Toxicology and Antidotes

• Definition: Clinical toxicology addresses xenobiotic exposures that produce organ injury; an antidote is an agent that reverses, competes with, chelates, or bypasses the toxic mechanism, as opposed to supportive care and decontamination/enhanced elimination.
• Why it matters: Only a minority of poisonings have a true antidote, so exam stems reward recognizing the toxidrome and the mechanism first. Airway, breathing, circulation, glucose check, and ECG precede any antidote decision.

Epidemiology worth recalling

• Volume: America's Poison Centers (National Poison Data System) logs on the order of two million human exposure calls annually in the US; the great majority are managed without hospitalization.
• Bimodal age pattern: Children under 6 account for the largest number of exposures — unintentional, exploratory, single-substance, low mortality. Adolescents and adults account for most deaths, driven by intentional self-harm and multi-drug ingestions.
• Acetaminophen: the most common single pharmaceutical in overdose and, per the US Acute Liver Failure Study Group and AASLD acute liver failure guidance, the leading cause of acute liver failure in the United States. Both intentional bolus ingestions and unintentional "therapeutic misadventure" (combination opioid–acetaminophen products, fasting, chronic alcohol use) contribute.
• Carbon monoxide: a leading cause of unintentional poisoning death in the US per CDC surveillance, clustering in winter months with faulty furnaces, generators after storms, and fires; smoke inhalation victims frequently have combined CO and cyanide toxicity.
• Organophosphates: uncommon in US households since residential chlorpyrifos restrictions, but a major cause of poisoning death worldwide (agricultural self-poisoning) and the mechanistic template for nerve-agent exposure — hence its persistence on examinations.
• Opioids: overdose mortality dominates US poisoning deaths overall, now largely from illicitly manufactured fentanyl analogues, which shapes naloxone dosing and redosing strategy.

Restoring a depleted cofactor

• N-acetylcysteine: cysteine donor that regenerates hepatic glutathione, directly conjugates NAPQI, and expands sulfate conjugation capacity; it is also a free-radical scavenger and improves microvascular oxygen delivery, which is why benefit persists late. NAPQI itself is generated by CYP2E1, induced by chronic ethanol and isoniazid — a pharmacokinetic interaction that raises risk at lower doses. IV and oral routes are both effective; IV avoids vomiting and gives a defined 21-hour protocol.

Blocking upstream metabolism

• Fomepizole: competitive inhibitor of alcohol dehydrogenase with far higher affinity than ethanol, preventing conversion of methanol to formate and ethylene glycol to glycolate/oxalate. It prolongs the parent alcohol's half-life dramatically, so the parent must be excreted renally or dialyzed off. Fomepizole induces its own metabolism after roughly two days, requiring dose escalation; ethanol is the alternative but has erratic kinetics, sedation, and hypoglycemia.

Direct binding/chelation

• Hydroxocobalamin: cobalt moiety binds cyanide to form cyanocobalamin, renally excreted. Sodium thiosulfate is a sulfur donor for rhodanese, converting cyanide to thiocyanate — slower onset, useful adjunct.
• Deferoxamine: siderophore chelating free ferric iron as ferrioxamine, cleared renally.

Receptor and enzyme pharmacology

• Atropine: competitive muscarinic antagonist; it reverses SLUDGE and bronchorrhea but has no effect at nicotinic receptors, so fasciculations, weakness, and diaphragmatic failure persist.
• Pralidoxime: nucleophile that reactivates phosphorylated acetylcholinesterase — effective only before aging (irreversible dealkylation of the enzyme–phosphate bond).
• Naloxone: competitive μ-opioid antagonist with a short duration relative to methadone or extended-release opioids, mandating redosing or infusion.
• Oxygen in CO poisoning: mass-action displacement of CO from hemoglobin and cytochrome oxidase; carboxyhemoglobin half-life falls from roughly 4–5 hours on room air to about an hour on 100% oxygen and shorter still with hyperbaric therapy.

