Haloperidol’s well-known toxicity
Haloperidol has been associated with numerous adverse effects:
Akathisia and other acute EPS. Treatment with even a single dose of IM haloperidol can result in acute EPS, including dystonia and akathisia. At best, such adverse effects are subjectively troubling and unpleasant; at worst, akathisia can exacerbate and be mistaken for agitation, leading to administration of more medication23 and the possible development of suicidal or violent behavior.24-25 In the studies reviewed above, the overall rate of EPS was as high as 21% after treatment with haloperidol,16 with parkinsonism occurring in up to 17% of patients,19 dystonia in up to 11%,7 and akathisia in up to 10%.15 However, because specific EPS were assessed inconsistently, and sometimes not at all, the rate of akathisia—arguably the most relevant and counter-therapeutic adverse effect related to agitation—remains unclear.
In another study that specifically assessed for akathisia in patients treated with haloperidol, up to 40% experienced akathisia 6 hours after a single oral dose of 5 mg.26 Even a single dose of IV prochlorperazine, another dopamine-antagonist routinely used to treat nausea in the emergency department (ED), has been reported to cause akathisia in up to 44% of patients.27 Such results suggest that when akathisia is carefully assessed, the rate with even brief FGA exposure may approach nearly half of treated patients. Because akathisia is typically dose-related, and considering that many patients receiving IM haloperidol may receive multiple injections in addition to standing doses of oral medications, akathisia may be underrecognized in patients who are agitated, with a much greater risk than is generally presumed.
Although anticholinergic medications or benzodiazepinesare often administered as part of a haloperidol “cocktail,” these medications often do not adequately resolve emergent akathisia.26,28 No clinical trials of IM haloperidol combined with benztropine or diphenhydramine have been published, but several studies suggest that combining haloperidol with promethazine—a phenothiazine with strong antihistaminergic and anticholinergic activity, but only weak antidopaminergic activity—can decrease the risk of dystonia relative to haloperidol alone.8,22,29,30 However, there have also been reports of promethazine causing dystonia.31,32 In addition, 1 trial of IM haloperidol, 2.5 mg, combined with promethazine reported that 74% of patients still had at least 1 form of EPS.30 Because the clinical trials of haloperidol with promethazine did not specifically assess for akathisia, promethazine’s ability to decrease the risk of akathisia remains unknown.
Cardiotoxicity. Although low-potency antipsychotic medications such as chlorpromazine are more sedating than haloperidol, the latter is preferred as an IM antipsychotic medication for agitation because of its lower risk of hypotension.2 In terms of cardiac effects, all antipsychotic medications carry a risk of QTc prolongation, with possible progression to the potentially lethal arrhythmia torsades de pointes as a result of interference with cardiac potassium channels.33 In 2007, the FDA added a “black-box” warning about this risk for haloperidol, in the wake of a disproportionately high number of reported cases associated with IV administration, sometimes even after a single dose.34
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