Plan B (levonorgestrel) works by delaying or preventing ovulation, and its most common effects on your body are nausea, changes to your next period, and fatigue. These side effects are temporary, typically lasting a few days at most. The pill does not affect future fertility or cause long-term hormonal changes.
How Plan B Works in Your Body
Plan B contains a synthetic version of progesterone, a hormone your body already produces. When you take it, the hormone surge stops or delays your ovary from releasing an egg. No egg means no fertilization. The FDA has concluded that Plan B works specifically by inhibiting ovulation and disrupting the midcycle hormonal changes that trigger egg release.
This is why timing matters so much. Plan B is most effective when taken before ovulation has occurred. If ovulation has already happened, the pill has little to work with. It does not end an existing pregnancy.
Common Side Effects
Most people who take Plan B experience at least one noticeable side effect, though they tend to resolve within a day or two. The most frequently reported ones include:
- Nausea: the most common complaint, sometimes accompanied by vomiting
- Fatigue and headache
- Breast tenderness
- Lower abdominal cramping
- Dizziness
If you vomit within two hours of taking the pill, it may not have been fully absorbed. In that case, you likely need another dose. A pharmacist can confirm whether a second pill is necessary.
Changes to Your Next Period
Plan B frequently shifts the timing and character of your next period. Some people get it a few days early, others a few days late, and some see no change at all. Having your period arrive up to a week earlier or later than expected is normal and not a cause for concern.
Research suggests that the earlier in your cycle you take Plan B, the more likely your next period will come early. Taking it later in your cycle tends to push your period back. You may also notice that bleeding is lighter or heavier than usual, and the period itself can last longer than a typical cycle. These changes are more pronounced if you’ve taken Plan B more than once in the same cycle.
If your period is more than a week late, take a pregnancy test. The most reliable result comes from testing at least 21 days after the unprotected sex that prompted you to take the pill.
Effectiveness by Timing
Plan B is not equally effective across its entire window. Taken within 24 hours of unprotected sex, it’s about 94% effective at preventing pregnancy. By the 72-hour mark, that drops to roughly 58%. Every hour you wait reduces your odds, so taking it as soon as possible makes a real difference.
Body weight also plays a role. Research indicates that Plan B becomes less effective in people with a BMI above 26 or a body weight over about 155 pounds (70 kg). If that applies to you, a different type of emergency contraception (one that uses a compound called ulipristal acetate, sold as ella) may be more effective. A copper IUD, placed by a provider within five days, is the most effective emergency contraception option regardless of weight.
Medications That Reduce Effectiveness
Certain medications speed up how quickly your liver breaks down levonorgestrel, which can cut the amount of active drug in your bloodstream significantly. The HIV medication efavirenz, for instance, reduces levonorgestrel levels by about 50%. Other medications that interfere include some drugs used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis medications like rifampicin, and the antifungal griseofulvin.
St. John’s wort, a widely available herbal supplement, has the same effect. Notably, this interaction can persist for up to four weeks after you stop taking any of these medications. If you’re on any of them, a different form of emergency contraception is a better option.
Effects on Future Fertility
Plan B does not affect your ability to get pregnant in the future. A systematic review of 22 studies looking at repeated use of emergency contraception found no impact on fertility. The most common consequence of frequent use was irregular periods, not lasting hormonal changes. Your cycle returns to its normal pattern after the hormonal effects of the pill wear off, usually within one to two cycles.
There is also no increased risk of ectopic pregnancy (a pregnancy that implants outside the uterus). When researchers compared ectopic pregnancy rates in the general population to rates after Plan B failure, the numbers were actually lower in the Plan B group: about 1% versus the general rate of 2%. Plan B reduces the absolute risk of ectopic pregnancy simply by reducing the chance of any pregnancy occurring.
Repeated Use
Taking Plan B more than once is safe, but it’s not ideal as a regular strategy. Each dose delivers a large burst of synthetic progesterone, which is why side effects like nausea and period disruption tend to be more intense with repeated use in the same cycle. The pill is also less effective than most ongoing contraceptive methods. Regular birth control options prevent pregnancy at rates above 99% with proper use, compared to Plan B’s best-case 94%. Plan B is designed as a backup, and it works well in that role, but relying on it routinely means accepting both more side effects and a higher chance of unintended pregnancy.