Plan B One-Step is an emergency contraceptive pill containing 1.5 mg of levonorgestrel, a synthetic form of progesterone. It’s a single tablet taken by mouth after unprotected sex or birth control failure to reduce the chance of pregnancy. It’s available over the counter in the United States with no age restriction and no prescription required.
How Plan B Prevents Pregnancy
Plan B works primarily by delaying or preventing ovulation, the release of an egg from the ovary. If your body hasn’t released an egg yet, sperm have nothing to fertilize, and pregnancy can’t happen. The pill delivers a large dose of the same hormone found in many daily birth control pills, just at a much higher concentration in a single dose.
Timing within your cycle matters enormously. If you take Plan B several days before ovulation is expected, it can effectively suppress or delay the hormonal surge that triggers egg release. But if you take it very close to ovulation (within a day or two), studies show it’s far less reliable at stopping the process. In one study, when levonorgestrel was given one to two days before ovulation, over 90% of women still showed signs of egg release. This is why Plan B is not a guarantee, and why taking it sooner rather than later improves your odds.
Even when ovulation does occur after taking the pill, researchers have observed what’s called “ovulatory dysfunction,” meaning the hormonal patterns around egg release are disrupted in ways that may make fertilization less likely. The egg may be released, but the conditions surrounding it are altered enough to reduce the chance of pregnancy.
How Effective It Is
Plan B reduces the risk of pregnancy by roughly 87% to 90% when taken within 72 hours (three days) of unprotected sex. That doesn’t mean 87% of women who take it avoid pregnancy. It means that out of the women who would have become pregnant without any intervention, 87% to 90% of those pregnancies are prevented. The actual pregnancy rate in clinical studies was about 0.8% for women who took it within 72 hours.
The pill still has some effect between 72 and 120 hours (three to five days), with effectiveness dropping to roughly 72% to 87% and a pregnancy rate of about 1.8%. Every hour you wait reduces how well it works. Taking it within the first 24 hours gives you the best chance.
Weight and Effectiveness
Body weight can affect how well Plan B works. Health Canada has stated that levonorgestrel may be less effective in women over 165 pounds and potentially ineffective in women over 176 pounds. That said, many experts consider this an oversimplification. The pill likely still offers some protection at higher weights, just less of it.
For women with a BMI over 30, an alternative emergency contraceptive called ella (ulipristal acetate) appears to be more effective, with an unintended pregnancy risk of about 2.5% compared to roughly 6% for levonorgestrel. A copper IUD, inserted by a healthcare provider within five days, is the most effective emergency contraception option regardless of weight.
Common Side Effects
Plan B delivers a concentrated hormone dose, so temporary side effects are common. In clinical studies, about 23% of women experienced nausea, 18% had abdominal pain, 17% reported fatigue or headache, and 6% experienced vomiting. These symptoms typically resolve within a day or two.
Your next period may arrive earlier or later than expected, and the flow may be heavier or lighter than usual. Spotting between periods is also common. If your period is more than a week late, it’s worth taking a pregnancy test. These menstrual changes are temporary and your cycle should return to its normal pattern within one to two months.
Can You Use It More Than Once?
Taking Plan B more than once won’t cause serious health problems, but it’s not designed to be a regular birth control method. The World Health Organization, along with major medical organizations, advises against repeated use as routine contraception for two reasons: it causes more side effects than standard hormonal birth control, and it’s less effective at preventing pregnancy over time compared to methods like the pill, IUD, or implant. If you find yourself needing emergency contraception frequently, that’s a good signal to explore an ongoing method that works better with less disruption to your body.
Safety for Breastfeeding
Plan B is considered safe while breastfeeding. Progestin-only methods like levonorgestrel are the preferred hormonal option for nursing mothers. Studies show it does not affect milk composition, milk supply, or infant growth and development. Only a tiny fraction of the dose, about 0.1%, reaches the infant through breast milk. Medical experts consider it acceptable at any point postpartum, and there appear to be no long-term effects on breastfeeding or the baby.
What Plan B Does Not Do
Plan B is not an abortion pill. It cannot end an existing pregnancy. If a fertilized egg has already implanted in the uterus, Plan B will have no effect. It also does not protect against sexually transmitted infections.
It’s a backup option, not a first-line method. It works best when taken as quickly as possible after unprotected sex, ideally within the first 24 hours, and becomes progressively less reliable as time passes. If more than five days have gone by, Plan B is unlikely to help, and a copper IUD is the only remaining emergency contraception option.