Plan B delivers a large, concentrated dose of a synthetic progesterone called levonorgestrel, which temporarily overrides your body’s normal hormonal signaling to prevent pregnancy. The single tablet contains 1.5 mg of levonorgestrel, far more than the amount found in a daily birth control pill. This sudden spike is what makes it work as emergency contraception, and it’s also what causes the short-term side effects many people experience afterward.
How the Hormone Surge Prevents Pregnancy
Your body’s natural menstrual cycle depends on a carefully timed chain of hormonal signals. Midway through each cycle, your brain releases a surge of luteinizing hormone (LH) that triggers ovulation, the release of an egg from the ovary. Plan B’s primary job is to disrupt that signal.
When you take Plan B before ovulation, the flood of levonorgestrel suppresses or blunts the LH surge. Without a strong enough LH peak, the ovary doesn’t release an egg, and fertilization can’t happen. Research shows that the strength of the LH surge directly affects the likelihood of conception: cycles with a robust, two-day surge result in pregnancy rates around 23%, while a weak, one-day surge drops that rate to roughly 6%. By flattening the surge entirely, Plan B can effectively shut down ovulation for that cycle.
This is why timing matters so much. If you’ve already ovulated, Plan B has far less to work with because its main mechanism is no longer available. There is some evidence that levonorgestrel thickens cervical mucus, making it harder for sperm to reach an egg, but this effect is better documented with ongoing low-dose hormonal methods than with a single emergency dose.
What Happens in Your Body After You Take It
Levonorgestrel is absorbed quickly. Blood levels of the hormone peak within about 1.5 to 2 hours of swallowing the pill. From that point, your body begins breaking it down, with an average elimination half-life of about 24 hours. That means roughly half the drug is cleared from your system in a day, and most of it is gone within three to five days. This is not a hormone that lingers in your body for weeks.
During those first few days, though, your system is responding to a progesterone level that’s far higher than normal. That temporary imbalance is responsible for the side effects people commonly report:
- Nausea or vomiting, which can start within hours
- Fatigue and dizziness, often lasting a day or two
- Headache
- Breast tenderness
- Spotting or unexpected bleeding between periods
These symptoms are a direct result of the hormonal spike and typically resolve on their own as levonorgestrel clears your system.
How Your Next Period May Change
One of the most noticeable hormonal effects is a shift in when your next period arrives and what it looks like. In a study tracking menstrual patterns after Plan B use, about 21% of participants had their cycle shortened by two or more days, while 24% had it lengthened by two or more days. Up to 13% experienced a delay of more than a week.
The direction of the shift depends on where you were in your cycle when you took the pill. If you took it before ovulation, your period tends to come about a day early. If you took it after ovulation, your period is more likely to be delayed by close to two days. Your flow may also be heavier or lighter than usual for that one cycle. These changes are temporary. By the following cycle, your hormones have typically returned to their normal rhythm.
No Lasting Effect on Your Hormones or Fertility
A review of the evidence found no impact on future fertility, miscarriage risk, fetal development, or ongoing menstrual regularity after Plan B use. The hormonal disruption is genuinely short-lived. Levonorgestrel doesn’t accumulate in your tissues, and it doesn’t reset your reproductive system in any lasting way. Your ovaries resume their normal hormone production once the drug is cleared. Using Plan B more than once also does not cause cumulative hormonal damage, though it’s less reliable than ongoing contraception and will keep triggering the same short-term side effects each time.
Why Body Weight Affects How Well It Works
The hormonal dose in Plan B is fixed at 1.5 mg, which means it produces different blood concentrations depending on body size. Research has found that people with a BMI over 30 have more than four times the risk of pregnancy after taking levonorgestrel emergency contraception compared to those with a BMI in the normal range. People with a BMI between 25 and 30 face roughly double the risk. One analysis found that the drug’s effectiveness appears to hit a ceiling at around 155 pounds (70 kg) and shows essentially no emergency contraception benefit at 176 pounds (80 kg) or above.
This doesn’t mean the hormone has no effect on your body at higher weights. You’ll still experience the side effects. But the levonorgestrel concentration in your blood may not reach the threshold needed to reliably suppress the LH surge. If you weigh over 155 pounds, a copper IUD or a different emergency contraceptive pill containing ulipristal acetate are generally more effective options.