Drug Interactions between haloperidol and Phenergan with Dextromethorphan

MONITOR CLOSELY: Haloperidol can cause dose-related prolongation of the QT interval. Theoretically, coadministration with other agents that can prolong the QT interval may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. Haloperidol treatment alone has been associated with a number of reported cases of torsade de pointes and sudden death. The majority of cases involved intravenous administration or use of higher than recommended dosages. In general, the risk of an individual agent or a combination of agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia). The extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s). In addition, certain agents with anticholinergic properties (e.g., sedating antihistamines; antispasmodics; neuroleptics; phenothiazines; skeletal muscle relaxants; tricyclic antidepressants) may have additive parasympatholytic and central nervous system-depressant effects when used in combination with haloperidol. Excessive parasympatholytic effects may include paralytic ileus, hyperthermia, mydriasis, blurred vision, tachycardia, urinary retention, psychosis, and seizures.

MANAGEMENT: Caution is recommended if haloperidol is used in combination with other drugs that can prolong the QT interval, particularly when administered intravenously or at higher than recommended dosages. Haloperidol is not approved by the FDA for intravenous administration. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope. In addition, if combination therapy with agents with anticholinergic properties is required, caution is advised, particularly in the elderly and those with underlying organic brain disease. Patients should be advised to notify their physician promptly if they experience potential symptoms of anticholinergic intoxication such as abdominal pain, fever, heat intolerance, blurred vision, confusion, and/or hallucinations. Ambulatory patients should be counseled to avoid activities requiring mental alertness until they know how these agents affect them. A reduction in anticholinergic dosages may be necessary if excessive adverse effects develop.

MONITOR: Coadministration with CYP450 2D6 inhibitors may increase the plasma concentrations of dextromethorphan in patients who are extensive metabolizers of this isoenzyme (approximately 93% of Caucasians and more than 98% of Asians and individuals of African descent). The proposed mechanism is inhibition of the CYP450 2D6-mediated O-demethylation of dextromethorphan. Increased plasma concentrations increase the risk of dextromethorphan-related adverse effects (e.g., agitation, confusion, tremor, insomnia, diarrhea, and respiratory depression) and serotonin syndrome. Coadministration of dextromethorphan (60 mg orally, once) with the CYP450 2D6 inhibitor panobinostat (20 mg orally once a day on days 3, 5, and 8) in 14 patients with advanced cancer had a highly variable effect on dextromethorphan levels, increasing the peak plasma concentration (Cmax) of dextromethorphan by 20% to 200%, and total systemic exposure (AUC 0 to infinity) by 20% to 130%, compared to dextromethorphan given alone. The moderate CYP450 2D6 inhibitor asunaprevir, given at 200 mg twice daily, also increased Cmax and AUC of a single 30 mg dose of dextromethorphan by 2.7- and 3.9-fold, respectively, in 17 study subjects.

MANAGEMENT: Caution should be exercised if these drugs must be used together. Patients should be monitored for signs of dextromethorphan adverse effects (e.g., agitation, confusion, tremor, insomnia, diarrhea, and respiratory depression) and serotonin syndrome, and advised to notify their health care professional if these adverse effects develop or worsen. Dose reduction of dextromethorphan may also be required.

MONITOR: Central nervous system- and/or respiratory-depressant effects may be additively or synergistically increased in patients taking multiple drugs that cause these effects, especially in elderly or debilitated patients. Sedation and impairment of attention, judgment, thinking, and psychomotor skills may increase.

MANAGEMENT: During concomitant use of these drugs, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression. Cautious dosage titration may be required, particularly at treatment initiation. Ambulatory patients should be counseled to avoid hazardous activities requiring mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.