Acetaminophen

• N-acetylcysteine is first-line. For a single acute ingestion with a known time, obtain a 4-hour post-ingestion level and plot it on the Rumack-Matthew nomogram (US treatment line 150 mcg/mL at 4 hours). Treat empirically when timing is unknown, the ingestion is staggered, or transaminases are rising. AASLD acute liver failure guidance supports NAC in acetaminophen-induced ALF regardless of interval and early transfer to a transplant center using King's College criteria (pH, INR, creatinine, encephalopathy grade).

Toxic alcohols

• Fomepizole first-line for suspected or confirmed methanol/ethylene glycol ingestion — high osmolal gap, anion-gap acidosis, or a measured concentration above the conventional 20 mg/dL threshold. Add hemodialysis for severe acidosis, visual loss, renal injury, or very high levels (EXTRIP/ACMT framing). Give folate/folinic acid for methanol and thiamine/pyridoxine for ethylene glycol to shunt metabolites.

Carbon monoxide

• High-flow 100% oxygen by non-rebreather for every suspected exposure, started before confirmatory co-oximetry. Hyperbaric oxygen is considered for syncope/loss of consciousness, neurologic deficits, myocardial ischemia, severe acidosis, markedly elevated carboxyhemoglobin, or pregnancy (fetal hemoglobin binds CO avidly); ACEP's clinical policy notes the evidence for HBO is not definitive, so it is an option rather than a mandate.

Cholinergic crisis

• Atropine titrated to drying of pulmonary secretions is the life-saving drug — respiratory failure, not bradycardia, kills. Pralidoxime is added early per WHO guidance for nicotinic features. Remove clothing and decontaminate skin; provide benzodiazepines for seizures.

Other board-standard pairings

• Opioids: naloxone titrated to adequate respirations, not to alertness; intranasal 4 mg is the standard community formulation.
• Sodium bicarbonate for TCA-associated QRS widening; digoxin-specific Fab for dysrhythmia or hyperkalemia; glucagon for beta-blocker and high-dose insulin euglycemia for calcium-channel-blocker shock; lipid emulsion for local anesthetic systemic toxicity (ASRA).

N-acetylcysteine

• Anaphylactoid reaction with IV administration — direct, non-IgE histamine release that is rate- and concentration-dependent, producing flushing, urticaria, and bronchospasm during the loading dose. Management is to pause the infusion, give an antihistamine, and restart at a slower rate; it is not an absolute contraindication. Oral NAC causes nausea/vomiting and a rotten-egg sulfur odor.

Toxic-alcohol antidotes

• Fomepizole: generally well tolerated (headache, nausea, mild transaminase elevation). Ethanol as substitute causes CNS depression, hypoglycemia (especially in children), and requires frequent level monitoring.

Cyanide antidotes

• Hydroxocobalamin: harmless red discoloration of skin and urine, hypertension, and colorimetric laboratory interference — it can falsely alter co-oximetry, hepatic panels, and trigger blood-leak alarms on hemodialysis machines.
• Sodium nitrite: induces methemoglobinemia and hypotension; avoid in smoke-inhalation victims with concurrent CO poisoning, where further reduction of oxygen-carrying capacity can be fatal. Methylene blue is the reversal agent for symptomatic methemoglobinemia (avoid in G6PD deficiency).

Cholinergic antidotes

• Atropine: anticholinergic delirium, hyperthermia, urinary retention, tachycardia — titrate to secretions rather than heart rate. Pralidoxime: rapid infusion causes hypertension, tachycardia, and transient neuromuscular blockade.

Chelators and antagonists

• Deferoxamine: hypotension with rapid infusion, ARDS with prolonged (beyond ~24 hour) therapy, and predisposition to Yersinia enterocolitica sepsis (the siderophore feeds the organism).
• Naloxone: precipitated withdrawal, agitation, and noncardiogenic pulmonary edema; anticipate re-sedation as it outlasts nothing.
• Flumazenil: contraindicated in chronic benzodiazepine users and suspected TCA co-ingestion — unopposed removal of GABAergic tone causes refractory seizures.
• Physostigmine: reserved for pure antimuscarinic delirium; avoid with TCA overdose (bradyasystole, seizures).
• Activated charcoal: aspiration pneumonitis; contraindicated with an unprotected airway, caustics, hydrocarbons, and metals/alcohols it does not adsorb.

