About This Calculator

The Fomepizole Dosing Calculator (Methanol/Ethylene Glycol) calculator provides weight-based intravenous (IV) dosing recommendations for fomepizole, a competitive inhibitor of alcohol dehydrogenase used as an antidote for toxic alcohol poisoning. This guide explains the calculator’s outputs, intended use, and the principles behind the dosing regimen.

Outputs Explained

The tool calculates the following key parameters based on patient weight and dialysis status:

• Loading Dose (mg and mL): An initial, larger dose to quickly achieve therapeutic blood concentrations. The dose is provided in milligrams and the corresponding volume in milliliters (based on a 1 g/mL concentration).
• Standard Maintenance Dose (mg): Subsequent doses for patients not on hemodialysis, administered every 12 hours. The regimen includes a step-up in dose after the first four maintenance doses due to auto-induction of metabolism.
• Hemodialysis Maintenance Dose (mg): A dose adjustment for patients actively undergoing hemodialysis. Because fomepizole is removed by dialysis, a more frequent dosing schedule (every 4 hours) is required to maintain effective levels.

How to Use This Tool

To ensure accurate dose calculation, follow these steps:

1. Enter Patient Weight: Input the patient’s weight. You can use either kilograms (kg) or pounds (lbs). The tool will convert lbs to kg for the calculation.
2. Select Renal/Dialysis Status: Choose the appropriate option. Select “Standard Dosing” for patients with normal or impaired renal function who are not on hemodialysis. Select “Patient is Undergoing Hemodialysis” only if a dialysis session is currently in progress.

Dosing Overview

Fomepizole therapy begins with a loading dose, followed by a series of maintenance doses. All doses should be diluted in at least 100 mL of 0.9% sodium chloride or 5% dextrose in water and administered as an IV infusion over 30 minutes.

• Loading Dose: 15 mg/kg.
• Maintenance Doses (Standard): Four doses of 10 mg/kg every 12 hours, then increase to 15 mg/kg every 12 hours thereafter.
• Maintenance Doses (During Hemodialysis): 10 mg/kg every 4 hours for the duration of the dialysis session.

Therapy should continue until ethylene glycol or methanol concentrations are undetectable or below 20 mg/dL and the patient is asymptomatic with a normal pH.

Switching Therapy

Historically, ethanol was used as a competitive inhibitor of alcohol dehydrogenase. Fomepizole is now the preferred antidote due to its easier administration, more predictable pharmacokinetics, and better safety profile. Switching from ethanol to fomepizole may be considered, but a loading dose of fomepizole is still required. There is no established protocol for switching from fomepizole to ethanol, and this is not recommended practice.

Missed Dose

If a scheduled maintenance dose is missed, it should be administered as soon as possible. The regular dosing schedule (every 12 hours or every 4 hours during dialysis) should be resumed from the time of the last administered dose. Do not double the dose to make up for a missed one. Timely administration is critical to prevent the metabolism of toxic alcohols.

Safety Alerts

Healthcare professionals should be aware of the following:

• Hypersensitivity: Use with caution in patients with a history of allergies to fomepizole or other pyrazoles.
• Adverse Effects: Common side effects include headache, nausea, and dizziness. Minor allergic reactions (e.g., rash) have been reported.
• Monitoring: Monitor for signs of efficacy (e.g., resolution of metabolic acidosis) and for adverse reactions throughout treatment. Frequent monitoring of toxic alcohol levels and acid-base status is essential.

Frequently Asked Questions

Why is the dose increased after the 5th dose in the standard regimen?

Fomepizole induces its own metabolism (auto-induction) via the cytochrome P450 system. After several doses, it is eliminated more rapidly. The dose is increased from 10 mg/kg to 15 mg/kg to maintain therapeutic concentrations.

Why is the dosing frequency increased during hemodialysis?

Fomepizole is significantly cleared by hemodialysis. To compensate for this rapid removal and maintain effective levels to block alcohol dehydrogenase, the dosing interval must be shortened from every 12 hours to every 4 hours.

Can this calculator be used for children?

While the dosing regimen (mg/kg) is the same for pediatric patients, the safety and efficacy of fomepizole have not been established in this population. Its use in children should be guided by a poison control center or toxicologist.

What should I do if the patient’s exact weight is unknown?

Use an estimated weight (e.g., using the Broselow tape for children or visual estimation for adults). It is generally better to slightly overestimate than underestimate the weight to ensure an adequate blocking dose.

How is fomepizole supplied?

Fomepizole is typically available as a sterile solution for injection in vials containing 1.5 g in 1.5 mL (1 g/mL). Always confirm the concentration of the product being used.

What is the duration of therapy?

Treatment should continue until the toxic alcohol is cleared from the blood (levels <20 mg/dL) and the associated metabolic acidosis has resolved.

Does a patient with renal failure who is not on dialysis need a dose adjustment?

No, the standard dosing regimen should be used. Fomepizole is primarily metabolized by the liver, and its metabolites are excreted by the kidneys. The parent drug itself does not require dose adjustment for renal impairment, but the dosing must be increased during active hemodialysis.

How fast should the IV infusion be?

Each dose, once diluted, should be infused intravenously over a 30-minute period. Rapid infusion may increase the risk of venous irritation and other side effects.