What Is the Duration of Plan B Side Effects?

Emergency contraception -- otherwise known as the morning-after pill -- is a safe and effective method to prevent pregnancy if your birth control method failed or you had unprotected sex. Plan B, a brand of emergency contraception, comes in 2 strengths. Regular Plan B consists of 2 pills taken 12 hours apart, each containing 0.75 mg of levonorgestrel, a progesteronelike hormone. Plan B One-Step consists of a single-dose pill containing 1.5 mg of levonorgestrel. Both are well-tolerated, yet there are some side effects, most of which resolve within 24 hours.

The most common side effects is nausea. (Image: Justin Sullivan/Getty Images News/Getty Images)

Nausea and Vomiting

The most common side effect associated with Plan B is nausea, which usually goes away within 24 hours. According to the American College of Obstetricians and Gynecologists, or ACOG, Plan B causes nausea in approximately 18 percent of women with only 4 percent experiencing vomiting. The original method of emergency contraception introduced in the 1970s and 1980s -- known as the Yuzpe method -- had significantly higher rates of nausea and vomiting, and anti-nausea medication was often needed. Because Plan B is associated with a much lower rate of nausea and vomiting, anti-nausea treatment is rarely necessary.

Irregular Bleeding

Plan B may cause irregular bleeding that can occur anytime in the month after treatment. Plan B appears to work by preventing ovulation, although the precise mechanism is still unclear. If ovulation is delayed, it could affect the timing of the first menstrual period after treatment. In some cases it occurs a week earlier than expected, and in other cases it can be a week late. In addition to an unpredictable cycle, some women experience irregular bleeding or spotting up to a month after treatment. According to ACOG, 16 percent of women noted bleeding in the first week after treatment.

Other Side Effects

Other possible short-term side effects associated with Plan B include breast tenderness, abdominal pain, dizziness, headache and fatigue. According to an April 2014 review article published in BMC Women's Health, women who took Plan B One-Step instead of the regular Plan B noted higher rates of headache, breast tenderness and irregular bleeding. With the exception of irregular bleeding, all of these side effects should resolve within a few days after treatment, and most go away with 24 hours.

Follow Up

Plan B is safe and effective for most women. According to ACOG, no deaths or lasting complications are associated with use of Plan B, and it has a success rate of 60 percent to 90 percent. Seek medical advice if your period is more than a week late or if you have irregular bleeding and pelvic pain that does not go away. Medical evaluation is needed as you may be pregnant or have a tubal pregnancy or a miscarriage.