Most Plan B side effects are mild and resolve within 24 hours. The one exception is changes to your menstrual cycle, which can show up days or even weeks later. The pill itself clears your body within about a week.
What the First 48 Hours Look Like
The most common side effects tend to hit within the first day or two after taking Plan B. These include nausea, headache, fatigue, dizziness, abdominal cramps, breast tenderness, and diarrhea. For most people, these symptoms fade within 24 hours. If any of them stick around past 48 hours or feel severe, that’s worth a call to your doctor.
Nausea is the side effect people worry about most, and for good reason. If you vomit within three hours of taking the pill, it may not have been fully absorbed. In that case, you should take another dose as soon as possible. Taking the pill with a small amount of food can help settle your stomach, and an over-the-counter anti-nausea medication beforehand is an option if you’re prone to nausea, though it isn’t routinely recommended.
Menstrual Changes Can Last Longer
Plan B delivers a large dose of a synthetic hormone that works by delaying ovulation. That hormonal surge can throw off your next period. You might notice spotting a few days after taking the pill, even if your period isn’t due yet. Your next period could also arrive earlier or later than expected, and the flow might be heavier or lighter than usual.
These menstrual changes are the one category of side effects that can stretch well beyond 48 hours. Spotting or irregular bleeding may continue on and off for days. Your cycle typically returns to its normal pattern within one to two cycles. If your period is more than a week late after taking Plan B, a pregnancy test is a reasonable next step.
Side Effect Timeline at a Glance
- Hours 0 to 24: Nausea, headache, fatigue, dizziness, cramps, breast tenderness, and diarrhea are most likely during this window. These are the most common symptoms and usually the shortest-lived.
- Hours 24 to 48: Most physical symptoms have faded by now. Any that persist past this point or feel unusually intense are worth discussing with a provider.
- Days to weeks later: Spotting, irregular bleeding, or a shift in your period’s timing or flow. These changes may not appear until your next cycle is due.
- Within one week: The medication itself has fully cleared your system.
What’s Normal vs. What’s Not
Mild cramping and nausea that resolve in a day are completely expected. Spotting between periods is also normal. What isn’t normal is severe abdominal pain, especially if it’s sharp and one-sided. This can be a sign of an ectopic pregnancy (a pregnancy that implants outside the uterus), which is a medical emergency regardless of whether you’ve taken Plan B.
Heavy bleeding that soaks through a pad every hour, a high fever, or side effects that keep getting worse rather than better also fall outside the range of typical reactions.
Making Side Effects More Manageable
Since most symptoms are concentrated in that first 24-hour window, a few simple strategies can help. Taking the pill with food reduces the chance of nausea. Staying hydrated helps with headaches and fatigue. Rest if you can, especially if dizziness is an issue. Over-the-counter pain relievers can handle cramps and headaches.
For the menstrual changes that come later, keeping a brief note of when you took Plan B and tracking your next period can save you from unnecessary anxiety. Knowing your cycle might shift by several days helps you interpret spotting or a late period for what it likely is: a temporary hormonal adjustment, not a sign that something went wrong.