Objective: Case report of a pharmacokinetic interaction between diltiazem and nortriptyline.
Methods: Determination of plasma nortriptyline concentrations by HPLC. Calculation of nortriptyline clearances and half-life by formulae used routinely in therapeutic drug monitoring.
Results: The average plasma concentration of nortriptyline at steady state (Css) divided by the amount of nortriptyline administered per time rose significantly in a patient with concomitant administration of diltiazem, suggesting increased bioavailability and/or decreased clearance of nortriptyline.
Conclusions: There is a significant pharmacokinetic interaction between diltiazem and nortriptyline, which is probably due to a reduction in the first pass clearance of nortriptyline, leading to an increase in its bioavailability.