• Pharmacokinetic studies between Days 9 and 12 of the menstrual cycle before and during treatment with St. John's wort or placebo [ Time Frame: Days 9 and 12 of the menstrual cycle ] [ Designated as safety issue: No ]
  
• ovulation defined as progesterone levels 3.0 ng/ml [ Time Frame: weekly intervals after dosing until menses ] [ Designated as safety issue: No ]
  

• Mean levels of follicle-stimulating hormone, estradiol-17b (E2), luteinizing hormone, inhibin, and glycodelin drawn at weekly intervals until next menses [ Time Frame: weekly intervals until next menses ] [ Designated as safety issue: No ]
  

In the last decade, St. John's wort has become one of the most commonly used botanicals. Levonorgestrel is a form of progesterone, a female hormone involved in conception. It can be given as both a pill and an injection and is used for contraception and for the treatment of endometriosis. However, evidence suggests that St. John's wort may reduce the effectiveness of the contraceptive hormone levonorgestrel. This study will determine whether interactions between St. John's wort and levonorgestrel reduce the effectiveness of the hormone. This study will also determine whether a higher dose of levonorgestrel will override the effects of St. John's wort.

All participants will receive a single dose of levonorgestrel between Days 9 and 12 of their first menstrual cycle after entering this study. Blood and urine collection will occur immediately after the levonorgestrel is given and every week until participants' next menstrual cycle to determine the levels of reproductive hormones in participants' bodies.

At the beginning of participants' next menstrual cycle, they will be randomly assigned to one of four groups and receive either St. John's wort or placebo for 6 weeks. Group 1 will receive a placebo; Groups 2 and 3 will receive a standard dose of St. John's wort (900 mg per day); and Group 4 will receive an increased dose of St. John's wort (1500 mg per day). After 6 weeks, Groups 1, 2, and 4 will receive 150 mcg levonorgestrel; and Group 3 will receive 225 mcg levonorgestrel. Blood and urine collection will occur immediately after levonorgestrel is given and every week until participants' next menstrual cycle.

Inclusion Criteria:

• Body mass index (BMI) between 20 and 25
• Regular menstrual cycles for at least 3 months prior to study entry

Exclusion Criteria:

• Current use of foods, herbs, vitamins, over-the-counter supplements, or any medications that could alter pharmacokinetics of other drugs
• Medical contraindications to the use of contraceptives