Haldol Drops Drug Interactions

As with other antipsychotics, caution is advised when prescribing haloperidol with medications known to prolong the QT interval.
Haloperidol is metabolized by several routes, including glucuronidation and the cytochrome P450 enzyme system (particularly CYP3A4 or CYP2D6). Inhibition of these routes of metabolism by another drug or a decrease in CYP2D6 enzyme activity may result in increased haloperidol concentrations and an increased risk of adverse events, including QT-prolongation. In pharmacokinetic studies, mild to moderately increased haloperidol concentrations have been reported when haloperidol was given concomitantly with drugs characterized as substrates or inhibitors of CYP3A4 or CYP2D6 isozymes, such as, itraconazole, nefazodone, buspirone, venlafaxine, alprazolam, fluvoxamine, quinidine, fluoxetine, sertraline, chlorpromazine, and promethazine. A decrease in CYP2D6 enzyme activity may result in increased haloperidol concentrations. Increases in QTc have been observed when haloperidol was given with a combination of the metabolic inhibitors ketoconazole (400 mg/day) and paroxetine (20 mg/day). It may be necessary to reduce the haloperidol dosage.
Caution is advised when used in combination with drugs known to cause electrolyte imbalance.
Effect of Other Drugs on Haloperidol: When prolonged treatment with enzyme-inducing drugs such as carbamazepine, phenobarbital, rifampicin is added to Haldol therapy, this results in a significant reduction of haloperidol plasma levels. Therefore, during combination treatment, the Haldol dose should be adjusted, when necessary. After stopping such drugs, it may be necessary to reduce the dosage of Haldol.
Sodium valproate, a drug known to inhibit glucuronidation, does not affect haloperidol plasma concentrations.
Effect of Haloperidol on Other Drugs: In common with all neuroleptics, Haldol can increase the central nervous system depression produced by other CNS-depressant drugs, including alcohol, hypnotics, sedatives or strong analgesics. An enhanced CNS effect, when combined with methyldopa, has also been reported.
Haldol may antagonise the action of adrenaline and other sympathomimetic agents and reverse the blood-pressure-lowering effects of adrenergic-blocking agents such as guanethidine.
Haldol may impair the antiparkinson effects of levodopa.
Haloperidol is an inhibitor of CYP2D6. Haldol inhibits the metabolization of tricyclic antidepressants, thereby increasing plasma levels of these drugs.
Other Forms of Interaction: In rare cases the following symptoms were reported during the concomitant use of lithium and haloperidol: encephalopathy, extrapyramidal symptoms, tardive dyskinesia, neuroleptic malignant syndrome, brain stem disorder, acute brain syndrome and coma. Most of these symptoms were reversible. It remains unclear whether this represents a distinct clinical entity.
Nonetheless, it is advised that in patients, who are treated concomitantly with lithium and Haldol, therapy should be stopped immediately if such symptoms occur.
Antagonism of the effect of the anticoagulant phenindione has been reported.