Plan B prevents pregnancy primarily by delaying or blocking ovulation. It contains a synthetic hormone that interferes with the chain of events your body needs to release an egg, essentially pressing pause on your fertility for long enough that any sperm already inside you die before they can reach an egg. This mechanism is most effective when the pill is taken before ovulation has started, which is why timing matters so much.
How Plan B Interrupts the Ovulation Process
To ovulate, your body needs to produce a sharp spike in a hormone called LH (luteinizing hormone). This spike triggers the dominant follicle in your ovary to rupture and release an egg. Plan B’s active ingredient, levonorgestrel, disrupts this process in two ways depending on where you are in your cycle.
When your lead follicle is still relatively small and days away from being ready, Plan B prevents or delays the follicle from rupturing for at least five days. Since sperm can only survive about five days inside your body, this delay is usually enough to outlast them entirely. When the follicle is larger and closer to maturity, Plan B works by suppressing the LH surge itself, preventing the hormonal trigger that would cause the egg’s release. Both mechanisms can work together in the same cycle.
The FDA has concluded that Plan B does not have a direct effect on fertilization or implantation. It works entirely on the ovulation side of the equation, acting well before an embryo could form or reach the uterus.
Why Timing Relative to Ovulation Matters
Plan B is most effective when taken before the LH surge begins. Once ovulation is already underway, the pill has little ability to stop it. In clinical trials, levonorgestrel performed no better than a placebo at preventing follicle rupture when given at an advanced stage of the LH surge. This is the core limitation of Plan B: if your body has already committed to releasing an egg, the drug can’t call it back.
The challenge is that most people don’t know exactly when they’re ovulating. Ovulation typically happens around the midpoint of your cycle, but it can shift by several days due to stress, illness, travel, or just normal variation. So when you take Plan B after unprotected sex, the outcome depends heavily on a biological event you likely can’t observe in real time. Taking it sooner gives it a better chance of getting ahead of ovulation, which is one reason the standard advice is to take it as quickly as possible.
How It Compares to Ella
A prescription alternative called ella (ulipristal acetate) works through a similar principle but maintains its effectiveness later in the ovulation timeline. In pooled data from three randomized trials, ella prevented follicle rupture in 79% of cycles even when taken at the time of the LH surge. Plan B succeeded only 14% of the time at that same stage, essentially matching the placebo rate of 10%.
Ella also delayed ovulation for a median of six days after treatment, compared to just two days for levonorgestrel. It successfully pushed ovulation back by at least five days in 59% of cycles. Neither drug worked when given on the actual day of the LH peak, but ella’s wider effective window makes it a stronger option when you suspect ovulation may be close.
Body Weight and Effectiveness
Plan B may be less effective for people with a BMI of 30 or higher. The CDC notes that limited evidence from secondary analyses shows an increased risk of pregnancy after levonorgestrel use among people in this weight range compared to those with a BMI under 25. The drug is still considered safe to take regardless of weight, but the reduced efficacy is worth knowing about. Ella or a copper IUD may be more reliable alternatives in this situation.
What to Expect After Taking It
Because Plan B works by manipulating the hormones that control ovulation, it often shifts the timing of your next period. Your cycle may arrive up to a week earlier or later than expected. This is a direct consequence of the drug rearranging your ovulation schedule, and it’s not a sign that something is wrong.
Common side effects include nausea, fatigue, headache, and spotting between periods. These are temporary and result from the large dose of synthetic hormone entering your system. Plan B reaches its peak concentration in your blood within about three hours and has a half-life of roughly 43 hours, meaning the hormone clears your system within a few days.
Using Plan B more than once in a cycle or across multiple cycles does not reduce its effectiveness or cause long-term side effects. However, repeated use can make your periods increasingly irregular and unpredictable, since each dose resets the ovulation clock. It’s not harmful as an occasional backup, but it’s a less reliable and more disruptive strategy than ongoing contraception.
When Plan B Won’t Help
If ovulation has already occurred and an egg has been released, Plan B has no meaningful way to prevent pregnancy. It does not block fertilization, alter the uterine lining to prevent implantation, or affect an existing pregnancy. Its entire mechanism depends on keeping the egg locked inside the follicle long enough for sperm to die off. Once that window has passed, the drug has missed its target. For people who suspect they’ve already ovulated, a copper IUD inserted within five days of unprotected sex is the most effective emergency option, as it works independently of ovulation timing.