Plan B is absorbed into your bloodstream quickly. The active ingredient reaches peak levels in your blood within about 1.5 to 2 hours, though it can take anywhere from 1 to 4 hours depending on the person. This timing matters most if you vomit after taking the pill, since throwing up before it’s fully absorbed can prevent it from working.
How Quickly Plan B Gets Into Your System
After you swallow Plan B, the hormone it contains (levonorgestrel) passes through your stomach and is absorbed in your small intestine, just like most oral medications. In a study of 30 healthy women who took a single 1.5 mg dose, blood levels of the hormone peaked at a median of 1.67 hours, with a range of 1 to 4 hours. That means for most people, the pill has done the bulk of its absorption work within about two hours.
This doesn’t mean the drug takes two hours to start working. Absorption begins within minutes of swallowing the tablet. The “peak” simply marks the point where the highest concentration is circulating in your blood. By then, the pill is already signaling your body to delay or prevent ovulation, which is how it prevents pregnancy.
The Two-Hour Vomiting Rule
This is likely the reason you searched this question. If you vomit within two hours of taking Plan B, there’s a real chance your body didn’t absorb enough of the hormone for it to work. The NHS recommends contacting a pharmacist about taking another dose if you throw up in that window.
If you vomit after the two-hour mark, you generally don’t need a second dose. By that point, enough of the drug has moved from your stomach into your intestines and bloodstream. Nausea is a common side effect of Plan B itself, so if you feel queasy but manage to keep the pill down for at least two hours, you’re in the clear from an absorption standpoint.
If you’re prone to nausea, eating a small amount of food before or with the pill can help settle your stomach without significantly slowing absorption.
Timing After Sex Matters More Than Digestion Speed
Once the pill is absorbed, the bigger factor in whether it works is how soon you took it after unprotected sex. Plan B is around 94% effective when taken within the first 24 hours. That number drops to about 58% by the 72-hour mark. The pill works by delaying or stopping your ovary from releasing an egg. If ovulation has already happened, the pill has little to no effect.
This is why speed matters so much. Taking it sooner gives the hormone more time to suppress ovulation before sperm can reach an egg. Sperm can survive in the reproductive tract for up to five days, so Plan B is essentially racing to shut down ovulation before fertilization occurs. It does not end an existing pregnancy.
Why Body Weight Can Affect Absorption
Your body size can change how effectively Plan B works, and this relates directly to how the drug is distributed once absorbed. Research from Oregon Health & Science University found that people with a BMI of 30 or higher had levonorgestrel blood levels that were 50% lower than those with a BMI under 25 after taking a standard dose. People in the higher BMI group experienced contraceptive failure four times as often.
Importantly, doubling the dose doesn’t solve this problem. The same research team tested whether taking two Plan B pills would compensate for the lower blood levels in people with higher BMIs, and it did not effectively prevent pregnancy. If your weight is above 176 pounds or your BMI is over 30, a copper IUD placed within five days of unprotected sex is the most reliable emergency contraception option. The prescription pill ella (ulipristal acetate) is another alternative that maintains better effectiveness across a wider weight range, though it still has limits.
Medications That Speed Up Clearance
Certain medications can cause your liver to break down levonorgestrel faster than normal, reducing how much active hormone stays in your system even after full absorption. This category includes some seizure medications, certain HIV treatments, and the herbal supplement St. John’s wort. If you take any of these regularly, Plan B may be less effective for you regardless of how well you digest it. A pharmacist can tell you whether your current medications fall into this category and suggest an alternative form of emergency contraception if needed.
What “Fully Digested” Actually Means Here
When people ask how long Plan B takes to digest, they usually mean one of two things: how long until it’s safe to vomit (two hours), or how long until it’s actively working. The hormone begins affecting your reproductive system as soon as it enters your bloodstream, which starts within minutes. By two hours, peak absorption has occurred for most people. The drug then continues circulating and doing its job over the following days, with a half-life of about 24 to 32 hours, meaning it takes roughly a day for your body to clear half of the dose.
The pill itself, as a physical tablet, dissolves in your stomach within 15 to 30 minutes. But dissolution and absorption are different things. The tablet breaking apart is just the first step. The hormone still needs to pass through your intestinal lining and into your blood before it can work. That full process is what takes up to two hours in most people and up to four in some.