The morning-after pill works best within the first 24 hours after unprotected sex, and its effectiveness drops steadily with each passing day. The maximum window is 3 days (72 hours) for levonorgestrel pills like Plan B, or 5 days (120 hours) for the prescription pill ulipristal acetate, sold as ella. Taking it sooner always gives you better odds.
Effectiveness by Day
Both types of morning-after pill become less reliable the longer you wait. Within the first 3 days, levonorgestrel and ulipristal acetate have similar effectiveness. The real difference shows up between days 3 and 5: ulipristal acetate continues to reduce pregnancy risk during that window, while levonorgestrel essentially stops working after 72 hours.
This declining effectiveness isn’t a quirk of the drug wearing off. It’s a matter of biology. The closer you get to ovulation, the harder it is for the pill to do its job, and every hour that passes after sex is another hour closer to that window.
How the Pill Actually Prevents Pregnancy
The morning-after pill works primarily by delaying or blocking ovulation. It does not end a pregnancy that has already begun. If you take it before your body releases an egg, the pill suppresses the hormonal surge that triggers ovulation, pushing it back by days or even weeks. In one study, some women experienced an ovulation delay of nearly 17 days.
Timing relative to ovulation matters enormously. In the same study, women who took the pill the day before or the day of their hormonal surge often ovulated normally anyway. The drug was essentially “too late” to stop an event already underway. This is why the pill is more effective earlier: the further you are from ovulation when you take it, the better the chance it can intervene.
There’s also evidence the pill affects the second half of the menstrual cycle in ways that may add a secondary layer of protection, even when ovulation isn’t fully blocked. But the primary mechanism is ovulation delay, which is why timing is everything.
Body Weight Can Reduce Effectiveness
Research has found that emergency contraception becomes less reliable at higher body weights, particularly for levonorgestrel (Plan B). A significant study found reduced effectiveness in women with a BMI above 26, which was an unexpected finding. Levonorgestrel is the most affected of the two options.
Ulipristal acetate may also be affected by weight, but not to the same degree, making it the better choice for women with a higher BMI. If neither pill is ideal, a copper IUD placed within 5 days of unprotected sex is the most effective form of emergency contraception regardless of body weight.
Medications That Interfere
Certain medications speed up how your liver breaks down levonorgestrel, which can cut its effectiveness dramatically. These include some drugs used to treat epilepsy, tuberculosis, HIV, and fungal infections. The herbal supplement St. John’s wort has the same effect. One HIV medication (efavirenz) reduces levonorgestrel levels in the blood by roughly 50%.
This interaction doesn’t just apply while you’re actively taking these medications. The enzyme changes they cause can persist for up to 4 weeks after you stop. If you’ve used any of these drugs in the past month, the standard dose of levonorgestrel may not be enough.
Taking It More Than Once
You can use the morning-after pill more than once in the same cycle if needed. A systematic review published in BMJ Sexual & Reproductive Health found no significant safety concerns with repeated use, though the overall evidence quality was low. No serious side effects were observed in studies of repeated ulipristal acetate use either.
One finding worth noting: among women who became pregnant despite using levonorgestrel emergency contraception, those who had used it multiple times in the same cycle had roughly 2.5 times higher odds of an ectopic pregnancy compared to those who used it once. This doesn’t mean repeated use causes ectopic pregnancies, but it’s a pattern researchers flagged. If you find yourself relying on emergency contraception frequently, a regular contraceptive method will be far more effective and predictable.
Side Effects and What to Expect After
Most people experience mild side effects, if any. Nausea is the most common. You may also notice spotting or bleeding between periods. Your next period could come early, late, or feel different than usual. These are all normal responses to a large dose of hormones.
If your period is more than 7 days late after taking the morning-after pill, take a pregnancy test. If you don’t know exactly when your period is due, the NHS recommends testing at least 21 days after the unprotected sex occurred. Testing earlier than that can give a false negative because pregnancy hormone levels may not yet be detectable.
Breastfeeding and the Morning-After Pill
Levonorgestrel passes into breast milk in very small amounts, and studies have found no differences in breastfeeding outcomes between women who took it and those who didn’t. Both the CDC and WHO have reviewed the evidence and found no special safety concerns for breastfeeding mothers using emergency contraception. Ulipristal acetate has less data available, but the existing evidence has not raised red flags either.