Other Antiemetics
Promethazine and metoclopramide both treat nausea, but one can cause a terrifying involuntary movement disorder — and the age group most at risk may surprise you.
Core Concept
Promethazine is a first-generation antihistamine (H1 blocker) and phenothiazine that suppresses the chemoreceptor trigger zone (CTZ) and vestibular system. It carries strong anticholinergic and sedating effects. It is contraindicated in children under 2 years due to risk of fatal respiratory depression. IV promethazine can cause severe tissue necrosis — the preferred route is deep IM; if given IV, it must be diluted and administered slowly through a large-bore, free-flowing IV line. Metoclopramide is a dopamine (D2) antagonist that blocks the CTZ and also increases upper GI motility (prokinetic effect), making it uniquely useful for gastroparesis and GERD in addition to nausea. Its dopamine-blocking action creates a significant risk: extrapyramidal symptoms (EPS) — acute dystonia, tardive dyskinesia — especially in young adults and with prolonged use. The FDA black box warning limits use to no more than 12 weeks due to tardive dyskinesia risk. Diphenhydramine (Benadryl) IV is the rescue treatment for acute EPS. Both drugs cause significant sedation and should not be combined with CNS depressants or alcohol.
Watch Out For
Don't confuse metoclopramide's prokinetic action with a simple antiemetic — it increases gastric motility, while promethazine does not. Students mix up promethazine (phenothiazine/antihistamine) with ondansetron (5-HT3 blocker); promethazine causes heavy sedation and anticholinergic effects, ondansetron does not. EPS risk belongs primarily to metoclopramide (dopamine blocker) due to its strong D2 antagonism — it carries the FDA black box warning for tardive dyskinesia. Promethazine, as a phenothiazine, has a lower but real EPS risk; however, metoclopramide's EPS risk is the high-yield NCLEX concern.
Clinical Pearl
Metoclopramide moves the gut AND blocks dopamine — think 'Metoclo-MOVES and Metoclo-SHAKES.' The moving is therapeutic; the shaking (EPS) is the danger.
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