Levonorgestrel is a synthetic form of progesterone that primarily works by delaying or preventing ovulation. When taken as emergency contraception (sold as Plan B and generics), a single 1.5 mg dose delivers a surge of this hormone that disrupts the signals your body needs to release an egg. If ovulation doesn’t happen, sperm have nothing to fertilize, and pregnancy can’t occur.
But the full picture is more nuanced than that one-liner suggests, especially when it comes to timing, body weight, and what the drug can and can’t do once ovulation has already happened.
Ovulation Delay: The Primary Mechanism
Your body releases an egg through a tightly coordinated hormonal sequence. A spike in luteinizing hormone (LH) triggers the ovary to release a mature egg, and levonorgestrel interferes with that spike. When taken before this hormone surge begins, it can suppress or delay it long enough for any sperm already in the reproductive tract to die off (sperm survive about five days). This is the main way it prevents pregnancy.
The timing matters enormously. If you take levonorgestrel well before ovulation, the drug is highly effective at blocking egg release. But if you take it in the day or two just before ovulation, when the LH surge is already underway, results are far less reliable. In one study, 80% of women who took levonorgestrel in the five days before ovulation still showed signs of follicular rupture (the physical release of the egg). That number climbed to 92% when the pill was taken just two days before ovulation. In other words, once the ovulation process is well underway, levonorgestrel often can’t stop it.
This is a key reason the drug becomes less effective the longer you wait after unprotected sex. It’s not just about the drug wearing off. It’s about the narrowing window in which it can still block ovulation before the process passes the point of no return.
Effects on Cervical Mucus and Sperm Transport
Levonorgestrel also thickens cervical mucus, creating a physical barrier that makes it harder for sperm to travel through the cervix and reach the egg. This effect is well documented in people using levonorgestrel-releasing IUDs, where the hormone is delivered continuously at low doses. In one study, zero percent of IUD users showed sperm penetration through their cervical mucus, compared to about 64% of people not using the device.
Whether a single emergency dose thickens mucus enough to meaningfully block sperm is less clear. The IUD delivers levonorgestrel directly to the uterus over months, creating sustained local concentrations that a one-time oral pill doesn’t replicate. Still, it likely contributes some barrier effect, particularly in combination with ovulation suppression.
What It Does Not Do: Implantation
For years, product labels suggested levonorgestrel might also prevent a fertilized egg from implanting in the uterine lining. The scientific evidence doesn’t support this. Nine out of ten studies examining the question found no difference in the uterine lining’s receptivity to implantation after levonorgestrel exposure compared to controls. A review of the evidence concluded that levonorgestrel taken after ovulation does not affect implantation and results in conception rates similar to placebo. In practical terms, if ovulation and fertilization have already occurred, the drug is unlikely to prevent pregnancy.
How Quickly It Works and How Long It Lasts
After swallowing the 1.5 mg tablet, levonorgestrel is absorbed rapidly. Blood levels peak at about two hours in most people. The drug then clears slowly, with a half-life averaging 30 hours for people at a normal weight. That means it takes roughly 30 hours for half the drug to be eliminated from your system, and several days for it to clear entirely.
This doesn’t mean protection lasts for days. The critical window for blocking ovulation is narrow, and the drug needs to reach effective levels before the LH surge progresses too far. This is why guidelines recommend taking it as soon as possible after unprotected sex rather than waiting.
Why Body Weight Affects Effectiveness
Levonorgestrel’s effectiveness drops as body weight increases. Health Canada has stated the drug may be less effective in women over 165 pounds and potentially ineffective over 176 pounds. People with a BMI over 25 may see reduced protection.
The pharmacokinetics explain why. In people with a higher BMI, the drug distributes across a larger volume of body tissue, which dilutes peak blood concentrations. The half-life also stretches considerably: from about 30 hours at a normal BMI to roughly 46 hours at a BMI of 40 or above. Lower peak levels mean the hormonal signal may not be strong enough to reliably suppress the LH surge.
For people with a BMI over 30, an alternative emergency contraceptive containing ulipristal acetate appears more effective. Studies suggest an unintended pregnancy rate of about 2.5% with ulipristal acetate compared to roughly 6% with levonorgestrel in this weight range. A copper IUD, inserted within five days of unprotected sex, is the most effective emergency option regardless of weight.
What to Expect Afterward
The most common side effect is nausea, reported by about 14% of users. Breast tenderness occurs in roughly 8%. These effects are temporary and typically resolve within a day or two.
Your next period will likely shift. About one in five people see their cycle shorten by two or more days, while about one in four see it lengthen by two or more days. The direction of the shift depends on when in your cycle you took the pill. Taking it before ovulation tends to shorten the cycle by about a day. Taking it after ovulation tends to push your period back by close to two days. If your period is more than a week late, a pregnancy test is a reasonable next step.
Medications That Reduce Effectiveness
Levonorgestrel is broken down in the liver by an enzyme called CYP 3A4. Medications that ramp up the activity of this enzyme can cause your body to clear levonorgestrel faster, reducing its effectiveness. The most common culprits are certain anti-seizure medications, some HIV treatments, the antibiotic rifampin, and the herbal supplement St. John’s wort. If you take any of these regularly, levonorgestrel may not provide reliable emergency contraception, and an alternative method like the copper IUD is a better option.