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Antidote / Vitamin Pregnancy: A (at prophylactic doses); antidote use in overdose justified regardless of category

Pyridoxine (Vitamin B6)

Brand names: Benadon, Nestrex

Adult dose

Dose: INH overdose: gram-for-gram with INH ingested IV (if unknown, 5 g IV). INH neuropathy prophylaxis: 10-40 mg/day PO
Route: IV / PO
Frequency: IV: single dose or repeat if seizures recur. PO: daily
For isoniazid (INH) overdose with refractory seizures: give 1g IV pyridoxine per estimated gram of INH ingested. If amount unknown, give 5g IV (70 mg/kg in children). Administer IV over 5 min or faster in active seizure.

Paediatric dose

Dose: 70 mg/kg
Route: IV
Frequency: Single dose; repeat if seizures continue
Max: 5000 mg (5 g)
70 mg/kg IV for INH overdose. Prophylaxis dose: 1-2 mg/kg/day PO (max 50 mg/day) during INH therapy.

Dose adjustments

Renal

No adjustment required for antidote use.

Paediatric weight-based calculator

70 mg/kg IV for INH overdose. Prophylaxis dose: 1-2 mg/kg/day PO (max 50 mg/day) during INH therapy.

Clinical pearls

  • INH seizures are refractory to standard benzodiazepines and phenytoin - pyridoxine is the definitive treatment.
  • Mechanism: INH blocks pyridoxal phosphate (active form of B6), reducing GABA synthesis - causing refractory seizures. Pyridoxine restores GABA production.
  • Give simultaneously with benzodiazepines, not instead of them.
  • Also used for: gyromitra mushroom poisoning, ethylene glycol adjunct, hydrazine toxicity.
  • INH prophylaxis dose: all patients on INH for TB treatment/prevention should receive 10-25 mg/day PO to prevent peripheral neuropathy (especially diabetics, pregnant, malnourished, HIV+, elderly).
  • IV formulation not always stocked - ensure pharmacy access or Toxbase emergency supply pathway.

Contraindications

  • No absolute contraindications in overdose/emergency setting
  • Chronic high-dose supplementation >200 mg/day may cause peripheral neuropathy (irrelevant to acute antidote use)

Side effects

  • Peripheral sensory neuropathy (only with chronic high-dose supplementation, not acute antidote use)
  • Nausea at high IV doses
  • Ataxia at very high doses

Interactions

  • Levodopa: pyridoxine reduces efficacy - relevant only in chronic Parkinson's management, not acute overdose
  • Phenobarbital / Phenytoin: may reduce serum levels with long-term co-administration

Monitoring

  • Seizure frequency and duration
  • Blood glucose
  • Arterial blood gas (acidosis from INH toxicity)
  • Neurological status

Reference: BNFc; TOXBASE; NPIS UK; WHO Model Formulary; BNF 84; NICE TB Guidelines NG33. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

Pathways