As with other dopaminergic antagonists, the most common side effects of prochlorperazine are EPRs (acute dystonias, akathisias, neuroleptic malignant syndrome [uncommon], and rarely, akinesias and dyskinesias) and sedation. Marked hypotension may also result if IV prochlorperazine is administered rapidly at high doses. Administration over at least 30 minutes appears adequate to prevent hypotensive episodes.[6,7,8]
Droperidol and haloperidol
Droperidol and haloperidol represent another class of dopaminergic (D2 subtype) receptor antagonists that are structurally and pharmacologically similar to the phenothiazines. While droperidol is used primarily as an adjunct to anesthesia induction, haloperidol is indicated as a neuroleptic antipsychotic drug; however, both agents have some antiemetic activity. Droperidol is administered IM or IV, typically from 1 to 2.5 mg every 2 to 6 hours, but higher doses (up to 10 mg) have been safely given.[9,10] Haloperidol is administered IM, IV, or orally, typically from 1 to 4 mg every 2 to 6 hours. Results of a small, uncontrolled, open-label study showed some efficacy for haloperidol in palliative care patients. Both agents may produce EPRs, akathisia, hypotension, and sedation.
Dopamine 2 Antagonists
Metoclopramide is a substituted benzamide, which, prior to the introduction of serotonin (5-HT3) receptor antagonists, was considered the most effective single antiemetic agent against highly emetogenic chemotherapy such as cisplatin. Although metoclopramide is a competitive antagonist at dopaminergic (D2) receptors, it is most effective against acute vomiting when given IV at high doses (e.g., 0.5–3 mg/kg/dose), probably because it is a weak competitive antagonist (relative to other serotonin antagonists) at 5-HT3 receptors. It may act on the CTZ and the periphery. Metoclopramide also increases lower esophageal sphincter pressure and enhances the rate of gastric emptying, which may factor into its overall antiemetic effect. It can be administered IV at the U.S. Food and Drug Administration (FDA)–approved dose of 1 to 2 mg/kg every 2 hours (or less frequently) for three to five doses. Metoclopramide has also been safely given by IV bolus injection at higher single doses (up to 6 mg/kg) and by continuous IV infusion, with or without a loading bolus dose, with efficacy comparable to multiple intermittent dosing schedules.[13,14,15]
Metoclopramide is associated with akathisia and dystonic extrapyramidal effects; akathisia is seen more frequently in patients older than 30 years, and dystonic extrapyramidal effects are seen more commonly in patients younger than 30 years. Diphenhydramine, benztropine mesylate, and trihexyphenidyl are commonly used prophylactically or therapeutically to pharmacologically antagonize EPRs.[7,16] While cogwheeling rigidity, acute dystonia, and tremor are responsive to anticholinergic medications, akathisia—the subjective sense of restlessness or inability to sit still—is best treated by the following:
- Switching to a lower potency neuroleptic for vomiting, if possible.
- Lowering the dose.
- Adding a benzodiazepine (e.g., lorazepam) or beta blocker (e.g., propranolol).