Does plan b effect periods?

It can: Affect bleeding a little bit by changing the lining of the uterus, that's how it works, but it doesn't affect ovulation (egg release) which affects a monthly cycle.

It certainly might.: Potential side effects w emergency contraception include: menstrual flow changes (spotting, bleeding, early or late period), nausea, vomiting, dizziness, fatigue, headache, breast tenderness & abdominal pain or cramps. Take pregnancy test & notify dr if your period is 7 or > days late.