Although the Plan B pill can affect the timing of the next period after its use, it should not stop menstruation once it has started. This pill can cause menstrual flow issues at the next period after its use, states WebMD.
Plan B can make the menstrual cycle come early or late by a week of its normally expected time if it has worked. If there is no menstruation within 4 weeks after using a morning after pill, it is important to take a pregnancy test, recommends Mayo Clinic. Plan B can also lead to either an increase or decrease in menstrual flow at the following period after using this pill, notes WebMD.
Plan B is referred to as a morning after pill because it is mainly used as emergency contraception after unprotected sex.