Taking Plan B is not bad for you. It is a safe, FDA-approved emergency contraceptive that does not cause long-term health effects, does not harm future fertility, and does not interfere with an existing pregnancy. The side effects are mild and temporary for most people, and there is no medical limit on how many times you can take it.
That said, “safe” doesn’t mean “no questions worth asking.” Here’s what actually happens in your body when you take Plan B, what side effects to expect, and the specific situations where it may not work as well.
How Plan B Works
Plan B contains a single dose of a synthetic hormone that your body already produces naturally during your menstrual cycle. Its primary job is to delay or block ovulation, the release of an egg from the ovary. If there’s no egg available, sperm has nothing to fertilize.
An FDA review of the full body of clinical evidence confirmed that Plan B works by interrupting the chain of hormonal events leading up to ovulation. It does not prevent a fertilized egg from implanting in the uterus, and it does not end an existing pregnancy. If you’re already pregnant when you take it, it simply won’t do anything.
Common Side Effects
Most side effects are short-lived and resolve within a day or two. In clinical trials, the most frequently reported ones were:
- Nausea: 13.7% of users
- Lower abdominal pain: 13.3%
- Fatigue: 13.3%
- Headache: 10.3%
So roughly 1 in 7 people will feel nauseous, and about 1 in 10 will get a headache. These numbers also mean the majority of users experience no notable side effects at all. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose.
What Happens to Your Period
Plan B can shift the timing of your next period. Some people get it a few days early, others a few days late, and some see no change. Having your period arrive up to a week earlier or later than expected is normal after taking it.
If your period is more than a week late, a pregnancy test is a reasonable next step. The delay doesn’t mean something is wrong with your body. It’s a temporary side effect of the hormone dose shifting your cycle’s timing.
Repeated Use Is Not Dangerous
One of the most common concerns is whether taking Plan B multiple times is harmful. There is no medical limit on how often you can take it. You can use it more than once in the same menstrual cycle if needed, and frequent use of the pill is not associated with any long-term side effects or complications. The World Health Organization confirms that repeated use poses no known health risks, though it can cause more menstrual irregularity than a single use.
That said, Plan B is less effective at preventing pregnancy than regular contraception like an IUD or daily birth control pills. It’s designed as a backup, not because it’s dangerous to use often, but because it’s simply not as reliable as other options for ongoing protection.
It Does Not Affect Future Fertility
Plan B does not harm your ability to get pregnant later. There is no delay in the return to fertility after taking it. Your ovulation cycle returns to normal once the hormone clears your system, which happens quickly. This holds true whether you’ve taken it once or multiple times.
When Plan B Is Less Effective
Plan B works best the sooner you take it after unprotected sex. It can be taken up to 72 hours (three days) afterward, but its effectiveness drops with each passing day. Taking it within the first 24 hours gives you the best chance of preventing pregnancy.
Body Weight Matters
Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason: blood levels of the active hormone were about 50% lower in people with higher BMIs after a standard dose, meaning the drug likely never reaches the concentration needed to block ovulation. Doubling the dose did not solve the problem in the study’s trial. If your weight is above 176 pounds, a copper IUD placed within five days of unprotected sex is a more reliable emergency contraceptive option.
Certain Medications Reduce Effectiveness
Some medications speed up how fast your liver breaks down Plan B’s active ingredient, leaving too little in your bloodstream to work. These include certain drugs used to treat epilepsy, tuberculosis, HIV, and fungal infections, as well as the herbal supplement St. John’s wort. One HIV medication (efavirenz) cuts blood levels of the hormone by roughly 50%. The effect on your liver can persist for up to four weeks after you stop taking the interfering medication, so even recently discontinued drugs can be a factor.
What Plan B Cannot Do
Plan B does not protect against sexually transmitted infections. It does not work as an abortion pill. It contains a completely different compound than medication used to end a pregnancy, and it has no effect once a fertilized egg has implanted. It also won’t provide ongoing contraceptive protection. If you have unprotected sex again after taking it, even the next day, you’re at risk of pregnancy from that new exposure.