The morning after pill works primarily by delaying or preventing ovulation, the release of an egg from the ovary. If there’s no egg available, sperm have nothing to fertilize, and pregnancy can’t occur. The pill can prevent up to 95% of pregnancies when taken within five days of unprotected sex, though it works best the sooner you take it.
There are two types of morning after pill available, and they work in slightly different ways. Understanding the difference matters because it affects which one will work best depending on your situation.
How Levonorgestrel (Plan B) Works
Levonorgestrel, sold as Plan B and several generic versions, is the most widely available morning after pill. You can buy it over the counter without a prescription. It contains a synthetic form of progesterone, a hormone your body naturally produces during your menstrual cycle.
Its main job is to block the hormonal surge that triggers ovulation. Your body releases a burst of luteinizing hormone (LH) right before an egg is released, and levonorgestrel interrupts that process. In clinical studies, when women took levonorgestrel before the LH surge began, ovulation was delayed or blocked in 71% to 91% of cases, compared to 0% to 45% in placebo groups. That’s a dramatic difference when the timing is right.
The catch is that timing has to be genuinely early. Once the LH surge is already underway or the dominant follicle in the ovary has grown past a certain size (about 18 mm), levonorgestrel has no significant effect on ovulation compared to a placebo. In one study of 12 women who took it before the LH peak, seven still ovulated normally, likely because the hormonal cascade was already too far along to stop. This is why the pill becomes less effective the longer you wait.
Researchers have also found that even when levonorgestrel doesn’t block ovulation entirely, it can shorten the luteal phase (the second half of the menstrual cycle) and reduce hormone levels during that window. This may create conditions less favorable for pregnancy, though the exact contribution of this effect isn’t fully understood.
How Ulipristal Acetate (ella) Works
Ulipristal acetate, sold under the brand name ella, requires a prescription. It works by blocking progesterone receptors in the body, which gives it a wider window of effectiveness. Its principal mechanism is still preventing or delaying the rupture of the follicle that releases the egg, but it can do this even after the LH surge has started, something levonorgestrel can’t reliably do.
This is why ella remains more effective than Plan B in the 72 to 120 hour window after unprotected sex. Both options show similar effectiveness in the first three days, but ella pulls ahead during days three through five. In a meta-analysis of two studies, women who used ella had a pregnancy rate of 1.2%. Studies of levonorgestrel showed pregnancy rates between 1.2% and 2.1%.
Researchers also believe ella may have effects beyond ovulation. Because it blocks progesterone receptors, it could alter the uterine lining in ways that make it less receptive. The full picture of these additional mechanisms is still being studied, but the primary, well-established action is preventing the egg from being released in the first place.
What It Doesn’t Do
The morning after pill is not an abortion pill. If a fertilized egg has already implanted in the uterus, the morning after pill will not end that pregnancy. Studies looking at women who took levonorgestrel after the LH surge found similar conception rates to women who took a placebo, meaning the pill didn’t prevent pregnancies that were already in progress.
It also won’t protect you from pregnancy if you have unprotected sex again after taking it. The pill addresses one specific window of risk, not ongoing exposure.
Why Timing Matters So Much
Emergency contraception can prevent over 95% of pregnancies when taken soon after intercourse, but that number drops with every passing day. The biological reason is straightforward: the closer you are to ovulation, the harder it is for the pill to stop the process. If you’ve already ovulated, the pill’s primary mechanism is no longer relevant.
Both types should be taken as early as possible, ideally within 120 hours (five days). If you’re choosing between them and it’s been more than three days, ella is the stronger option during that later window.
Body Weight and Effectiveness
Body weight can reduce how well the morning after pill works. Research has found that emergency contraception appears less likely to work for women with a BMI above 26, which was an unexpected finding when first identified. Levonorgestrel seems to be more affected by weight than ulipristal acetate, which is one reason clinicians sometimes recommend ella for women with higher BMIs.
A copper IUD, placed within five days of unprotected sex, is the most effective form of emergency contraception regardless of body weight. It can also be kept in place as ongoing birth control for years afterward.
Medications That Reduce Effectiveness
Certain medications speed up how quickly your liver breaks down levonorgestrel, which can cut its effectiveness significantly. The HIV medication efavirenz, for example, reduces levonorgestrel blood levels by about 50%. Other medications that have the same effect include some drugs used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis medications like rifampicin, and the herbal supplement St. John’s wort.
This interaction can persist for up to four weeks after you stop taking the interfering medication, so it’s not enough to simply skip a dose. If you take any of these, a copper IUD is a more reliable emergency option.
Side Effects and Your Next Period
The most common side effects are nausea, abdominal cramping, and changes to your next menstrual cycle. Your period may come earlier or later than expected. These effects are temporary and generally resolve on their own within a few days.
The older combined estrogen-progestin regimen (known as the Yuzpe method) causes significantly more nausea and vomiting than either Plan B or ella, which is one reason it’s rarely used today.
What to Do About Ongoing Contraception
After taking Plan B or a combined regimen, you can start or resume your regular hormonal birth control immediately. You’ll need to use condoms or avoid sex for seven days while the regular method takes effect.
After taking ella, the rules are different. You should wait at least five days before starting hormonal contraception, because hormonal birth control can actually interfere with how ella works. During that waiting period and for seven days after restarting your regular method, use condoms. Non-hormonal methods like the copper IUD can be started right away after ella.