In methanol poisoning, what is the primary mechanism by which ethanol acts as an antidote?

Explanation

## Ethanol as Antidote in Methanol Poisoning **Key Point:** Ethanol works as a competitive inhibitor of alcohol dehydrogenase (ADH), not as a neutralizing agent or renal enhancer. ### Mechanism of Action 1. **Competitive Inhibition:** Ethanol has higher affinity for ADH than methanol 2. **Substrate Competition:** When ethanol is present, ADH preferentially metabolizes ethanol over methanol 3. **Result:** Methanol accumulation is prevented, allowing it to be excreted unchanged in urine and breath 4. **Toxic Metabolite Prevention:** By blocking ADH, ethanol prevents formation of formaldehyde and formic acid ### Why Other Mechanisms Are Incorrect | Mechanism | Why It's Wrong | |-----------|---------------| | Direct neutralization | Ethanol is a weak base; formic acid is a strong organic acid. No acid-base reaction occurs. | | Enhanced renal excretion | Ethanol does not increase methanol clearance by the kidney. It works by preventing metabolism. | | Cytochrome P450 inhibition | Ethanol is not a significant P450 inhibitor; ADH is the target enzyme. | **High-Yield:** The goal of ethanol therapy is to **maintain a blood ethanol level of 100–150 mg/dL** to keep ADH saturated with ethanol, preventing methanol metabolism. **Clinical Pearl:** Fomepizole (4-methylpyrazole) is a more selective ADH inhibitor now preferred over ethanol in many centres because it does not cause intoxication and has more predictable pharmacokinetics. **Mnemonic:** **COMPETE** = **C**ompetitive inhibition of **ADH** by **E**thanol prevents **T**oxic metabolite formation, allowing **E**xcretion of methanol ```mermaid flowchart TD A[Methanol poisoning] --> B{Ethanol present?}:::decision B -->|No| C[ADH metabolizes methanol]:::action C --> D[Formaldehyde formed]:::outcome D --> E[Formic acid formed]:::outcome E --> F[Metabolic acidosis<br/>Optic nerve damage]:::urgent B -->|Yes| G[Ethanol competes for ADH]:::action G --> H[Methanol NOT metabolized]:::action H --> I[Methanol excreted unchanged<br/>in urine & breath]:::outcome I --> J[Toxicity prevented]:::action ```