Associated with adverse drug interactions - vomiting

Red flags

Vomiting + altered consciousness - aspiration risk, needs airway protection
Haematemesis - GI bleeding drug cause (NSAID, anticoagulant)
Vomiting + abdominal pain + acidosis - salicylate/metformin (lactic acidosis) toxicity
Vomiting as prodrome of a toxic ingestion (paracetamol, iron, colchicine) - do not dismiss as "just nausea"
Cyclical/refractory vomiting in a cannabis user - cannabinoid hyperemesis (relieved by hot showers)

Direct GI irritant / mucosal injury

Chemoreceptor trigger zone (CTZ) stimulation

Opioids, digoxin toxicity, dopamine agonists, chemotherapy (highly emetogenic - cisplatin), SSRIs (early treatment)

Metabolic/toxic

Salicylate toxicity (early feature, with tinnitus, hyperventilation)
Theophylline toxicity
Digoxin toxicity - often the earliest sign, with visual disturbance (xanthopsia)
Metformin-associated lactic acidosis

Withdrawal

Idiosyncratic

Cannabinoid hyperemesis syndrome - chronic heavy cannabis use, compulsive hot showering

Timing relative to dose, new drug or dose increase
Associated symptoms - tinnitus (salicylate), visual change (digoxin), abdominal pain (iron, colchicine)
Chronic heavy cannabis use with relief on hot showers
Deliberate self-poisoning quantity and timing

Hydration status, postural BP
Abdominal tenderness, peritonism (iron, colchicine can cause bowel injury)
Hyperventilation (salicylate), tremor (theophylline), visual field/colour testing (digoxin)

Treat the cause, not just the symptom - withhold the causative agent where safe
Antiemetic choice matters: ondansetron first-line in most drug-induced vomiting; avoid metoclopramide in Parkinson's or with other dopamine antagonists (extrapyramidal risk)
Opioid-induced - dose reduction, rotate, or add antiemetic; naloxone if overdose
Digoxin toxicity - stop drug, correct K, DigiFab if arrhythmia/severe toxicity
Salicylate toxicity - urinary alkalinisation, haemodialysis if severe
Cannabinoid hyperemesis - supportive, topical capsaicin cream, cessation counselling; benzodiazepines helpful acutely

Vomiting is often the first and only sign of a serious ingestion (paracetamol, iron, colchicine) before the dangerous phase - don't reassure and discharge without considering levels
Cannabinoid hyperemesis is frequently mislabelled as cyclical vomiting syndrome or gastroenteritis, delaying diagnosis by years
Metoclopramide/prochlorperazine can precipitate acute dystonia, especially in the young

1. Ask what changed

"New drug or dose change is my first question - vomiting is often the earliest toxic sign."

2. Don't just treat the symptom

"I choose an antiemetic that fits the mechanism and check for the ingestion underneath it."

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