This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The drug interaction between oral contraceptives and oral antibiotics may reduce the exposure of the body to the contraceptive steroids leading to the legitimate concern that contraceptive efficacy may be reduced or lost. This potential for reduced efficacy has become more likely in recent years because of the lower recommended doses of oral contraceptives and the frequency with which antibiotics and oral contraceptives are co-prescribed. An interaction with doxycycline is of particular interest because of the expected widespread use of this antibiotic during an outbreak of anthrax infection following a bioterrorism attack. We plan to quantify the effect long-term doxycycline dosing on the pharmacokinetics ethinylestradiol and norethindrone and on the plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and progesterone as surrogate markers of ovulation. The frequency of breakthrough bleeding will be determined as an additional index of oral contraceptive efficacy. This will be an open-label, fixed order, two-period (two cycle per period), study of 24 volunteers.
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