Plan B should be taken within 72 hours (three days) of unprotected sex, but sooner is significantly better. Taken within the first 24 hours, it prevents about 94% of pregnancies. By the 72-hour mark, that drops to roughly 58%. It’s a single pill, taken once, not an ongoing medication.
Why Every Hour Matters
Plan B works by delivering a high dose of a hormone that delays or prevents your ovary from releasing an egg. If ovulation hasn’t happened yet, the pill can pause the process long enough for sperm (which survive up to five days) to die off before an egg becomes available. It does not end a pregnancy that has already started.
This is why the clock matters so much. The longer you wait, the more likely your body has already ovulated, and once that egg is released, Plan B has little effect. There’s no magic cutoff where it suddenly stops working at exactly 72 hours, but the decline in effectiveness is steep enough that taking it the same day makes a real difference compared to waiting until day three.
If You’re Past 72 Hours
A prescription alternative called ella (ulipristal acetate) works for up to 120 hours, or five full days, after unprotected sex. The American College of Obstetricians and Gynecologists notes that ella is more effective than Plan B at every time point within that window, and across all body weights. If you’re approaching or past the 72-hour mark, contacting a provider for an ella prescription is worth the effort. A copper IUD inserted within five days is another highly effective option.
Body Weight and Effectiveness
Plan B becomes less effective if you weigh more than 165 pounds. The pill still contains the same fixed dose regardless of body size, so at higher weights, blood levels of the hormone may not reach the threshold needed to reliably delay ovulation. If this applies to you, ella or a copper IUD are stronger choices, since ella maintains its effectiveness across a wider weight range.
Taking Plan B More Than Once
You can take Plan B more than once in the same menstrual cycle if you have another episode of unprotected sex. The American College of Obstetricians and Gynecologists endorses repeated use within the same cycle when needed, and studies have not found increased health risks from doing so. Your period may become irregular, but those changes are typically mild. Each dose only covers the sexual encounter that prompted it, since the pill works by delaying ovulation for a limited time. If you have sex again days later, you’d need another dose.
That said, if you’re relying on Plan B frequently, a regular birth control method will be far more effective and less expensive over time.
What to Expect Afterward
Plan B is a single pill. You take it, and that’s it. Common side effects include nausea, fatigue, headache, and breast tenderness, which typically resolve within a day or two. Your next period may come a few days early or late. If it’s more than a week late, take a pregnancy test.
Some spotting or light bleeding in the days after taking the pill is normal and isn’t your period. It’s a hormonal response to the large dose of progestin.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down Plan B’s active ingredient, which can cut its effectiveness significantly. One HIV medication (efavirenz) reduces blood levels of the hormone by about 50%. Other medications that interfere include some treatments for epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampin, and the herbal supplement St. John’s wort. This interference can persist for up to four weeks after you stop taking the interacting medication. If you’re on any of these, a copper IUD or ella (after discussing with a provider) may be better options for emergency contraception.
Breastfeeding and Plan B
Only small amounts of the hormone pass into breast milk, and it isn’t expected to harm infants. Pumping and dumping is unnecessary. If you want extra reassurance, you can nurse or pump right before taking the pill, then resume your normal feeding schedule.