Most Plan B side effects are short-lived, clearing up within 24 to 48 hours after you take the pill. Some effects, particularly changes to your menstrual cycle, can linger for several weeks. Here’s what to expect and when each symptom typically resolves.
Common Side Effects and How Long They Last
Plan B delivers a large, single dose of a synthetic hormone that your body processes relatively quickly. In clinical trials of over 1,350 women, the three most frequently reported side effects were nausea (13.7%), lower abdominal pain (13.3%), and fatigue (13.3%). Headache and dizziness are also common. These symptoms tend to peak within the first several hours after taking the pill and fade as your body clears the hormone, usually within one to two days.
Nausea is the side effect most people worry about. If it does hit, it typically starts within a few hours and resolves by the next day. If you vomit within three hours of taking Plan B, the pill may not have been fully absorbed, and you may need to take another dose. Eating a small meal before or after taking the pill can help settle your stomach.
Spotting and Irregular Bleeding
Light bleeding or spotting can show up in the days following Plan B, even if your period isn’t due. This happens because the hormone surge can disrupt the lining of your uterus. Not everyone experiences it, and it’s not a sign that anything is wrong. This spotting is typically lighter than a normal period and lasts anywhere from a day to several days.
The key distinction is between this mid-cycle spotting and your actual period. Spotting after Plan B is usually light pink or brown and doesn’t fill a pad. If bleeding becomes heavy enough to soak through a pad every hour for two hours or longer, that falls outside the normal range and warrants a call to your doctor.
Changes to Your Next Period
This is the side effect that lasts the longest and causes the most anxiety. Plan B can shift the timing of your next period, making it come earlier or later than expected. A delay of a few days is common. Your flow may also be heavier or lighter than usual, and you might notice more cramping than normal.
If your period arrives within a week of its expected date, that’s considered within the normal range of variation after emergency contraception. If your period is more than seven days late, take a home pregnancy test. A late period doesn’t necessarily mean Plan B failed, but it’s the simplest way to rule out pregnancy and put your mind at ease.
Your cycle should return to its normal pattern by the second month. Plan B does not cause lasting changes to your fertility or long-term menstrual regularity.
Side Effects Timeline at a Glance
- Nausea, headache, dizziness, fatigue: typically resolve within 24 to 48 hours
- Lower abdominal pain or cramping: usually fades within one to two days
- Spotting or light bleeding: can occur for a few days after taking the pill
- Breast tenderness: may last a few days
- Menstrual cycle changes: your next period may shift by up to a week, with your cycle normalizing by the following month
Symptoms That Need Medical Attention
While Plan B side effects are overwhelmingly mild, a few symptoms fall outside the expected range. Severe abdominal pain, especially if it’s sharp and located on one side of your lower abdomen, could be a sign of an ectopic pregnancy (a pregnancy that implants outside the uterus). This is rare but requires prompt evaluation. Heavy bleeding that soaks a pad every hour for two or more hours is another reason to contact a healthcare provider.
If you haven’t gotten your period within three weeks of taking Plan B, take a pregnancy test. Emergency contraception is effective but not perfect, and catching a pregnancy early matters for your options and your health.