anything is wrong. It’s a predictable consequence of temporarily interrupting your hormonal cycle, and things typically return to normal by the following month.
No Evidence of Long-Term Harm
A 2022 review examining levonorgestrel emergency contraception across multiple studies found no evidence that it affects fetal development, increases miscarriage risk, causes stillbirth, or disrupts menstrual cycles beyond the immediate month of use. Researchers specifically looked at ovulation, conception, implantation, ectopic pregnancy, and fertility in subsequent cycles. None showed lasting effects.
The question of ectopic pregnancy risk comes up often. A large case-control study tracking over 2,400 ectopic pregnancies found no increased odds of ectopic pregnancy with emergency contraception use, even among people who had used it five or more times in a year. There was no upward trend in risk with more frequent use.
Using It More Than Once
The American College of Obstetricians and Gynecologists states that oral emergency contraception can be used more than once, even within the same menstrual cycle. Long-term data on very frequent use over extended periods is limited, but repeated use is not considered harmful. It’s simply less effective and more expensive than regular contraception, which is why it’s not recommended as a primary method.
Each time you take it, you’re likely to experience the same short-term side effects. Frequent use means frequent hormone surges, which can make your periods increasingly unpredictable for a stretch. But “unpredictable” is not the same as “damaged.” Your cycle will regulate once you stop.
Weight Affects How Well It Works
This is one of the most under-discussed aspects of Plan B. Research shows that its effectiveness drops significantly at higher body weights. Studies have found that levonorgestrel-based emergency contraception begins losing efficacy at around 70 kg (about 154 pounds) and appears to have essentially no efficacy at 80 kg (about 176 pounds) and above.
The reason is straightforward: people with a BMI of 30 or higher achieve peak blood levels of the hormone that are roughly 50% lower than those in people with a BMI under 25. The standard dose simply doesn’t deliver enough of the hormone to reliably block ovulation. UK clinical guidelines suggest doubling the dose to 3 mg for people at higher weights, which has been shown to correct the difference in blood levels. If this applies to you, a copper IUD placed within five days of unprotected sex is the most effective emergency option regardless of weight.
Drug Interactions That Reduce Effectiveness
Certain medications speed up how quickly your liver breaks down levonorgestrel, which can cut its effectiveness significantly. One HIV medication (efavirenz) reduces blood levels of the hormone by about 50%. Other categories that cause similar problems include some epilepsy medications, tuberculosis drugs, certain antifungals, and the herbal supplement St. John’s wort.
This effect can linger. Elevated liver enzyme activity persists for up to four weeks after stopping one of these medications, so even if you’re no longer taking them, the interaction may still apply. If you’ve used any of these drugs in the past month, the standard Plan B dose may not be enough.
Safe During Breastfeeding
Levonorgestrel passes into breast milk in minimal quantities. Research supports the safety of taking Plan B while breastfeeding without needing to pump and dump or pause nursing. Some product packaging suggests waiting eight hours before breastfeeding, but this recommendation is not supported by the available evidence. Studies have found no measurable risk to infants, and breastfeeding can continue as normal.