According to WebMD, a woman can experience menstrual changes after taking Plan B. Most commonly, a Plan B user experiences lighter periods, heavier periods or irregular spotting for a few weeks after taking the drug.
Plan B's official websites notes that women should abstain from unprotected sex until their period arrives after using the emergency contraceptive. Plan B warns that women who are over a week late or three weeks total without a period after taking the emergency contraceptive should take a pregnancy test to ensure that they are not pregnant. Additionally, women who have taken Plan B shortly before a Pap smear should let their doctor know, as the drug can affect the results.