Plan B is effective for up to 72 hours (3 days) after unprotected sex, but it works best the sooner you take it. The pill doesn’t provide ongoing protection. It works as a single-use intervention to prevent one pregnancy, and its protective effect is tied to that one window.
Most people searching this question want to know one of three things: how long they have to take it, how long it stays in their system, or how long side effects stick around. Here’s what we know about each.
How Long the Effectiveness Window Lasts
Plan B can reduce the risk of pregnancy when taken within 72 hours of unprotected sex. The closer to the event you take it, the better it works. Taking it within the first 24 hours gives you the highest level of protection, and effectiveness drops with each passing day. By the 72-hour mark, it still offers some benefit, but significantly less than if you’d taken it right away.
Plan B works by delaying ovulation, the release of an egg from your ovaries. If your body hasn’t released an egg yet, the hormone in Plan B can pause that process long enough for sperm (which survive about five days) to die off before they ever meet an egg. That’s the entire mechanism. It does not affect ovulation that’s already underway. So if you’ve already ovulated, Plan B won’t prevent pregnancy, regardless of how quickly you take it.
How Long It Stays in Your Body
The active hormone in Plan B has a half-life of roughly 27.5 hours. That means about half the dose is cleared from your system in just over a day. After five to six half-lives, a drug is considered essentially eliminated, which puts full clearance at roughly six to seven days. About 45% of the hormone and its byproducts leave through urine, and about 32% through stool.
This doesn’t mean Plan B is “working” for a full week. Its ability to delay ovulation depends on blood levels being high enough at the right moment. Once levels drop below a certain threshold, the ovulation-blocking effect fades. The drug lingering in trace amounts doesn’t translate to continued pregnancy prevention.
How Long Side Effects Last
Common side effects include nausea, fatigue, headaches, dizziness, breast tenderness, abdominal cramping, and light spotting. For most people, these are mild and resolve within a day or two. Nausea tends to hit first and fade fastest. If you vomit within two hours of taking the pill, it may not have been fully absorbed, and you may need another dose.
The one side effect that can linger is a change to your menstrual cycle. Plan B can make your next period come earlier or later than expected, and the flow may be lighter or heavier than usual. This irregularity typically resolves by the following cycle. If your period is more than a week late, a pregnancy test is a reasonable next step.
Factors That Can Reduce How Well It Works
Body Weight
Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason appears to be straightforward: blood levels of the hormone were about 50% lower in people with higher BMIs after a standard dose. The drug simply never reaches the concentration needed to reliably block ovulation. Doubling the dose doesn’t fix this problem either. If your BMI is 30 or above, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception option.
Certain Medications and Supplements
Some medications speed up how quickly your liver breaks down Plan B’s active hormone, potentially cutting its blood levels in half or more. These include certain drugs used to treat epilepsy, tuberculosis, HIV, and fungal infections. St. John’s wort, a common herbal supplement, has the same effect. The impact can persist for up to four weeks after you stop taking the interacting medication, so even recently discontinued drugs can be relevant.
Plan B Doesn’t Protect You Going Forward
Plan B is a one-time dose for a single event. It does not provide any ongoing contraceptive protection. If you have unprotected sex again, even the next day, you’re at risk of pregnancy again. You can take another dose if needed, but relying on Plan B repeatedly is less effective and more expensive than regular contraception.
If you want to start or restart hormonal birth control after taking Plan B, you can do so immediately. You don’t need to wait for your next period. However, you’ll need to use a backup method like condoms for the first seven days after starting your regular contraception, since it takes that long for most hormonal methods to become fully effective.
If you took a different emergency contraceptive containing ulipristal acetate (sold as ella), the timing is different. You should wait six days before starting any hormonal birth control, because the two medications can interfere with each other’s effectiveness.