Prescribing 2.5mg Methotrexate Tablets to Achieve 7.5mg Weekly Dose
To achieve a 7.5mg weekly dose using 2.5mg tablets, prescribe three tablets (2.5mg × 3 = 7.5mg) taken as a single dose once weekly on the same day each week. 1, 2, 3
Dosing Schedule
- Prescribe 3 tablets of 2.5mg methotrexate to be taken together once weekly (total 7.5mg per week) 1, 2
- The single weekly dose is preferred over divided dosing for rheumatoid arthritis and psoriasis, as it provides better efficacy and simpler adherence 1
- Instruct the patient to take all three tablets on the same day each week (e.g., every Monday morning) 1, 3
Alternative Split-Dosing (If GI Intolerance Occurs)
- If gastrointestinal side effects are problematic with single-dose administration, the 7.5mg weekly dose can be divided into three doses of 2.5mg each, given at 12-hour intervals over 24 hours once weekly 1, 3
- This split-dosing schedule (one 2.5mg tablet every 12 hours for three doses) may improve GI tolerability in some patients 1
Critical Prescribing Instructions
- Clearly specify "ONCE WEEKLY" on the prescription to prevent the dangerous error of daily dosing, which can lead to severe toxicity including bone marrow suppression and death 2, 3, 4
- Document the specific day of the week for administration (e.g., "Take 3 tablets every Monday") 2
- Counsel the patient explicitly that this is a weekly medication, not daily 2, 3
Mandatory Folate Supplementation
- Prescribe folic acid 1-5mg daily on all days EXCEPT the day methotrexate is taken 1, 2
- Folate supplementation reduces gastrointestinal side effects, hepatic abnormalities, and potentially hematologic toxicity without compromising efficacy 1, 2
- Most commonly, folic acid 1mg daily is prescribed for 6 days per week (skipping the methotrexate day) 1, 2
Baseline and Monitoring Requirements
- Obtain baseline complete blood count, liver function tests, and renal function (creatinine clearance) before initiating therapy 2, 3, 4
- Consider a test dose of 2.5mg in patients with renal impairment, elderly patients (>70 years), or those at high risk for toxicity, waiting 5-7 days to check CBC before proceeding to full dose 2, 3
- Methotrexate is contraindicated if estimated glomerular filtration rate (eGFR) is <30 mL/minute 4
Common Pitfall to Avoid
- The most critical error is daily administration instead of weekly dosing, which can result in fatal toxicity 2, 3, 4
- Use clear verbal and written instructions emphasizing weekly dosing, and consider having the patient repeat back the instructions 2
- Mark the prescription with "WEEKLY DOSE - NOT DAILY" to alert pharmacists 2