Most Plan B side effects resolve within 24 to 48 hours. The hormone in Plan B has a half-life of about 26 to 27 hours, meaning your body eliminates half of it roughly every day. By the second or third day after taking the pill, the vast majority of people feel completely back to normal. The exception is menstrual cycle changes, which can take a full cycle to sort themselves out.
What Side Effects Are Most Common
In clinical trials submitted to the FDA, the most frequently reported side effect was heavier menstrual bleeding, affecting about 31% of users. Nausea and lower abdominal pain each occurred in roughly 14% of women, and fatigue hit at about the same rate. Headache affected around 10%, dizziness close to 10%, and breast tenderness about 8%.
These numbers are worth keeping in perspective. The majority of users don’t experience most of these effects, and when they do, the symptoms tend to be mild.
The First 48 Hours
The first day after taking Plan B is when side effects peak. Nausea typically shows up within a few hours and fades by the following day. Fatigue, headache, dizziness, breast tenderness, and lower abdominal cramping follow a similar pattern. If you’re going to feel off, it will almost certainly be during this window.
By day three, most people notice no lingering symptoms at all. If nausea, headache, fatigue, or pain continues past the 48-hour mark, that’s unusual and worth checking out with a healthcare provider.
One practical note on nausea: if you vomit within three hours of taking the pill, the medication may not have been fully absorbed. The CDC recommends taking another dose as soon as possible in that situation.
Bleeding and Spotting
Irregular bleeding is the one side effect that can linger well beyond that initial two-day window. Some people experience light spotting between periods in the weeks after taking Plan B, while others notice a heavier flow than usual when their next period arrives. Most people who take Plan B don’t experience any unusual bleeding at all, but for those who do, it can last up to a month.
This bleeding is not dangerous. It’s a predictable response to a sudden surge of synthetic hormone disrupting the normal cycle. If spotting or unusual bleeding continues beyond one to two months, something other than Plan B is likely responsible.
Changes to Your Next Period
Plan B can shift the timing of your next period. Your period might arrive a few days early or a few days late. In the FDA clinical trial, about 4.5% of users experienced a delay of more than seven days. A period that’s a week late after taking Plan B is within the range of normal, but it understandably causes anxiety for people who took emergency contraception specifically to avoid pregnancy.
If your period is more than a week late, a pregnancy test is a reasonable next step for peace of mind. By the cycle after that, your period should return to its normal pattern. Plan B delivers a one-time dose of hormone that clears your system within days, so it doesn’t have a lasting effect on your fertility or your cycle beyond that initial disruption.
Why the Hormone Clears Quickly
Plan B contains a single 1.5 mg dose of a synthetic hormone that peaks in your bloodstream about two hours after you swallow the pill. From there, levels drop steadily with a half-life of roughly 26 to 27 hours. That means about half the hormone is gone after one day, three-quarters after two days, and so on. Within a few days, the amount remaining is negligible. This rapid clearance is why the acute side effects (nausea, fatigue, headache) resolve so quickly compared to ongoing hormonal birth control, which delivers a continuous supply.
Side Effects That Aren’t Normal
Severe abdominal pain that doesn’t improve, very heavy bleeding that soaks through a pad every hour, or any side effect that worsens after the first couple of days falls outside the expected pattern. These symptoms don’t necessarily mean something is wrong, but they do warrant a call to a healthcare provider. The typical Plan B experience is mild discomfort for a day, some cycle irregularity over the following weeks, and then a return to normal.