Plan B starts working almost immediately after you swallow it. The active hormone reaches its peak concentration in your bloodstream within about 1.5 to 2 hours, and its primary job, delaying ovulation, begins as soon as those levels rise. But “working” and “guaranteeing protection” aren’t the same thing, and the timing of when you take it relative to your cycle matters far more than most people realize.
What Happens in Your Body After Taking It
Plan B delivers a large dose of a synthetic hormone similar to progesterone. After you take the pill, your body absorbs it rapidly, with blood levels peaking at roughly 1.6 hours on average. Once absorbed, the hormone sends a signal to your brain to pause the process that releases an egg from the ovary. If no egg is released, sperm have nothing to fertilize, and pregnancy can’t occur.
This is the key detail most people miss: Plan B works by delaying ovulation, not by doing anything after ovulation has already happened. In women who took it before their hormonal surge triggered egg release, ovulation was delayed by an average of nearly 17 days. But when taken on the day of that surge or after, it performed no better than a placebo at preventing the egg from being released. So the pill is actively working in your body within a couple of hours, but whether that matters depends entirely on where you are in your menstrual cycle.
Why Taking It Sooner Matters
Plan B can be taken up to 5 days (120 hours) after unprotected sex, but its effectiveness drops noticeably with each day you wait. When taken within the first 3 days, it works comparably to other emergency contraception options. After 3 days, pregnancy rates climb. The CDC notes that while Plan B still reduces risk through day 5, the rates of pregnancy are “slightly higher” when taken after the 72-hour mark.
The reason is straightforward. The longer you wait, the closer you may be to ovulation, and once that process is underway, Plan B can’t reliably stop it. Taking it within the first 24 hours gives it the best chance of getting ahead of your cycle.
Body Weight Affects How Well It Works
If you weigh more than about 176 pounds or have a BMI of 30 or higher, Plan B may be significantly less effective. Research from Oregon Health and Science University found that people with a BMI of 30 experienced emergency contraception failure four times as often as those with a BMI under 25. The reason: blood levels of the hormone were roughly 50% lower in higher-BMI individuals after a standard dose, meaning it never reached the concentration needed to reliably block ovulation.
Doubling the dose doesn’t solve the problem either. The same research group tested a double dose in people with BMIs above 30 and found it was not effective at preventing pregnancy. If your weight falls in this range, a copper IUD placed within 5 days is the most reliable emergency option, or a different emergency contraception pill that uses a different hormone and maintains effectiveness at higher body weights.
Medications That Can Reduce Its Effectiveness
Certain medications speed up how quickly your liver breaks down Plan B’s hormone, which can lower blood levels enough to make it ineffective. These include some medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampicin, several HIV medications including efavirenz (which cuts hormone levels by about 50%), and the herbal supplement St. John’s wort.
If you’ve taken any of these in the past 4 weeks, a standard Plan B dose may not provide adequate protection. UK drug safety guidelines recommend doubling the dose in this situation, though a copper IUD remains the most effective alternative.
Side Effects and What to Expect Afterward
Most side effects show up within hours of taking the pill and resolve within a few days. Nausea is the most common, along with fatigue, headache, and lower abdominal discomfort. Your next period may come earlier or later than expected, and the bleeding itself might be heavier or lighter than usual. These changes are a normal response to the large hormone dose and don’t indicate whether the pill worked or failed.
Because Plan B can shift your cycle, you can’t rely on your period’s timing alone to know if it worked. The reliable way to confirm is a pregnancy test taken 3 weeks after you took the pill. That gives enough time for pregnancy hormone levels to be detectable if conception occurred.
Plan B vs. Other Emergency Options
Another emergency contraception pill uses a different active compound that has one major advantage: it can still prevent egg release even after the hormonal surge has begun. In studies comparing the two, this alternative blocked ovulation 79% of the time when taken at the surge, while Plan B succeeded only 14% of the time, essentially the same as a placebo. Both pills are similarly effective when taken within the first 3 days, but the alternative maintains better efficacy through day 5 and in people with higher BMIs. It does require a prescription in many areas, while Plan B is available over the counter.
A copper IUD inserted within 5 days of unprotected sex is the most effective emergency contraception available, regardless of body weight or cycle timing. It works by creating an environment that prevents fertilization and implantation, so it doesn’t depend on blocking ovulation at all.