Clinical Research Directory

Inclusion Criteria: 1. Provide written informed consent. 2. Pre-menopausal female. 3. 18 - 35 years of age. 4. Naturally cycling (i.e., not using any form of hormonal contraception for ≥ 3 months). 5. Eumenorrheic (i.e., experiencing a predictable menstrual cycle lasting 21-45 days). 6. In the surrounding Hamilton area, or able to visit McMaster for study visits. 7. Not pregnant. Exclusion Criteria: 1. Known hypersensitivity to the active substance levonorgestrel or to any ingredient in this formulation (i.e., levonorgestrel colloidal silicon dioxide, potato starch, magnesium stearate, talc, cornstarch, lactose monohydrate, or any ingredient/excipient in the placebo capsule. 2. Known or suspected pregnancy. 3. At risk of the development of unwanted pregnancy within the past 72 hours. 4. Known, suspected, or history of cardiovascular, metabolic, or musculoskeletal disease. 5. Undiagnosed abnormal vaginal bleeding. 6. Use of anti-HIV drugs, anticonvulsant drugs, antibiotics, rifampicin/rifabutin, ulipristal acetate, Griseofulvin, Hypericum perforatum (St John's wort) or any other drug that may interfere with the normal pharmacokinetic or pharmacodynamic characteristics of the study intervention. 7. Active or history of migraine. 8. A condition or factor that would prevent the use of Plan B®.