BACKGROUND: Methotrexate is the drug of choice for moderate to severe rheumatoid arthritis (RA), as recommended by American College of Rheumatology and European Alliance of Associations for Rheumatology guidelines, when no contraindications exist. These guidelines endorse methotrexate as a first-line treatment due to its efficacy in controlling disease activity and preventing joint damage, as well as for its cost-effectiveness. These guidelines emphasize the importance of regular monitoring and dose adjustments to minimize toxicity while maximizing therapeutic benefits. Despite its efficacy, complex dosing regimens can lead to medication errors, and overdose can result in severe hematologic and gastrointestinal toxicity.

CASE REPORT: This case report presents a 77-year-old Saudi woman with a history of RA, hypertension, hypothyroidism, lumbar spine stenosis, and gastroesophageal reflux disease, who mistakenly took methotrexate daily instead of weekly after confusing it with her daily dosing regimen for omeprazole. As a result, she experienced an adverse reaction to methotrexate, which led to pancytopenia, acute kidney injury, and elevated liver enzymes. She also experienced severe gastrointestinal symptoms, including abdominal pain, vomiting, diarrhea, and mucositis. Additionally, computed tomography imaging showed colonic inflammation, further indicating gastrointestinal involvement. The patient initially received an immediate platelet transfusion. Due to persistent pancytopenia, she subsequently received leucovorin rescue therapy and supportive treatment. Her condition improved, and she was discharged with a follow-up plan that included repeated complete blood count, liver function testing, and symptoms assessment.

CONCLUSIONS: This case underscores the importance of medication counseling and reconciliation for patients who are taking multiple medications, especially in the case of elderly individuals. It also underscores the need for increased awareness among healthcare providers regarding methotrexate dosing for RA, toxicity, and early intervention strategies in case of drug toxicity, to improve patient outcomes.

Keywords: Arthritis, Rheumatoid, Medication Errors, Methotrexate, Pancytopenia, Saudi Arabia