Who should not take levonorgestrel?

Women who have a known history of allergy to levonorgestrel should not take it, nor should women who think they are already pregnant. Girls who have not yet had a menstrual cycle and postmenopausal women should not use levonorgestrel. In addition, levonorgestrel may affect your blood sugar, so women with diabetes should be monitored closely while taking this medication.

You should avoid taking levonorgestrel altogether if you're taking St. John's wort, rifampin, or nevirapine, or if you have breast cancer, a blood clotting disorder, have had a stroke, or have had instances of brain bleeding or irregular vaginal bleeding.