Hello, Plan B is emergency contraception. It contains a type of progesterone, the same steroid hormone that’s in oral contraceptives, but at a higher dose. Though Plan B is generally considered very safe, it may cause a few potential side effects. Taking Plan B can cause your period to be lighter or heavier, or arrive earlier or come late. “These side effects are due to the high-dose progesterone acting on the lining of the uterus at variable points in time during the menstrual cycle. But don t worry because these changes are always temporary, especially when the pill is used correctly, i.e. very rarely/occasionally and not regularly and repeatedly. Your next period should be normal and come at the expected time, or within a week of the expected time. If this does not happen, I suggest consulting a local obgyn for an ultrasound and some specific tests. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Elona Xhardo, Endocrinologist