Methotrexate Dosing Guidelines

Introduction to Methotrexate Dosing

The British Journal of Dermatology recommends calculating methotrexate dose based on body surface area (BSA) using the formula: Dose (mg) = Prescribed dose per m² × Patient's BSA (m²), with BSA typically calculated using the Mosteller formula 1

The Mosteller formula is the most widely accepted method for calculating BSA: BSA (m²) = √[(height in cm × weight in kg) / 3600], as recommended by the British Journal of Dermatology 1
Alternative validated BSA calculators, such as DuBois and Haycock, may also be used, though the Mosteller formula is preferred in clinical practice, according to the Journal of Crohn's and Colitis 2

For pediatric inflammatory skin disease, the Journal of the American Academy of Dermatology suggests that weight-based dosing may be easier to calculate than BSA-based dosing, with a maximum dose of 1 mg/kg (not exceeding 25 mg/week) regardless of calculated BSA 3, 4
For pediatric Crohn's disease, the Journal of Crohn's and Colitis recommends prescribing methotrexate at 15 mg/m² once weekly, with a maximum dose of 25 mg 2
For psoriasis, the Journal of the American Academy of Dermatology recommends typical dosing ranges from 7.5 mg to 25 mg weekly, with BSA-based dosing less commonly used for this indication 5

To calculate the methotrexate dose for a patient, first calculate the patient's BSA using height and weight, then multiply BSA by the prescribed dose per m², and round to the nearest available tablet strength, as described in the Journal of the American Academy of Dermatology 5, 6

The Journal of the American Academy of Dermatology notes that parenteral (subcutaneous or intramuscular) administration likely results in higher bioavailability and may increase efficacy compared to oral dosing, with 0.1 mL of 25 mg/mL injection solution equivalent to a 2.5 mg oral tablet 3, 6
Methotrexate is usually given as a single weekly dose, according to the Journal of the American Academy of Dermatology and the Journal of Crohn's and Colitis 2, 6
Patients with renal impairment may need lower doses, with a recommended dose reduction of 50% for creatinine clearance 20-50 mL/min, as suggested by the British Journal of Dermatology and the Journal of the American Academy of Dermatology 1, 7
Folic acid supplementation (1-5 mg daily on non-methotrexate days) is mandatory to reduce toxicity, as recommended by the Journal of the American Academy of Dermatology, the British Journal of Dermatology, and other guidelines 1, 5, 8

The American Academy of Dermatology recommends a typical weekly dosing of 7.5 to 25 mg per week for adults with psoriasis and atopic dermatitis 9, 10, 11
The Lancet Child and Adolescent Health suggests a starting dose of 10 mg/m² BSA per week for children with juvenile idiopathic arthritis (polyarticular), with a maximum recommended dose of 15 mg/m² BSA per week 12

The American Academy of Dermatology notes that parenteral (subcutaneous or intramuscular) administration has higher bioavailability than oral dosing 9
The Lancet Child and Adolescent Health recommends using the subcutaneous route for doses ≥15 mg/m² BSA per week to ensure adequate absorption 12
The American Academy of Dermatology suggests a test dose of 2.5 or 5 mg with CBC evaluation 5-6 days later, which is mandatory in patients with decreased glomerular filtration rate or significant risk factors for hematologic toxicity 10

The American Academy of Dermatology recommends folic acid 1-5 mg daily on non-methotrexate days to reduce toxicity 10, 11
The American Academy of Dermatology suggests monitoring baseline CBC, liver function tests, and renal function, with monthly liver function tests for the first 6 months and every 3-6 months thereafter 10, 11