Potential drug-drug interaction between warfarin and norethindrone in adolescent females: A case series.

This article focuses on a potential drug interaction between warfarin, a vitamin K–dependent oral anticoagulant commonly used in pediatric patients with mechanical heart valves, and norethindrone, a progestin-only hormonal therapy used to manage heavy menstrual bleeding (HMB) in adolescent females. A case series of three adolescent females with mechanical mitral valve replacements demonstrated that initiation of norethindrone at doses used for HMB led to significant increases in their international normalized ratio (INR), necessitating warfarin dose reductions of 29% to 50% over several weeks. The interaction appears to have a delayed onset and offset, with stable warfarin dosing achieved 5 to 12 weeks after starting or stopping norethindrone. The authors recommend close INR monitoring and aggressive warfarin dose adjustments during norethindrone initiation and discontinuation to maintain therapeutic anticoagulation.