How Effective Is Plan B?

Plan B (levonorgestrel) is an emergency contraceptive pill that can help prevent pregnancy after unprotected sex. When taken correctly, it's effective for most people—but timing matters a lot.

How Effective Is Plan B?

Plan B is about 89% effective when taken within 72 hours (3 days) after unprotected sex. However, the sooner you take it, the better it works. When taken within 24 hours, Plan B is up to 95% effective.

Effectiveness by timing: within 24 hours (95%), 24-48 hours (85%), 48-72 hours (61%). After 72 hours, Plan B is much less effective and generally not recommended.

Does Plan B Work During Ovulation?

Plan B works by delaying or stopping ovulation—the release of an egg from the ovary. If you have already ovulated, Plan B likely will not work. Research shows that women who took Plan B on the day of ovulation or after had a much higher pregnancy rate than those who took it before ovulation.

Since it's hard to know exactly when you're ovulating, experts still recommend taking Plan B as soon as possible—even if you're unsure of your timing.

Does Body Weight Affect Plan B?

Some research suggests Plan B may be less effective for people with higher body weight. Studies show pregnancy risk may increase for women weighing over 165 pounds. However, the FDA says current evidence is not strong enough to set an official weight limit.

If you have concerns about weight and effectiveness, talk to a healthcare provider about alternatives like ella (ulipristal acetate) or the copper IUD, which remain effective regardless of body weight.

What Are More Effective Alternatives?

The copper IUD (Paragard) is the most effective form of emergency contraception—over 99% effective when inserted within 5 days of unprotected sex. Ella (ulipristal acetate) is a prescription pill that works for up to 5 days and is more effective than Plan B, especially around ovulation.

How to Take Plan B

Plan B is a single 1.5 mg pill taken by mouth. It's available without a prescription at pharmacies. Take it as soon as possible after unprotected sex—don't wait. Common side effects include nausea, headache, and irregular bleeding.

Plan B does not end an existing pregnancy and is not an abortion pill. If your period is more than a week late after taking Plan B, take a pregnancy test.