Erythromycin and other macrolides increase risk of bleeding when taking Warfarin. 2 possibilities 1) change gut flora, so decrease amt of VitK, so increase action of Warfarin. 2) inhibit liver enzyme responsible for Warfarin metabolism.

Also: increases Digoxin bioavailability by the above 2 mechanisms and also maybe inhibitin p-glycopritein transport system in kidney for Dig.