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Pharmacokinetics of the levonorgestrel-only emergency contraception regimen in normal-weight, obese and extremely obese users: A pilot study

Melissa Natavio, University of Southern California, 2012

Project abstract

Obesity has been reported as a risk factor for emergency contraception failure. A paucity of studies has shown that obesity alters the pharmacokinetic behavior of steroid hormones, however, it is unclear if these changes result in a decrease in their efficacy. No study to date has examined the effects of obesity on the pharmacokinetic behavior of levonorgestrel (LNG) emergency contraception (EC).

Thirty healthy women between 18-35 years of age with regular menstrual cycles who are non-smokers will be enrolled in this pilot study to determine the pharmacokinetic profile of normal weight, obese and extremely obese users of LNG-EC. Excluded will be women using hormonal contraception or using a cytochrome P450 inhibitor medication.

Participants will undergo a collection of 14 venous blood samples for LNG during the first 96 hours after administration of 1.5mg LNG on day 8 of their cycle. Participant's serum albumin and sex-steroid binding hormone will be measured in order to then calculate free LNG using a novel algorithm. Serum follicular-stimulating hormone, luteinizing hormone, estradiol and progesterone will be measured to determine if ovulation inhibition is altered among the groups.

The proposed research will begin to elucidate the mechanism(s) underlying the increased failure rate of LNG-EC reported in obese women. Findings from this study will be critically important to support future grant proposals to interrogate the effects of obesity on EC efficacy. Information gained from studies on the effects of obesity on LNG-EC efficacy could influence recommendations for EC use in the obese population, such as increasing the dose of LNG when used for EC.

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