You should take the morning after pill as soon as possible after unprotected sex. Every hour matters with the most common type (levonorgestrel, sold as Plan B and generics), while the prescription option (ulipristal acetate, sold as ella) maintains steady effectiveness for up to five days. But “when” isn’t just about the clock. Your weight, where you are in your cycle, and which pill you choose all affect whether it will work.
How Timing Affects Each Type
There are two types of morning after pill, and they behave very differently as time passes.
Levonorgestrel (Plan B, Take Action, and other generics) works best within the first 24 hours and becomes progressively less effective after that. Its overall effectiveness drops significantly by 72 hours, and it’s not recommended beyond that point. This is the one available over the counter at pharmacies without a prescription or age restriction.
Ulipristal acetate (ella) can be taken up to 120 hours (five full days) after unprotected sex. In clinical trials reviewed by the FDA, the pregnancy rate stayed remarkably consistent across the entire window: roughly 1.6% in the first 24 hours, 2.1% at 25 to 48 hours, and similar rates all the way through day five. In one trial, no pregnancies occurred in the 73- to 120-hour window at all. This consistency is ella’s biggest advantage, but it requires a prescription.
So if you’re within the first 12 to 24 hours, either pill is a reasonable option. If you’re past 48 hours, ella is the significantly better choice.
Why Your Cycle Stage Matters More Than You Think
The morning after pill works primarily by delaying or preventing ovulation, the release of an egg from the ovary. It does this by suppressing the hormonal surge that triggers that release. This means the pill is most effective when taken before ovulation happens.
A study tracking women’s actual ovulation timing found striking results. Among 103 women who took levonorgestrel before ovulation, zero pregnancies occurred out of 16 expected. Among 45 women who took it on the day of ovulation or after, 8 pregnancies occurred out of 8.7 expected. In other words, the pill provided essentially no protection once the egg had already been released. The same study confirmed that levonorgestrel does not prevent a fertilized egg from implanting.
The practical problem is that most people don’t know exactly when they ovulate. If you’re in the middle of your cycle (roughly days 10 to 16 of a 28-day cycle), the egg may already be on its way. Ella has an advantage here because it can still delay ovulation even when the hormonal surge has already started, while levonorgestrel cannot. This is one reason ella tends to outperform Plan B in real-world use, especially when taken later in the window.
Weight Can Reduce Effectiveness
Body weight is one of the most underappreciated factors in emergency contraception. Research shows levonorgestrel starts losing effectiveness at around 70 kg (about 154 pounds) and may have essentially no efficacy at 80 kg (176 pounds) or above. Women with a BMI of 30 or higher who took levonorgestrel had more than four times the risk of pregnancy compared to women with a BMI under 25.
Ella performs better at higher weights but isn’t immune to the effect. The pregnancy rate for women with a BMI over 30 taking ella was around 2.5%, compared to nearly 6% for those using levonorgestrel. That’s a meaningful difference if you weigh more than about 165 pounds.
For people above these weight thresholds, a copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception regardless of body weight. It requires a clinic visit, but it’s worth considering if weight is a factor and you’re past the first 24 hours.
Taking It More Than Once in a Cycle
You can take the morning after pill more than once in the same menstrual cycle if needed. A systematic review of the available evidence found no serious safety concerns with repeated use of either levonorgestrel or ulipristal acetate. One study tracked women taking ulipristal acetate four to six times per month and found no serious adverse events, no abnormal lab results, and normal uterine lining on biopsy.
One signal worth noting: among women who became pregnant after using levonorgestrel multiple times in the same cycle, there was a modestly higher odds of ectopic pregnancy (a pregnancy outside the uterus) compared to those who used it only once. The evidence was rated very low certainty, but it’s a reason to pay attention to unusual symptoms like sharp one-sided pelvic pain if you’ve taken the pill more than once and suspect it didn’t work.
Repeated use is safe as an emergency measure, but it’s far less effective than regular contraception. If you’re using it frequently, that’s a practical signal to explore a routine method.
What to Expect After Taking It
Side effects are generally mild. The most common ones include nausea, fatigue, headache, dizziness, breast tenderness, and lower abdominal cramping. These typically resolve within a day or two.
Your next period may arrive earlier or later than expected, and you might notice some spotting in between. A period that’s more than a week late is a reason to take a pregnancy test.
Restarting Birth Control Afterward
If you take levonorgestrel (Plan B), you can start or resume hormonal birth control (the pill, patch, ring, or shot) immediately. There’s no interaction to worry about.
If you take ella, the situation is different. The CDC recommends waiting at least five days before starting or resuming hormonal contraception. Ella works by blocking progesterone receptors, and hormonal birth control contains progesterone or its synthetic equivalent. Taking them together can reduce ella’s effectiveness. During that five-day gap, use condoms or another barrier method.
Choosing the Right Option
- Within 24 hours, under 155 pounds: Either Plan B (available over the counter, no prescription needed) or ella will work well. Plan B is faster to access since you can buy it at most pharmacies.
- 24 to 72 hours: Ella is the stronger choice, especially if you’re mid-cycle or weigh more than 155 pounds. You’ll need a prescription, which some telehealth services can provide quickly.
- 72 to 120 hours: Ella is the only pill option. Plan B is not recommended this late.
- Over 176 pounds: Ella is preferable to Plan B at any time point. A copper IUD is the most reliable option if accessible.
The bottom line on timing: take whichever pill you can access as quickly as possible, but don’t assume it’s too late if a day or two has passed. Ella in particular maintains its effectiveness across the full five-day window in a way that levonorgestrel simply does not.