An 8-year-old male child with acute lymphoblastic leukemia (ALL) developed signs of methotrexate (MTX) toxicity—such as vomiting, chest tightness, and rapidly elevated serum creatinine and uric acid levels—on the second day after his first high-dose methotrexate (HD-MTX) treatment. The toxicity is considered due to delayed excretion of methotrexate. The clinical pharmacist assisted the medical team in formulating a treatment plan that included adequate hydration and alkalinization, leucovorin rescue, and subsequent dose adjustment of MTX, based on therapeutic drug monitoring and pharmacogenetic testing results. By day 11, the patient's MTX plasma concentration, serum creatinine, and uric acid levels had returned to safe ranges. In this case, the clinical pharmacists used pharmaceutical knowledge to analyze potential factors contributing to delayed MTX elimination, and assisted the treatment team to improve the safety and efficacy of drug therapy. This case provides valuable experience for the standardized management of similar pediatric patients.

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