• The single best next step in any acetaminophen stem is a 4-hour post-ingestion level (plus transaminases and INR) — not immediate charcoal, not a psychiatric consult. Below-line levels on the Rumack-Matthew nomogram still warrant NAC if the ingestion was staggered or the timing is unknown.
• Normal pulse oximetry does not exclude carbon monoxide poisoning. Standard two-wavelength oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin, and PaO₂ is normal because dissolved oxygen is unaffected. Order co-oximetry. The classic cherry-red skin is a late, uncommon finding and a frequent distractor; headache, nausea, and confusion in multiple housemates or a winter generator exposure is the real clue.
• Smoke inhalation with profound, persistent lactic acidosis and hypotension despite oxygen = cyanide. Give hydroxocobalamin empirically and avoid nitrites, which add methemoglobin to existing carboxyhemoglobin.
• In organophosphate poisoning, atropine saves lives and is titrated to dry lungs, not to heart rate. Pralidoxime must precede enzyme aging; dimethyl compounds age within hours, so the correct answer is always "give it now."
• Osmolal gap early, anion gap late in toxic alcohols: the parent alcohol raises osmolality, the acid metabolite raises the anion gap. Methanol → formate → optic injury and "snowfield" vision loss; ethylene glycol → oxalate → hypocalcemia and calcium oxalate crystalluria with acute kidney injury.
• Titrate naloxone to respiratory rate, not consciousness, and expect re-sedation with methadone, extended-release formulations, or fentanyl analogues — an infusion or observation period is often the tested answer.
• Do not give flumazenil to a patient with chronic benzodiazepine use or possible TCA co-ingestion; seizure risk outweighs any diagnostic benefit.
• Chronic alcohol use, isoniazid, and fasting amplify acetaminophen hepatotoxicity through CYP2E1 induction and glutathione depletion — hepatotoxicity at seemingly therapeutic doses.

• Acetaminophen toxicity peaks at 72 hours (fulminant hepatic failure); N-acetylcysteine (NAC) is antidote
• Cyanide binds cytochrome c oxidase; treat with hydroxocobalamin or sodium thiosulfate
• Methanol/ethylene glycol cause metabolic acidosis with anion gap; treat with fomepizole (alcohol dehydrogenase inhibitor)
• Iron overdose causes GI hemorrhage and cardiogenic shock; deferoxamine chelates iron
• Organophosphates inhibit acetylcholinesterase; treat with atropine + pralidoxime (2-PAM)

1. 1.Acetaminophen timing myth: Don't assume "too late for NAC." NAC benefits extend beyond 24 hours, especially in fulminant hepatic failure. Nomogram interpretation requires accurate ingestion time.

1. 1.Methanol vs. ethylene glycol confusion: Both cause anion gap + osmolar gap, but methanol → formate (blindness/optic neuritis), ethylene glycol → oxalate (acute kidney injury with calcium oxalate crystals). Fomepizole treats both.

1. 1.Organophosphate atropine dosing: Atropine is titrated to dry secretions (not standard dosing); pralidoxime must be given early (<24-48 hours) to reactivate acetylcholinesterase—delayed administration is ineffective.

• Acetaminophen: NAC IV/oral (loading dose 150 mg/kg)
• Methanol/Ethylene glycol: Fomepizole 15 mg/kg IV load, then 10 mg/kg q12h
• Cyanide: Hydroxocobalamin 70 mg/kg IV (can give empirically in cyanide-exposed patients)
• Organophosphates: Atropine IV titrated to drying of secretions + pralidoxime 1-2 g IV
• Iron: Deferoxamine IM/IV titrated to urine color (rose-colored = adequate chelation)
• Opioids: Naloxone 0.4-2 mg IV; repeat q2-3