本案例為62 歲男性病患，2009 年起使用phenytoin 治療中風引發之癲癇，治療期間phenytoin 血中濃度皆維持在治療區間，且未有不良反應發生。近期病患因為結腸癌合併肝轉移，輔助治療使用capecitabine 治療，併用二週後發生明顯意識變化，雖立即降低capecitabine 劑量，但病患仍發生呼吸衰竭與心搏徐緩而轉入加護病房治療，經檢查發現phenytoin 血中濃度上升至69.6 μg/mL，立即停止使用phenytoin，5 天後再行測量phenytoin 血中濃度仍為44.1 μg/mL，直至二週後phenytoin 血中濃度才降至15.4 μg/mL。
研究指出capecitabine 會抑制CYP2C9代謝酶，導致phenytoin代謝降低，造成血中濃度上升和毒性反應，此交互作用為延遲性且與capecitabine 劑量成正相關係，因此當臨床上合併此二種藥物進行治療時，需要持續性長期監測病患phenytoin 血中濃度，並依病患臨床之反應調整phenytoin 劑量。

A 62-years-old patient was stable on phenytoin to control epilepsy caused by stroke since 2009. Recently,due to his colon cancer with liver metastasis, the patient was given an additional treatment with capecitabine;however, the interaction of phenytoin and capecitabine caused obvious consciousness change after two weeks.
Although the dose of capecitabine was reduced immediately, the patient still had respiratory failure and bradycardia, and the phenytoin level increased to 69 μg/mL. The patient was sent to ICU and stopped takingphenytoin. After five days, the phenytoin level was still 44.1 μg/mL, and then reduced to 15.4 μg/mL after two weeks.
According to the literature, capecitabine increased serum phenytoin level and toxicity by inhibiting CYP2C9 enzyme. The drug interaction was delayed and had positive relationship with capecitabine dose.Therefore, it is important to continue monitor the phenytoin level of the patient when capecitabine and phenytoin combination therapy, and to adjust the dose of phenytoin according to clinical responses.