Plan B is absorbed into your bloodstream quickly. The active ingredient reaches peak levels in about 1.6 hours, and the critical window for keeping the pill down is 2 hours. If you haven’t vomited within 2 hours of swallowing the tablet, the medication has been absorbed enough to work.
What “Digested” Really Means for Plan B
When most people ask how long Plan B takes to digest, they’re really asking: how long until it’s safely in my system? The answer is roughly 2 hours. Plan B contains levonorgestrel, a synthetic hormone that passes through the stomach lining and into the bloodstream rapidly after you swallow it. FDA pharmacology data shows the drug reaches peak blood concentration in 1 to 2 hours, with an average of about 1.6 hours.
This doesn’t mean the pill is fully broken down in 2 hours. It means enough of the drug has entered your bloodstream by that point to do its job. The hormone then stays active in your body with a half-life of about 20 hours, meaning it takes roughly a full day for levels to drop by half.
The 2-Hour Vomiting Rule
The reason the 2-hour mark matters so much is vomiting. If you throw up within 2 hours of taking Plan B, the tablet may not have been fully absorbed, and the FDA recommends considering a repeat dose. After 2 hours, vomiting won’t affect the pill’s effectiveness because the drug is already circulating in your blood.
About 14% of women experience nausea after taking Plan B, so this isn’t a rare concern. Other common side effects include lower abdominal pain, fatigue, and headache, each affecting roughly 13% of users. If you feel nauseous but haven’t actually vomited, the pill is still working normally. Taking it with a small snack may help settle your stomach, though no formal studies have tested whether food changes how the drug is absorbed.
How Plan B Works Once Absorbed
Once levonorgestrel enters your bloodstream, it suppresses the surge of luteinizing hormone that triggers ovulation. In simple terms, it delays or prevents your ovary from releasing an egg. If there’s no egg available, sperm can’t fertilize anything. Research shows that Plan B works by disrupting ovulation, not by interfering with a fertilized egg or preventing implantation.
This is why timing matters so much. The pill is most effective when taken before ovulation has already occurred. If you’ve already ovulated, the drug has less ability to prevent pregnancy.
Why Taking It Sooner Matters
Plan B’s effectiveness drops significantly with each day you wait. Taken within the first 24 hours after unprotected sex, it prevents pregnancy about 94% of the time. By 72 hours (3 days), that number falls to around 58%. You can technically take it up to 5 days after unprotected sex, but effectiveness continues to decline.
The drop in effectiveness isn’t about digestion speed. The pill absorbs just as fast whether you take it 6 hours or 60 hours later. The issue is biological timing: the closer you get to ovulation, the harder it is for the drug to stop the process. Taking it as soon as possible gives the hormone more time to suppress ovulation before it happens.
Body Weight and Absorption
Your weight can affect how well Plan B works. Research from Oregon Health & Science University found that levonorgestrel-based emergency contraception becomes less reliable for women with a BMI above 26, which corresponds to roughly 155 to 165 pounds depending on height. The hormone appears to be diluted at higher body weights, reducing its ability to suppress ovulation effectively.
This doesn’t mean the pill won’t work at all at higher weights, but the failure rate increases. If your BMI is above 26, a copper IUD placed within 5 days is the most effective emergency contraception option regardless of weight. Another prescription emergency contraceptive pill (sold under the brand name ella) also maintains effectiveness at higher BMIs better than levonorgestrel does.
What to Expect Afterward
Once Plan B is absorbed, your body processes and eliminates the hormone over the next several days. Most women notice some change in their next period. It may come a few days early or a few days late, and bleeding can be heavier or lighter than usual. These shifts are normal and result from the temporary hormonal disruption.
If your period is more than a week late after taking Plan B, a pregnancy test is a reasonable next step. The hormone itself clears your system within a few days and has no effect on a future pregnancy or your long-term fertility.