Plan B prevents pregnancy by delaying ovulation, the release of an egg from the ovary. It contains a synthetic hormone called levonorgestrel, and it works best when taken as soon as possible after unprotected sex, with effectiveness ranging from 81% to 90% depending on how quickly you take it. It is not an abortion pill, and it will not end an existing pregnancy.
How Plan B Prevents Pregnancy
Your body releases an egg roughly once per menstrual cycle, triggered by a hormonal surge. Plan B works by blocking or delaying that surge so the egg never leaves the ovary. Without an egg available, sperm has nothing to fertilize.
The timing within your cycle matters. If you haven’t ovulated yet, Plan B can suppress the hormonal signal entirely. If your body is closer to ovulation but hasn’t quite reached it, the pill can delay the physical release of the egg long enough for sperm (which survive about five days) to die off before fertilization can happen.
If you’ve already ovulated before taking Plan B, the pill is essentially ineffective. FDA data confirms that when levonorgestrel is taken after ovulation, the pregnancy rate is the same as if nothing had been taken at all. The totality of evidence also shows that Plan B does not alter the uterine lining in any clinically meaningful way that would prevent a fertilized egg from implanting. Its mechanism is prevention of ovulation, not interference with an established pregnancy.
Plan B Is Not an Abortion Pill
This is one of the most common points of confusion. Emergency contraception prevents a pregnancy from starting. Abortion medication ends a pregnancy that has already begun, defined medically as a fertilized egg that has implanted in the uterus. These are entirely different drugs with different mechanisms. Abortion medications work by blocking progesterone (causing the embryo to detach from the uterine wall) and triggering uterine contractions. Plan B does neither of those things. It simply stops ovulation.
How Timing Affects Effectiveness
Plan B is approved for use up to 72 hours (three days) after unprotected sex, and it can still be taken up to 120 hours (five days) afterward, though effectiveness drops significantly in that later window. The sooner you take it, the better it works. At its most effective, taken within the first 24 hours, it prevents roughly 85% to 90% of expected pregnancies. By the 48- to 72-hour mark, that number drops closer to the lower end of the range.
If more than five days have passed, Plan B is not a reliable option. A copper IUD, which can be inserted by a healthcare provider up to five days after unprotected sex, is the most effective form of emergency contraception at that point.
Body Weight Can Reduce Effectiveness
Research shows that Plan B becomes significantly less effective at higher body weights. Clinical data indicates that people with a BMI of 30 or above had more than four times the risk of pregnancy compared to those with a BMI under 25. The decline in effectiveness tracks with absolute weight: Plan B appears to hit a ceiling of efficacy around 155 pounds (70 kg), and studies suggest essentially no efficacy for people weighing 176 pounds (80 kg) or more.
The reason is straightforward. At higher body weights, the standard 1.5 mg dose produces peak blood concentrations about 50% lower than in lighter individuals, meaning the hormone may not reach the level needed to suppress ovulation. If you weigh more than 155 pounds, a different emergency contraceptive called ella (which uses a different active compound and works up to five days) or a copper IUD may be more reliable options.
Common Side Effects
Plan B delivers a concentrated dose of synthetic hormone, so short-term side effects are common and typically resolve within a few days. In clinical trials, the most frequently reported effects were:
- Heavier menstrual bleeding: 30.9%
- Nausea: 13.7%
- Lower abdominal pain: 13.3%
- Fatigue: 13.3%
- Headache: 10.3%
- Dizziness: 9.6%
- Breast tenderness: 8.2%
- Period delayed more than 7 days: 4.5%
If you vomit within two hours of taking the pill, the dose may not have been fully absorbed. In that case, you likely need to take another dose. A pharmacist can help you determine whether a repeat dose is necessary.
What to Expect With Your Next Period
Plan B commonly shifts the timing of your next period. It can arrive a few days early or be delayed by up to a week. The flow itself may be heavier or lighter than usual, and some people experience light spotting between periods. These changes are temporary and typically resolve by the following cycle. If your period is more than a week late, a pregnancy test is a reasonable next step.
Who Can Buy It and Where
Plan B One-Step has been available over the counter without a prescription and without age restrictions since 2013. You can find it at most pharmacies, often on the shelf rather than behind the counter. Multiple generic versions exist under names like My Way, Take Action, and AfterPill, all approved for the same nonprescription, no-age-limit access. Generics contain the same 1.5 mg dose and work identically.
Medications That Can Interfere
Certain medications speed up how your liver processes levonorgestrel, potentially clearing it from your system before it can work. Drugs used to treat seizures (such as carbamazepine), some HIV medications, and the herbal supplement St. John’s wort are among the most notable. If you take any of these regularly and need emergency contraception, a copper IUD or ella may be more appropriate since their effectiveness is less affected by these interactions.