Drug interactions involving protease inhibitors are common. Protease inhibitors 
are well known inhibitors of the 3A4 isozyme of cytochrome P450. Select protease 
inhibitors, including co-formulated lopinavir/ritonavir, may induce 
glucuronidation or the activity of other CYP450 isozymes. We describe the case 
of a patient taking warfarin who experienced a significantly decreased 
international normalized ratio after the initiation of antiretroviral therapy 
that included lopinavir/ritonavir. We review the possible mechanisms of this 
interaction and the reported interactions between warfarin and other protease 
inhibitors.