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Vitamin B1 Supplement Pregnancy: Safe and necessary in pregnancy if deficient — thiamine requirements increase in pregnancy; hyperemesis gravidarum (severe vomiting) is a significant risk factor for Wernicke's encephalopathy in pregnancy.

Thiamine (Vitamin B1)

Brand names: Pabrinex (IV/IM — high-potency), Thiamine Tablets 100mg (oral)

Adult dose

Dose: Wernicke's encephalopathy (treatment — Pabrinex IV): 2 pairs of Pabrinex ampoules (500mg thiamine IV) infused in 100mL sodium chloride 0.9% over 30 minutes, THREE times daily for 3–5 days. Prevention of Wernicke's in at-risk patients (Pabrinex): 1 pair of ampoules IV/IM once or twice daily for 3–5 days. Oral supplementation (chronic alcohol, malnutrition): 100mg oral TDS.
Route: IV (Pabrinex — emergency) / IM / Oral
Frequency: Three times daily (Pabrinex — treatment); once or twice daily (prevention); three times daily (oral)
Max: 1500mg daily IV (Wernicke's treatment — 3 × 500mg)
CRITICAL: IV thiamine MUST be given BEFORE IV glucose in alcohol-dependent patients presenting to ED. Giving glucose without thiamine precipitates Wernicke's encephalopathy by depleting remaining thiamine stores. Pabrinex must be given IV (not IM) in Wernicke's encephalopathy — IM Pabrinex is inadequate for treatment (too slow absorption). Anaphylaxis risk with IV — resuscitation facilities must be available.

Paediatric dose

Route: IV / Oral
Frequency: Three times daily (IV); once to twice daily (oral)
Max: Individualised
Seek specialist paediatric opinion for thiamine deficiency in children. Infantile Wernicke's (breastfed infants of thiamine-deficient mothers): 50–100mg IV thiamine urgently — seek neonatology/paediatric opinion.

Dose adjustments

Renal

No dose adjustment required.

Hepatic

No dose adjustment required — liver disease increases thiamine requirement (depleted stores); ensure adequate replacement.

Clinical pearls

  • GLUCOSE BEFORE THIAMINE = WERNICKE'S: in any malnourished or alcohol-dependent patient presenting to ED, always administer IV Pabrinex (thiamine) before IV glucose/dextrose — glucose utilisation consumes thiamine, precipitating acute Wernicke's encephalopathy (confusion, ophthalmoplegia, ataxia — Wernicke's triad)
  • Wernicke's triad: only present in 10% of cases — treat empirically with high-dose IV thiamine if any suspicion (alcohol history, malnutrition, confusion). Do not wait for all three features
  • Oral thiamine is poorly absorbed and inadequate for acute Wernicke's — always use Pabrinex IV 2 pairs TDS for established Wernicke's; IM is only acceptable for prevention
  • Furosemide depletion: long-term loop diuretic use increases renal thiamine excretion — heart failure patients on high-dose furosemide (common) are at risk of thiamine deficiency; consider oral supplementation 100mg TDS

Contraindications

  • Hypersensitivity to thiamine (anaphylaxis risk with IV preparation — have resuscitation equipment available)

Side effects

  • Anaphylaxis (rare — IV preparation; most reactions are mild flushing; severe anaphylaxis rare at current Pabrinex formulation)
  • Local irritation at injection site (IM)
  • Flushing, pruritus (IV infusion)

Interactions

  • No significant pharmacokinetic drug interactions
  • Loop diuretics (furosemide) — chronic use depletes thiamine; monitor in heart failure patients on long-term furosemide

Monitoring

  • Clinical response to IV thiamine (improvement in confusion, ophthalmoplegia within hours — confirms diagnosis)
  • Serum thiamine (erythrocyte transketolase activity — pre- and post-thiamine if available)
  • Blood glucose (monitor during replacement)

Reference: BNFc; BNF 90; Royal College of Physicians Alcohol Guidelines; NICE CG100 (Alcohol Use Disorders); SIGN 74. Verify against your local formulary and the latest BNF before prescribing.

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Curated clinical cross-links plus same-class fallbacks.

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