Plan B's common side effects are nausea, vomiting, headache, dizziness, tiredness, breast tenderness, and lower belly pain, plus a next period that may come early, late, heavier, or lighter.
Most resolve within a day or two. Three situations need action. Vomiting within 2 hours of the dose means the pill likely did not absorb, so retake it. A period missing after 3 weeks means take a pregnancy test. And severe lower abdominal pain 3 to 5 weeks after taking levonorgestrel is the one true red flag, since it can indicate an ectopic pregnancy; that needs urgent care. Menstrual changes are the most lingering effect: your cycle should normalize the following month. Taking Plan B repeatedly is not known to be dangerous, but it reliably scrambles cycle timing, which is a practical reason to set up a regular method rather than leaning on emergency contraception.
What to do
Common questions
What are Plan B side effects?
Plan B's side effects are common, mild, and short-lived for most people. The ones listed across the label and MedlinePlus: nausea (the most frequent), vomiting, headache, dizziness, tiredness, breast pain or tenderness, and lower abdominal pain. Diarrhea shows up for some. Your next period is the other big one: it can come earlier or later than expected and be heavier, lighter, or spottier than usual, all normal for one cycle. Things to actually act on: if you vomit within 2 hours of taking the pill, it probably did not absorb, so take another dose. If your period is more than a week late, or has not come within 3 weeks, take a pregnancy test, since a failed EC attempt often announces itself that way. The one serious red flag: severe lower abdominal pain 3 to 5 weeks after taking levonorgestrel, which can signal an ectopic pregnancy and needs immediate medical attention. Side effects typically fade within a day or two. Frequent use is not dangerous per se, but it makes periods irregular, which is one reason regular birth control beats repeat emergency contraception.