Drug Interactions between propoxyphene and trimipramine

MONITOR CLOSELY: Concomitant use of propoxyphene and tricyclic and tetracyclic antidepressants (TCAs) may result in additive central nervous system (CNS) depressant effects. Misuse of propoxyphene, either alone or in combination with other CNS depressants, has been a major cause of drug-related deaths, particularly in patients with a history of emotional disturbances, suicidal ideation, or alcohol and drug abuse. In a large Canadian study, propoxyphene use was also associated with a 60% increased risk of hip fracture in the elderly, and the risk was further increased by concomitant use of psychotropic agents including antidepressants, presumably due to additive psychomotor impairment. Therefore, these drugs may constitute a dangerous combination in certain susceptible populations. Pharmacokinetically, propoxyphene is a CYP450 2D6 inhibitor and may increase the plasma concentrations of TCAs such as clomipramine, desipramine, doxepin, maprotiline, and nortriptyline that are primarily metabolized by the isoenzyme. In one case report, doxepin serum concentrations doubled in an 89-year-old patient following the addition of propoxyphene, resulting in lethargy. A retrospective study also found that the plasma concentration/dose ratio of nortriptyline was nearly 40% higher in patients treated concomitantly with propoxyphene than in patients treated with nortriptyline alone.

MANAGEMENT: Caution is advised if propoxyphene is prescribed with TCAs, particularly in the elderly and in patients with a history of emotional disturbances, suicidal ideation, or alcohol and drug abuse. Serum TCA levels should be monitored more closely following the addition or withdrawal of propoxyphene, and the TCA dosage adjusted as necessary. Patients should be warned not to exceed recommended dosages of either drug, to avoid alcohol, and to notify their physician if they experience symptoms of toxicity such as lethargy, excessive sedation, dizziness, syncope, seizures, and/or irregular heartbeat. In addition, they should avoid activities requiring mental alertness until they know how these agents affect them.