Plan B works by delaying or preventing ovulation, the moment when an ovary releases an egg. If there’s no egg available to meet sperm, pregnancy can’t happen. It contains a single dose of a synthetic hormone similar to progesterone, and its effectiveness depends heavily on timing: the sooner you take it after unprotected sex, the better it works.
The Core Mechanism: Blocking Ovulation
Each menstrual cycle, your body builds toward a hormonal spike called the LH surge, which triggers an ovary to release an egg. Plan B interrupts that process. The synthetic hormone in the pill either blocks the surge entirely or delays it long enough that any sperm already in the reproductive tract die before an egg appears. Sperm typically survive about five days inside the body, so even a short delay in ovulation can close the window for fertilization.
This is why timing matters so much. If you take Plan B before the LH surge has started, it’s highly effective. If ovulation has already happened, the pill has little to offer because its primary job is already done by your body.
It Does Not End a Pregnancy
One of the most persistent misunderstandings about Plan B is that it works by preventing a fertilized egg from implanting in the uterus. Research from the Karolinska Institutet tested this directly using three-dimensional endometrial cell cultures (lab-grown models of the uterine lining) and found that Plan B’s active ingredient does not impair embryo attachment. It also showed no changes to the uterine lining markers involved in implantation.
A review published in Human Reproduction Update reached the same conclusion: at the dose used in emergency contraception, the drug acts by interfering with ovulation rather than blocking fertilization or implantation. The World Health Organization states plainly that emergency contraceptive pills do not induce an abortion. If you’re already pregnant when you take Plan B, the pill will not affect the pregnancy.
How Effective It Is, and When
Taken within 72 hours (three days) of unprotected sex, Plan B prevents an estimated 60% to 94% of expected pregnancies. That range is wide because effectiveness drops with every passing hour. Between 0.6% and 2.6% of women who take it within that window will still become pregnant.
The highest effectiveness comes from taking it as early as possible, ideally within the first 12 to 24 hours. By the time you reach the 72-hour mark, you’re at the lower end of that range. Some evidence suggests a small benefit even between 72 and 120 hours, but effectiveness at that point is substantially reduced.
Body Weight Affects How Well It Works
Plan B becomes significantly less effective at higher body weights. A reanalysis of clinical trial data found that the pill appears to hit a ceiling of effectiveness at around 70 kg (about 154 pounds), and may offer little to no emergency contraceptive benefit for people weighing 80 kg (176 pounds) or more.
When broken down by BMI, the numbers are striking. People with a BMI over 30 had more than four times the risk of pregnancy compared to those in the normal BMI range. People with a BMI between 25 and 29.9 were about twice as likely to experience failure. This happens because higher body weight changes how the drug is absorbed and distributed, resulting in lower blood levels of the active hormone.
A clinical trial tested whether doubling the dose could overcome this problem in people with obesity, but found it did not meaningfully improve outcomes. If your weight falls in these ranges, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception option, regardless of body weight. Another oral option, which contains a different active ingredient (sold as ella), maintains better efficacy at higher weights than Plan B does, though it requires a prescription.
Common Side Effects
Plan B’s side effects are temporary and generally mild. In clinical trials, the most frequently reported effects were:
- Heavier menstrual bleeding: 30.9% of users
- Nausea: 13.7%
- Lower abdominal pain: 13.3%
- Fatigue: 13.3%
- Headache: 10.3%
- Delayed period (more than 7 days late): 4.5%
Your next period may come earlier or later than expected, and the flow may be heavier or lighter than normal. These changes typically resolve within one to two cycles. If your period is more than a week late, a pregnancy test is a reasonable next step.
What to Do if You Vomit After Taking It
If you vomit within three hours of swallowing the pill, your body may not have absorbed enough of the hormone. The CDC recommends taking another dose as soon as possible. Taking an anti-nausea medication beforehand can help if you’re prone to stomach upset or feeling nauseated.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down Plan B’s active ingredient, lowering the amount that reaches your bloodstream. These include medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampicin, some HIV medications (including efavirenz, which cuts Plan B blood levels by roughly 50%), and certain antifungal drugs.
St. John’s wort, a common herbal supplement used for mood, also reduces the hormone’s levels in the blood. If you take any of these regularly, Plan B may not work well enough on its own, and a copper IUD is a more reliable emergency option.
How It Differs From the Abortion Pill
Plan B and medication abortion involve completely different drugs that do completely different things. Plan B prevents a pregnancy from starting by delaying ovulation. The medications used for abortion (mifepristone at a much higher dose, combined with misoprostol) end an established pregnancy by blocking progesterone’s effects on the uterus and causing it to contract.
At the low dose used in emergency contraception, mifepristone actually works similarly to Plan B, primarily by disrupting ovulation. It’s only at the much higher doses used in abortion protocols that it affects the uterine lining and prevents implantation. Plan B contains neither of these abortion medications. It is a single, high dose of a synthetic progesterone-like hormone, and it cannot terminate a pregnancy that has already begun.