How Does Plan B Affect Your Period?

Plan B One-Step is an emergency contraceptive medication used to prevent pregnancy after unprotected sexual intercourse or a contraceptive failure. It is available by prescription for women younger than 17 years old, and is available as an over-the-counter product for women older than 17 years and older.

Common Menstrual Changes

One of the common adverse reactions to this medication is a change in menstrual periods. Approximately 30 percent of women in clinical trials experienced heavier menstrual bleeding after taking a Plan B tablet, according to the Plan B prescribing information. The Plan B prescribing information also notes that approximately 4 percent of women experienced a late menstrual period after taking Plan B. Menstruation for these women occurred over seven days later than expected.

Other Menstrual Changes

Some women also experience periods that are lighter than usual or come earlier than expected, according to the official Plan B website, but these reactions do not happen as frequently. Spotting may also occur.


Plan B may also cause other temporary side effects, some of which may accompany the altered menstruation or spotting and be mistaken for changes to the period. These side effects may include nausea, tiredness, headache, dizziness, breast pain or vomiting, according to Women who vomit within one hour of taking the Plan B tablet should contact a doctor, as the medication may not have not had time to work and they may not be protected against pregnancy. Any woman with questions or concerns about side effects should contact a doctor.


Women who have severe abdominal pain in addition to vaginal bleeding or spotting may not be experiencing menstruation, but an ectopic pregnancy. This occurs when a fertilized egg implants in a fallopian tube or another undesirable location instead of in the uterus. Ectopic pregnancies can pose serious and life-threatening complications to the mother. Any woman experiencing severe abdominal pain--with or without bleeding-- should call a doctor or seek immediate medical attention.


Although Plan B medication is effective at preventing unintended pregnancy, it does not work for everyone. Women who have not gotten their menstrual period within one week of the expected time may not be experiencing a side effect of the medication, but may actually be pregnant. Women who have a period that is a week late should always take a pregnancy test or see a doctor to rule out pregnancy.