Haloperidol, drug interactions

It may be fair to ask, Is this much ado about nothing Although test tube studies have shown that particular drugs interact, it is seldom that this causes problems for patients taking the medications. For example, the antidepressant fluvoxamine inhibits the enzyme that deactivates the antipsychotic haloperidol (Haldol). Does this mean that fluvoxamine and haloperidol cannot be taken together By no means. Although this would probably raise the blood level of haloperidol somewhat, the main effect if any would be that a smaller dose of haloperidol would be more effective. 

Hypersensitivity to the drug or any other component of the product (cross-sensitivity between phenothiazines may occur) comatose or greatly depressed states caused by CNS depressants or from any other cause (phenothiazines, clozapine, loxapine, molindone, pimozide, haloperidol) coadministration with other drugs that prolong the QT interval and in patients with congenital long QT syndrome or history of cardiac arrhythmias (mesoridazine, thioridazine, pimozide, ziprasidone see Drug..Interactions). 

Clinicians should be aware of a few drug interactions with Zolpidem. Flumazenil acts as an antagonist to the hypnotic effects of zolpidem. There is decreased alertness when zolpidem is combined with cimetidine. There is an increase in anterograde amnesia in volunteers treated with a combination of imipramine and zolpidem. Haloperidol, ranitidine, chlorpromazine, warfarin, and digoxin, along with cimetidine and flumazenil, do not alter the pharmacokinetics of zolpidem (Salva and Costa, 1995). 

The use of two or more agents concurrently has the potential for possibly deleterious drug interactions. Thus, as noted in subsequent chapters, the addition or elimination of an agent may significantly alter the activity of the concurrent drug treatment (e.g., carbamazepine lowering haloperidol plasma levels nefazodone increasing the levels of triazolam). 

A pharmacodynamic drug interaction could not be excluded when a 60-year-old man developed delirium at a serum lithium concentration of 0.97 mmol/1 when taking lithium and haloperidol (158). 

The addition of low-dose fluvoxamine (50-100 mg/day) to neuroleptic drug treatment may improve the negative symptoms in patients with schizophrenia, but involves a risk of a drug interaction. In 12 in-patients with schizophrenia receiving 6 mg/day of haloperidol, incremental doses of fluvoxamine (25, 75, and 150 mg/day for 2 weeks each) respectively increased haloperidol plasma concentrations by 120%, 139%, and 160% of those before fluvoxamine co-administration in spite of the increase, there were no particular adverse effects (50). 

Iwahashi K, Miyatake R, Suwaki H, Hosokawa K, Ichikawa Y. The drug-drug interaction effects of haloperidol on plasma carbamazepine levels. Clin Neuropharmacol 1995 18(3) 233—6. 

Figure 10.10 Top antipsychotics haloperidol and risperidone. Bottom H3 receptor antagonists/inverse agonists ciproxifan and ABT-239 (Zhang, M., et ah Lack of cataleptogenic potentiation with non-imidazole H3 receptor antagonists reveals potential drug-drug interactions between imidazole-based H3 receptor antagonists and antipsychotic drugs. Brain Res. 2005, 1045, 142-149).

The increased hypotension and orthostasis that can occur if phenothiazines are used with antihypertensive drugs such as clonidine is established. Note that, of the phenothiazines, levomepromazine is particularly associated with postural hypotension. One report suggests that haloperidol may interact similarly. Monitor, particularly during the initial stages of concurrent use, and warn patients that if they feel faint and dizzy they should lie down, and that they should remain lying down until symptoms abate completely. Dosage adjustment may be necessary. 

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