Drug Interactions between Infumorph and Regroton

MONITOR CLOSELY: Coadministration with P-glycoprotein (P-gp) inhibitors may increase the plasma concentrations of morphine and its risk of adverse effects, including hypotension, respiratory and CNS depression, profound sedation, coma, and death. The proposed mechanism may involve inhibition of the intestinal P-gp efflux transporter, resulting in enhanced oral bioavailability of morphine, a P-gp substrate. The interaction may be more significant for orally administered morphine. According to some authorities, the concomitant use of a P-gp inhibitor with oral morphine may increase the systemic exposure of morphine by approximately 2-fold. However, in a pharmacokinetic study involving 12 healthy subjects, itraconazole (200 mg daily for 4 days), a strong P-gp inhibitor, increased the morphine (0.3 mg/kg as a single oral dose) peak plasma concentration (Cmax) and systemic exposure (AUC) by 28% and 22%, respectively. Plasma concentrations of intravenously administered morphine appear less affected. In a crossover study involving 14 healthy volunteers, intravenous administration of P-gp inhibitor cyclosporine followed by an intravenous infusion of morphine 0.1 mg/kg led to a minimal increase of morphine AUC to 100 ng/mL*h compared to 85 ng/mL*h when administered after control (no infusion). In the same study, cyclosporine appeared to prolong morphine-induced miosis.

MANAGEMENT: Caution is recommended whenever morphine, particularly orally administered morphine, is used concomitantly with a P-gp inhibitor. Close clinical and laboratory monitoring should be considered whenever a P-gp inhibitor is added to or withdrawn from therapy, and the morphine dosage adjusted as necessary. Patients should be monitored closely for signs and symptoms of respiratory depression and sedation that may be greater than otherwise expected. In addition, patients should be advised to avoid driving or operating hazardous machinery until they know how these medications affect them.

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MONITOR: Many psychotherapeutic and CNS-active agents (e.g., anxiolytics, sedatives, hypnotics, antidepressants, antipsychotics, opioids, alcohol, muscle relaxants) exhibit hypotensive effects, especially during initiation of therapy and dose escalation. Coadministration with antihypertensives and other hypotensive agents, in particular vasodilators and alpha-blockers, may result in additive effects on blood pressure and orthostasis.

MANAGEMENT: Caution and close monitoring for development of hypotension is advised during coadministration of these agents. Some authorities recommend avoiding alcohol in patients receiving vasodilating antihypertensive drugs. Patients should be advised to avoid rising abruptly from a sitting or recumbent position and to notify their physician if they experience dizziness, lightheadedness, syncope, orthostasis, or tachycardia.

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MONITOR: The hypotensive effects of thiazide diuretics and alpha-adrenergic blockers may be additive. Postural hypotension may occur.

MANAGEMENT: Hemodynamic responses should be monitored during coadministration, especially during the first few weeks of therapy. Patients should be advised to take the alpha-blocker at bedtime and to notify their physician if they experience dizziness or syncope while awake.

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