Although the patient should be informed that their next period may be heavier or lighter, earlier or later, 95% of patients have their menstrual period
at the usual time or within ±7 days.
The sooner it is taken after the unprotected intercourse, the more effective it is. If taken within 72 hours, it will prevent 7 out of 8 undesired pregnancies. If you are more than a week late, do a preg test.