ToxicologyEmergencyInfectious Disease
Isoniazid (INH) overdose
Refractory seizures and metabolic acidosis from isoniazid — pyridoxine antidote in gram-for-gram dosing.
Source: TOXBASE/NPIS; AACT/EAPCCT; BNF
Step 1 of ~4
warning
Recognise the toxic triad
Isoniazid toxicity classically presents with the triad:
1. Refractory seizures (usually within 30–120 min of ingestion)
2. Severe metabolic acidosis (high anion gap, lactic)
3. Coma
Also: hepatotoxicity (delayed days), peripheral neuropathy (chronic), lupus-like reaction.
Mechanism: inhibits pyridoxine-dependent enzymes including glutamate decarboxylase → ↓ GABA → seizures. Also blocks NAD regeneration → lactic acidosis.
ABCDE; secure airway early (seizures); IV access; bloods: VBG, U&E, LFTs, glucose, paracetamol/salicylate, INR, lactate.
Related
Curated clinical cross-links plus same-class fallbacks.
Drugs
- IsoniazidRecommendedAnti-Tuberculosis Antibiotic
- LorazepamRecommendedBenzodiazepine
- MidazolamRecommendedBenzodiazepine
- DiazepamRecommendedBenzodiazepine (long-acting)
Same class
Pathways
- Paracetamol overdose · TOXBASE/NPIS; MHRA DSU 2012/2024; SNAP regimen (Lancet 2014); BNF
- TCA overdose · TOXBASE/NPIS; AACT/EAPCCT position statements; Resuscitation Council UK ALS
- Opioid overdose · TOXBASE/NPIS; Resuscitation Council UK; BNF
- Anticholinergic toxidrome · TOXBASE/NPIS; AACT/EAPCCT; BNF
- Benzodiazepine overdose · TOXBASE/NPIS; AACT/EAPCCT; BNF
- β-blocker overdose · TOXBASE/NPIS; AACT/EAPCCT; ESC; BNF
Decision support only. Always apply local guidelines and clinical judgement.