Drug Interaction Report

Anticholinergic agents may increase the absorption and oral bioavailability of nitrofurantoin. The proposed mechanism involves increased gastrointestinal transit time due to reduction of stomach and intestinal motility by anticholinergic agents. In six healthy volunteers, pretreatment with propantheline 45 minutes before a single 100 mg dose of nitrofurantoin significantly increased the oral bioavailability of nitrofurantoin as indicated by a 68% increase in urinary recovery. In another study, atropine 500 mcg given subcutaneously one-half hour before a single 100 mg dose of nitrofurantoin delayed the absorption and urinary excretion of nitrofurantoin but did not affect its overall bioavailability in ten healthy volunteers. The clinical significance of these changes is unknown.

The consumption of grapefruit juice may be associated with increased plasma concentrations of darifenacin. The mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. The clinical significance is unknown.