Methotrexate Weekly Dosing Error Caught at Verification
A handwritten prescription arrives for a 62-year-old woman with rheumatoid arthritis: "Methotrexate 2.5 mg tablets - take 3 tablets daily." The prescription is signed by a rheumatologist and dated today. The patient's profile shows methotrexate 2.5 mg, 3 tablets once weekly, with the last fill 5 weeks ago, plus folic acid 1 mg daily and etanercept 50 mg weekly. She has been on methotrexate for two years. The prescriber's office is closed for lunch and will reopen in 45 minutes. The patient is waiting at the counter and says she "just needs a refill." The prescription quantity is 90 tablets.
1. What is the problem with this prescription, and what is the risk to the patient?
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Problem: Methotrexate is dosed weekly, not daily, for rheumatoid arthritis. The prescription says "3 tablets daily" (7.5 mg daily = 52.5 mg/week), which is a sevenfold overdose compared with her usual 7.5 mg weekly. This is a classic and potentially fatal prescribing error.
Risk: Daily methotrexate causes cumulative folate antagonism, leading to bone marrow suppression, mucositis, hepatic toxicity, and renal injury. At 7.5 mg daily, serious toxicity could develop within days to weeks. She is also on etanercept, increasing immunosuppression risk.
Verification duty: A pharmacist must not dispense a prescription that is inconsistent with the patient's established regimen and with standard dosing, even if it is signed. The quantity of 90 tablets would also supply 30 days at the erroneous daily dose, extending exposure.
Action: Do not dispense as written. Hold the prescription, attempt to reach the prescriber, and counsel the patient not to take any doses from an old supply according to the new instructions until clarified.
2. The prescriber cannot be reached for 45 minutes and the patient is waiting. How do you manage the situation?
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Immediate steps:
- Tell the patient there is a dosing discrepancy that needs clarification and that you cannot release the medication yet. Be calm and specific: "The instructions on this prescription differ from your usual weekly dose, and I need to confirm with your doctor."
- Attempt contact: phone the prescriber's office, try the after-hours line or fax, and leave a clear message describing the discrepancy.
- Check the patient's understanding: ask how she usually takes methotrexate. If she says weekly, that confirms the prescription is likely an error and reinforces the need to clarify.
- Offer to call her when resolved rather than making her wait, if she prefers.
Do not: dispense with a verbal correction from a non-prescriber, alter the prescription yourself, or tell her to "take it weekly anyway" without prescriber confirmation.
If the prescriber confirms weekly dosing: document the conversation, correct the label to weekly, and counsel accordingly. If the prescriber insists on daily dosing, escalate to the prescriber's supervisor or medical director and document.
3. What documentation and follow-up should accompany this event?
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Documentation:
- The prescription as received ("3 tablets daily"), the discrepancy identified, and the standard weekly dosing.
- Time-stamped attempts to contact the prescriber and the outcome.
- The patient counselling provided and her response.
- The final resolution: corrected instructions, who authorized the change, and the date/time.
- If an error was prevented, consider whether a near-miss report should be filed per pharmacy policy.
Follow-up:
- Confirm the patient understands weekly dosing and has a clear written schedule; consider a pill organizer or calendar.
- Ask about folic acid adherence, since it reduces methotrexate toxicity.
- Remind her of monitoring (CBC, liver and renal function) and to report mouth sores, fever, or unusual bruising.
- If she had already taken any doses from an old supply per the erroneous instructions, arrange urgent assessment.