Rx for vomiting – Orris AI

Rx for vomiting

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Here is a comprehensive overview of antiemetic (anti-vomiting) pharmacotherapy, drawn from Katzung's Basic and Clinical Pharmacology (16th ed.) and supporting emergency medicine sources:

Antiemetic Drug Classes & Their Prescriptions

1. 5-HT₃ Receptor Antagonists (First-line for most settings)

Drug	Dose	Notes
Ondansetron	4–8 mg PO/IV/ODT q6–8h	Drug of choice; ODT formulation preferred for active vomiting
Granisetron	1 mg PO/IV	Chemotherapy-induced N/V
Palonosetron	0.25 mg IV (single dose)	Long half-life; CINV prevention

Mechanism: Block 5-HT₃ receptors in the gut and chemoreceptor trigger zone (CTZ).

2. Dopamine (D₂) Antagonists / Prokinetics

Drug	Dose	Notes
Metoclopramide	10 mg PO/IV/IM q6h	Also a prokinetic; first-line in pregnancy (oral preferred)
Prochlorperazine	5–10 mg PO/IV/IM q6h; 25 mg PR	Phenothiazine; useful 2nd line; rectal suppository available
Promethazine	12.5–25 mg PO/IV/IM/PR q4–6h	Sedating; IV carries vascular damage risk — prefer IM or PR
Haloperidol	0.5–2 mg PO/SC	Useful when phenothiazines fail

Mechanism: Block D₂ receptors in the CTZ; some also block muscarinic/histamine receptors.

Risk: All D₂ antagonists can cause extrapyramidal side effects (EPS), including acute dystonia — treat with diphenhydramine 25–50 mg IV.

3. Antihistamines (H₁ Blockers)

Drug	Dose	Notes
Dimenhydrinate (Dramamine)	25–50 mg IV/PO q4–6h	Motion sickness, vestibular causes
Diphenhydramine	25–50 mg PO/IV/IM q6h	Also used for EPS prophylaxis
Meclizine	25 mg PO q6h	Motion sickness

Mechanism: H₁ + muscarinic blockade in the vestibular pathway.

4. NK1 (Neurokinin) Receptor Antagonists (Chemotherapy-induced)

Drug	Dose	Notes
Aprepitant	125 mg PO day 1, then 80 mg/day × 2 days	CYP3A4 inhibitor — check interactions
Fosaprepitant	115 mg IV (single dose, day 1)	IV prodrug of aprepitant
Rolapitant	180 mg PO (single dose)	180-hr half-life
Netupitant/palonosetron	300/0.5 mg PO (single dose)	Combo product (Akynzeo)

Mechanism: Block NK1 receptors in the area postrema centrally.

Combined NK1 + 5-HT₃ antagonist + dexamethasone prevents acute emesis in 80–90% of patients on highly emetogenic chemotherapy.

5. Corticosteroids (Adjunct, especially for CINV)

Drug	Dose
Dexamethasone	8–20 mg IV/PO before chemotherapy, then 8 mg/day × 2–4 days

Mechanism: Unknown; synergistic with 5-HT₃ antagonists.

6. Olanzapine (Adjunct for refractory/CINV)

Dose: 10 mg PO on days 1–4 of chemotherapy
Reduces acute and delayed N/V by an additional 15–30% on top of triple therapy.

7. Anticholinergics (Motion sickness / vestibular)

Drug	Dose	Notes
Scopolamine (hyoscine)	1.5 mg transdermal patch q72h	Motion sickness; apply behind ear

Quick Clinical Guide by Etiology

Cause	Preferred Agent(s)
Postoperative N/V	Ondansetron, dexamethasone, prochlorperazine
Chemotherapy (CINV)	Ondansetron + dexamethasone + aprepitant ± olanzapine
Pregnancy (hyperemesis)	Metoclopramide IV, dimenhydrinate, pyridoxine (B6)
Gastroenteritis/ED	Ondansetron ODT, metoclopramide IV
Motion sickness / vestibular	Scopolamine patch, meclizine, dimenhydrinate
Opioid-induced	Metoclopramide, ondansetron, prochlorperazine
Refractory (all others failed)	Haloperidol 0.5–2 mg PO/SC

Key point on route of administration: When patients are actively vomiting, prefer IV, ODT (orally disintegrating tablet), or IM/PR routes. Ondansetron ODT has largely replaced rectal suppositories as the preferred out-of-IV option. — Katzung's Basic and Clinical Pharmacology, 16th Ed.; Roberts and Hedges' Clinical Procedures in Emergency Medicine

Rx for vomiting

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Here is the prescribing guide for general/acute vomiting, sourced directly from Rosen's Emergency Medicine and Textbook of Family Medicine:

Antiemetics for Acute/General Vomiting

Step 1 — First-line

Drug	Class	Dose	Route	Notes
Ondansetron (Zofran)	5-HT₃ antagonist	4–8 mg (up to 16 mg)	IV / PO / ODT	Preferred first-line; ODT works even during active vomiting; may cause headache
Metoclopramide (Reglan)	D₂ + 5-HT₃ antagonist	10–20 mg	IV / IM; q6h	Also a prokinetic; preferred in gastroparesis; black box for tardive dyskinesia with prolonged use

Step 2 — If first-line fails or sedation is acceptable

Drug	Class	Dose	Route	Notes
Prochlorperazine (Compazine)	D₁/D₂ antagonist	5–10 mg IV/IM/PO; 25 mg PR q12h	IV / IM / PR	Risk of EPS and dystonia
Promethazine (Phenergan)	H₁ antihistamine	12.5–25 mg q4h	IM / PR preferred	IV extravasation risk — always dilute in 10–20 mL NS over 10–15 min if IV used; causes sedation
Dimenhydrinate (Dramamine)	H₁ antihistamine	25–50 mg q6h	IV / IM / PO	Sedating; useful for vestibular component

Step 3 — Refractory vomiting

Drug	Class	Dose	Notes
Haloperidol (Haldol)	D₂ antagonist	0.5–2 mg PO/SC/IV q4–8h	Use when phenothiazines have failed
Droperidol	Butyrophenone	0.625–1.25 mg IV	Highly effective; check QTc first

Practical Prescribing Points

Active vomiting with no IV access: ondansetron ODT dissolves on the tongue without needing to swallow — start here.
EPS / dystonia from any D₂ blocker: treat with diphenhydramine 25–50 mg IV.
Do not combine promethazine + dimenhydrinate — additive sedation.
Rehydrate: add IV fluids (NS or D5½NS ~1 L) if patient is dehydrated — this alone can reduce nausea.
Persistent nausea: schedule antiemetics around the clock rather than PRN; consider combining agents with different mechanisms (e.g., ondansetron + metoclopramide).
Bowel obstruction: avoid metoclopramide — worsens pain and cramping.

Always Rule Out a Reversible Cause First

Acute vomiting without abdominal pain is most commonly from gastroenteritis, medications, migraine, or pregnancy. Acute vomiting with severe abdominal pain warrants urgent workup (obstruction, ischemia, pancreatitis, appendicitis). — Goldman-Cecil Medicine; Textbook of Family Medicine 9e; Rosen's Emergency Medicine

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