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