The Influence of Plan B® on Cardiovascular Health.

The goal of this double-blind randomized controlled trial is to learn about the impacts of a single dose of Plan B® on the cardiovascular system in young, health, premenopausal females. The main questions it aims to answer are: 1. What is the short-term affect of Plan B® affect on the venous system? 2. What is the short-term affect of Plan B® affect on the health of arteries? Researchers will compare a single dose of Plan B® (1.5 mg of levonorgestrel) to a placebo to see if Plan B® has unique effects on the cardiovascular system. Participants will: 1. Undergo baseline measurements of venous thrombosis markers, blood pressure, and measures of arterial function using ultrasound. 2. Be randomized to either take a Plan B® or placebo pill 3. Repeat baseline measurements of venous thrombosis markers, blood pressure, and measures of arterial function using ultrasound each hour, for 6-hours, and then 24- and 48- hours after taking the Plan B® or placebo pill.

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®.