Renal Function Declines with Age
Kidney clearance drops in older adults, often halving after age 65 due to reduced glomerular filtration rate (GFR). Methotrexate, primarily excreted renally, requires dose reduction—typically 25-50% if creatinine clearance falls below 60 mL/min. Calculate using Cockcroft-Gault formula adjusted for age, weight, and serum creatinine.[1]
Higher Toxicity Risk in Elderly
Patients over 70 face 2-3 times higher rates of severe toxicities like mucositis, myelosuppression, and hepatotoxicity from slower drug metabolism via cytochrome P450 and reduced folate stores. Start at 50-75% of standard dose (e.g., 7.5-10 mg/week for rheumatoid arthritis vs. 15 mg) and titrate slowly with weekly monitoring of CBC, liver enzymes, and creatinine.[2]
Body Composition Changes
Older adults have lower lean body mass and higher fat percentage, altering volume of distribution. Use ideal body weight for dosing in obese elderly to avoid overdose; actual weight overestimates clearance.[1]
Drug Interactions and Comorbidities
Polypharmacy common in age 65+ increases interactions—e.g., NSAIDs or proton pump inhibitors boost methotrexate levels by competing for renal secretion. Comorbidities like diabetes or heart failure worsen renal/hepatic function, necessitating 20-30% dose cuts. Screen for these before starting.[3]
Pediatric Age Adjustments
In children (under 18), higher clearance requires higher mg/m² doses than adults for equal exposure—e.g., 15-30 mg/m²/week for juvenile idiopathic arthritis vs. fixed mg/week in adults. Adjust downward near puberty as adult-like pharmacokinetics emerge.[2]
For precise dosing, consult guidelines like those from the American College of Rheumatology, factoring individual GFR and response.
[1]: Lexicomp - Methotrexate Dosing Adjustments
[2]: American College of Rheumatology Guidelines for Methotrexate in RA
[3]: UpToDate - Methotrexate in Older Adults