Plan B works best within the first 24 hours after unprotected sex, when it’s about 94% effective. It can still be taken up to 72 hours (3 days) afterward, but its effectiveness drops significantly with each passing day, falling to around 58% by the 72-hour mark. The sooner you take it, the better it works.
How Effectiveness Changes Hour by Hour
Plan B contains a hormone called levonorgestrel that works by delaying or preventing the release of an egg from the ovaries. If the egg has already been released, the pill is far less likely to prevent pregnancy. That’s why timing matters so much: every hour you wait is an hour closer to ovulation potentially happening on its own.
Here’s the general breakdown:
- Within 24 hours: approximately 94% effective
- Within 72 hours (3 days): approximately 58% effective
There’s no sharp cutoff at exactly 72 hours where the pill suddenly stops doing anything, but effectiveness drops enough after that point that other options become worth considering. The CDC’s 2024 guidelines note that levonorgestrel emergency contraception pills “work less well after three days.”
What If It’s Been More Than 72 Hours
If you’re past the 3-day window, a different emergency contraceptive called ella (ulipristal acetate) is the better option. Ella can be taken up to 120 hours (5 days) after unprotected sex and maintains its effectiveness throughout that entire window, unlike Plan B, which declines steadily. Ella requires a prescription in most states.
A copper IUD is another option that works up to 5 days after unprotected sex and is the single most effective form of emergency contraception available. It also doubles as long-term birth control once placed. Both ella and the copper IUD stay equally effective across the full 5-day window, which gives you more breathing room than Plan B does.
Body Weight Can Affect How Well It Works
Plan B becomes less reliable at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced morning-after pill failure four times as often as those with a BMI under 25. The reason is straightforward: levonorgestrel blood levels were about 50% lower in people with a higher BMI after taking a standard dose, meaning the drug never reaches the concentration needed to reliably block ovulation.
Doubling the dose doesn’t solve the problem. The same research team found that taking two Plan B pills was not effective at improving pregnancy prevention for people with higher BMIs. If you weigh more than 165 pounds, Planned Parenthood recommends ella as the more effective morning-after pill option.
What to Expect After Taking It
Nausea is one of the more common side effects. If you vomit within 2 hours of taking the pill, your body may not have absorbed enough of the medication, and you may need to take another dose. A pharmacist can help you figure out whether a second pill is necessary.
Your next period may come earlier or later than expected. Some people experience spotting, breast tenderness, headaches, or fatigue in the days after taking Plan B. These effects are temporary and typically resolve within a few days.
If your period is more than a week late after taking Plan B, it’s worth taking a pregnancy test. The pill isn’t 100% effective at any point in the window, and a late period could mean it didn’t work.
Starting Regular Contraception Afterward
If you took Plan B and want to start or restart a regular birth control method, you can do so immediately. You don’t need to wait. However, you’ll need to use condoms or avoid sex for the next 7 days after starting your regular method, since hormonal contraception takes about a week to become fully effective.
If you took ella instead, the timeline is different. You should wait at least 5 days before starting or resuming hormonal birth control, because hormonal contraception can interfere with how ella works. During that waiting period and for 7 days after restarting your method, use condoms as a backup.