BackgroundTwo pharmacokinetic studies were performed to investigate whether there is any interaction between etonogestrel or ethinylestradiol released from the combined contraceptive vaginal ring NuvaRing®and concomitant treatment with orally administered amoxicillin or doxycycline.MethodsIn one study, healthy women were randomised to receive either NuvaRing®alone for 21 days or NuvaRing®for 21 days plus amoxicillin on days 1–10. After a 7-day ring-free washout period, women were crossed over to the alternate regimen for a further 21-day treatment period. The other study used an identical design except that women received doxycycline instead of amoxicillin. The amoxicillin study measured serum etonogestrel and ethinylestradiol levels and area under the serum concentration-time curve (AUC) values over the initial 12 hours on days 1 (AUC12) and 10 (AUC216–228) and the whole of days 1–11 (AUC240) and 1–22 (AUC504). The doxycycline study measured AUC values over the initial 24 hours on days 1 (AUC24) and 10 (AUC216–240) and the whole of days 1–11 (AUC240) and 1–22 (AUC504).ResultsNo differences in etonogestrel or ethinylestradiol serum concentrations were observed between subjects using NuvaRing®alone versus those receiving the ring plus either of the antibiotics. Calculation of etonogestrel and ethinylestradiol interaction/control ratios confirmed the absence of pharmacokinetic interactions.ConclusionThe results from these studies demonstrate the absence of pharmacokinetic interactions between etonogestrel and ethinylestradiol released from NuvaRing®and the oral antibiotics amoxicillin and doxycycline, suggesting that contraceptive efficacy would also be unaffected.