Plan B, also known as the "morning after pill," is actually two pills that contain the hormone levonorgestrel. The first pill is taken within 72 hours of unprotected sex; the second pill is taken 12 hours later. A newer formulation--Plan B OneStep--consists of one pill taken within 72 hours of unprotected sex. Both Plan B and Plan B OneStep are known as "emergency contraception" and can help prevent unplanned pregnancy. According to the online pharmacology reference MicroMedex, the pregnancy rate after taking Plan B is only 1.1 percent. However, several side effects do occur; patients should be aware of them.
Hormone-Related Side Effects
Since Plan B is itself a hormone, it is not surprising that some of its side effects relate to the endocrine, or hormone, system. If a woman takes Plan B and is expecting her period within the first few days after taking the medication, she may find that her period is later than anticipated. Nearly one-third of women may find that their periods, after taking Plan B, are heavier than usual. Finally, about 10 percent of women may experience breast tenderness.
Digestive System-Related Side Effects
Nausea is a relatively common side effect of Plan B. According to the online medical reference UpToDate, between 14 and 23 percent of women experience this annoying effect. Abdominal pain is also common; the Plan B website lists "lower abdominal pain" as one of the prominent possible side effects and UpToDate reports that between 12 and 18 percent of women may suffer from this effect.
Central Nervous System-Related Side Effects
Within the first few days after taking Plan B, approximately 10 to 17 percent of patients may complain of headaches. According to the "Clinician's Pocket Reference," fatigue may also be an issue. Finally, dizziness is another side effect that is listed on the Plan B website. UpToDate indicates that around 10 percent of women taking the morning-after pill will have to deal with this side effect.