Most Plan B side effects last 24 to 48 hours. The most common ones, like nausea, headache, and fatigue, typically peak on the first day after taking the pill and fade by day two or three. Changes to your menstrual cycle can take longer to resolve, sometimes lasting into the following cycle.
What Side Effects to Expect
Plan B delivers a large, single dose of a synthetic hormone that prevents or delays ovulation. That hormonal surge is what causes side effects, and it affects most of the body’s systems to some degree. In clinical trials involving 1,359 women, the most frequently reported side effects were lower abdominal pain (13.3%), fatigue (13.3%), and breast tenderness (8.2%). Other common effects include nausea, vomiting, headache, dizziness, and light spotting between periods.
These symptoms tend to cluster in the first day or two. On day one, you’re most likely to feel nauseous, crampy, or unusually tired. By day three, most people report no lingering side effects at all.
A Day-by-Day Timeline
Here’s a general picture of what the first few days look like for most people:
- Hours 1 to 6: The hormone is absorbed rapidly, peaking in your bloodstream around the two-hour mark. Nausea and dizziness are most likely during this window. If you vomit within two hours of taking the pill, the dose may not have been fully absorbed, and you may need to take another one.
- Hours 6 to 24: Cramping, headache, fatigue, and breast tenderness can develop or continue. These are the most commonly reported symptoms in this window.
- Day 2 (24 to 48 hours): Most side effects are fading. Some people still feel mild fatigue or breast tenderness.
- Day 3 and beyond: The majority of people feel back to normal. If symptoms persist past 48 hours or feel severe, that’s worth a call to your provider.
The hormone itself has a half-life of about 30 hours in most people, meaning half of it has been cleared from your bloodstream by that point. Within roughly four days, levels drop to very low concentrations. This is why physical side effects rarely linger past the first couple of days.
Why Body Weight Can Affect Duration
The hormone in Plan B is cleared more slowly in people with a higher body weight. Research published in the journal Contraception found that the half-life averaged 30 hours in normal-weight participants but stretched to 41 to 46 hours in obese participants. When researchers looked at the unbound (active) form of the hormone, the difference was even larger: 37 hours versus 59 to 70 hours. This means side effects could potentially last a bit longer if you’re in a higher weight range, simply because the hormone stays in your system longer.
How Plan B Affects Your Period
Changes to your menstrual cycle are the one side effect that can extend well beyond the first 48 hours. In clinical trials, 31% of participants experienced some shift in their cycle after taking Plan B. Your period might come a few days early or a few days late, and the flow may be lighter or heavier than usual. Most women get their next period within two days to one week of when they expected it.
The timing of these changes depends on where you were in your cycle when you took the pill. If you took it early in your cycle (before ovulation), your period is more likely to come early. If you took it later, a short delay is more common. Either way, a study on menstrual bleeding patterns after emergency contraception found that the majority of cycle changes disappeared by the following period. So even if your first post-pill period is off, the one after that should look normal.
If your period is more than a week late, take a pregnancy test. A late period alone isn’t a reason to panic, since Plan B commonly shifts timing, but confirming is straightforward and worth the peace of mind.
Managing Nausea and Cramping
Nausea is the side effect most people worry about, and it’s also the most time-sensitive. Taking Plan B with a small meal or snack can reduce the chance of an upset stomach. If nausea hits anyway, it usually passes within a few hours. Over-the-counter anti-nausea medication taken 30 minutes before the pill can also help, though this isn’t always practical in the moment.
For cramping and headaches, standard over-the-counter pain relievers work well. Rest and staying hydrated also help your body process the hormone more comfortably. These are short-lived symptoms, so simple measures are usually enough.
Signs That Something Else Is Going On
Plan B’s side effects are mild for the vast majority of people. But a few symptoms warrant attention because they can signal something unrelated to the pill itself. Severe abdominal pain, especially if it’s sharp and one-sided, can be a sign of an ectopic pregnancy. Heavy vaginal bleeding that soaks through a pad in an hour, or bleeding that starts after you’ve already missed a period, is also worth reporting to a provider. These situations are uncommon, but they’re distinct from the mild cramping and spotting that Plan B normally causes.