Antidote Pharmacology
Antidotes reverse drug toxicity through specific mechanisms: receptor antagonism, metabolic rescue, chelation, or antibody neutralization. Timely administration saves lives.
Overview
Antidotes are drugs that counteract the effects of a poison or overdose through specific pharmacological mechanisms. While supportive care remains the foundation of toxicology management, timely antidote administration can be lifesaving. Only a minority of poisonings have specific antidotes, making their knowledge particularly valuable.
Receptor Antagonists
Naloxone competitively antagonizes mu, kappa, and delta opioid receptors. Onset is 1-2 minutes IV with a duration of 30-90 minutes — critically shorter than most opioids, necessitating repeated dosing or continuous infusion. Initial dose 0.04-0.4 mg IV; titrate to respiratory adequacy without full reversal (to avoid withdrawal and pain). For suspected fentanyl overdose, higher doses (up to 10-20 mg) may be needed.
Flumazenil competitively antagonizes the benzodiazepine binding site on GABA-A receptors. Use is limited by seizure risk in chronic benzodiazepine users and mixed overdoses with pro-convulsant agents. It is most appropriate for iatrogenic benzodiazepine oversedation in benzodiazepine-naive patients.
Atropine blocks muscarinic acetylcholine receptors, countering the SLUDGE syndrome of organophosphate and carbamate poisoning. Doses far exceeding standard doses are often needed (2-4 mg IV initially, doubling every 3-5 minutes until secretions dry).
Metabolic Rescue
N-acetylcysteine (NAC) is the antidote for acetaminophen toxicity. It replenishes glutathione stores, directly scavenges NAPQI, and enhances sulfate conjugation. Maximum benefit occurs within 8 hours of ingestion, but efficacy persists up to 24 hours. The 21-hour IV protocol (150 mg/kg loading, then 50 mg/kg over 4 hours, then 100 mg/kg over 16 hours) is standard.
Fomepizole inhibits alcohol dehydrogenase, blocking conversion of methanol to formic acid and ethylene glycol to glycolic/oxalic acid. It has replaced ethanol infusion as the preferred antidote due to predictable pharmacokinetics and lack of CNS depression.
Pyridoxine (vitamin B6) treats isoniazid-induced seizures by repleting the B6 required for GABA synthesis. Dose: gram-for-gram equivalent of isoniazid ingested (or 5 g empirically if dose unknown).
Chelation Therapy
Chelators form water-soluble complexes with metals for renal excretion:
- Deferoxamine: Iron chelation (acute iron poisoning)
- Succimer (DMSA): Lead chelation (oral, pediatric-friendly)
- Calcium disodium EDTA: Lead chelation (parenteral, severe cases)
- Penicillamine: Copper chelation (Wilson's disease)
- Prussian blue: Thallium and radioactive cesium binding
Antibody-Based Antidotes
Digoxin-specific Fab fragments (DigiFab) bind free digoxin, reversing cardiac toxicity within 30 minutes. Each vial neutralizes approximately 0.5 mg of digoxin. Serum digoxin levels become unreliable after Fab administration because assays measure total (bound + free) digoxin.
CroFab and Anavip are antivenom Fab/F(ab')2 fragments for crotalid (pit viper) envenomation.
Key Takeaways
- Naloxone duration is shorter than most opioids — re-dosing or infusion is often needed
- NAC is most effective within 8 hours of acetaminophen ingestion but should be given up to 24 hours
- Fomepizole has replaced ethanol as the antidote for toxic alcohol poisoning
- Atropine doses in organophosphate poisoning far exceed standard doses
- Digoxin-specific Fab fragments invalidate subsequent serum digoxin level measurements