The condom broke, you missed a pill, or the moment got ahead of the plan, and now you are staring at a box of Plan B wondering what it actually does inside your body. The good news: it is a simple, well-studied medication, and understanding how it works can take a lot of the panic out of the next 72 hours.
Plan B is emergency contraception, not an abortion pill, and it is most effective the sooner you take it. Here is exactly how it works, how well it works, and when you should reach for a different option.
How does Plan B work?
Plan B works mainly by stopping or delaying the release of an egg from your ovary. To ovulate, your body needs a hormonal trigger called the luteinizing hormone (LH) surge. The levonorgestrel in Plan B blocks or delays that LH surge when it is taken before the surge happens, so no egg is released and there is nothing for sperm to fertilize.
It may also have backup effects, such as thickening cervical mucus or altering the lining of the uterus, which can make fertilization or attachment less likely. Importantly, there is no single mechanism that applies in every case. Which effect does the work depends on where you are in your menstrual cycle when you have sex and when you take the pill.
What Plan B does not do is end an existing pregnancy. Once implantation has begun, levonorgestrel is not effective and does not disrupt an established pregnancy. This is why it is not the same as the abortion pill (RU-486), and why it will not work if you are already pregnant.
How fast do you need to take it?
This is the part that matters most: Plan B works best the sooner you take it. It is labeled for use within 72 hours (3 days) of unprotected sex, but efficacy declines steadily across that window, so do not wait.
Here is the general timeline to keep in mind:
- Plan B is a single 1.5 mg levonorgestrel tablet you take as soon as possible after unprotected sex.
- It is roughly 94% effective when taken within the first 24 hours.
- Effectiveness declines toward the 72-hour mark, so earlier is always better.
- Overall, levonorgestrel morning-after pills are about 89% effective at preventing pregnancy when taken within the 72-hour window.
How effective is Plan B, really?
Across the labeled window, levonorgestrel pills like Plan B are about 89% effective at preventing pregnancy. Taken within the first 24 hours, that figure rises to roughly 94%. No emergency contraception is 100% effective, which is why a missed or unusually late next period is worth a pregnancy test.
One factor that surprises many people is body weight. Levonorgestrel becomes less effective at higher body weight or BMI. In a pooled analysis of clinical trials, the estimated pregnancy rate rose from 1.4% in women weighing 65 to 75 kg to 6.4% in the 75 to 85 kg group, with the risk climbing steeply starting around 70 to 80 kg.
If weight may reduce how well Plan B works for you, there are stronger options. ACOG notes that the copper IUD and ulipristal acetate (a prescription morning-after pill) retain their effectiveness better at higher weights, and the copper IUD is the most effective method of emergency contraception overall.
Plan B vs. the abortion pill: what's the difference?
This is one of the most common and most important points of confusion. Plan B prevents a pregnancy from starting. It works before or around the time of ovulation and fertilization. It has no effect on a pregnancy that is already established.
The abortion pill (RU-486, or mifepristone) is a completely different medication used to end an existing, confirmed pregnancy. Plan B cannot do this, and it will not harm or interrupt a pregnancy that has already implanted.
Put simply: Plan B is a backup form of birth control, not a method of abortion. It is available over the counter with no age restriction, which reflects how safe and straightforward it is to use.
What to expect after taking it
Plan B is generally well tolerated, but short-lived side effects are common and not a sign that something is wrong. You may notice nausea, fatigue, or a headache in the day or two afterward.
It is also normal for your next period to come a little earlier or later than usual, or to be heavier or lighter than you expect. Spotting before your period can happen too. These shifts settle on their own.
Because Plan B can change your cycle, the most reliable sign that it worked is a period that arrives within a week or so of when you expected it. If your period is more than a week late, take a pregnancy test.
When to see a doctor or clinician
For most people, Plan B is a self-managed over-the-counter option. But there are clear moments to get professional care rather than going it alone.
Reach out to a clinician if your period is more than about a week late, if you have severe lower abdominal pain in the weeks after (which can signal an ectopic pregnancy and needs prompt evaluation), or if you vomit within a couple of hours of taking the pill and are unsure whether it absorbed.
It is also worth a conversation if weight may reduce Plan B's effectiveness for you, if you need emergency contraception often, or if you want a reliable ongoing birth control plan. A clinician can point you toward the copper IUD, ulipristal acetate, or a regular contraceptive method that fits your body and your life. If you would rather start that conversation from home, Nolla can connect you with a clinician for personalized guidance.
Medical Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider before starting any new skincare treatment, especially if you have underlying health conditions, are pregnant, or are taking medications.
