血栓性事件(thromboembolic event)的高危險群包括心房顫動病史，年齡大於75 歲，曾有中風或缺血性中風病史者，瓣膜性心臟病，全身性血栓及左心室功能不佳者。與每天使用aspirin 325 mg 相比，高危險群使用warfarin 更能降低中風的風險，但醫療人員須注意藥物間交互作用，以及監測病人international normalized ratio (INR)。本文報告一位78 歲女性，曾患有心房顫動、高血壓、高血脂症及中風後局部性癲癇，同時併用rosuvastatin 10 mg 每天一次、phenytoin 100/200 mg 早晚各一次和warfarin 2.5 mg 每天一次後，發生四肢無力、步態不穩，於家中浴室跌倒，被家屬送至醫院急診室。斷層掃描顯示有右側硬腦膜下出血，phenytoin 血中濃度高達44.76 mg/dl，INR 高達5.22。INR 升高懷疑是因為與warfarin 相關的藥物交互作用所導致，另討論phenytoin 過量導致步態不穩的可能性。臨床藥師經過藥物動力學計算，給予醫師phenytoin 藥物動力學資訊。在病人無出血風險之後，建議醫師重新開立warfarin，並定期監測INR。
 

High risk factors for thromboembolic events include age greater than 75 years, previous stroke/transient ischemic attack or systemic embolus, valvular heart disease, and poor left ventricular systolic function. Compared to aspirin 325 mg/day, warfarin reduces stroke risk even more. However, medical staffs should pay attention to drug-drug interactions and international normalized ratio (INR) monitoring. We reported a case of a 78-year-old woman with history of atrial fibrillation, hypertension,hyperlipidemia, and post-stoke focal seizure. After coadministration of rosuvastatin 10 mg qd,phenytoin 100 mg/200 mg bid and warfarin 2.5 mg qd, she suffered from limb weakness and ataxia.She fell down in her bathroom and was sent to emergency room by her families. Computerized tomography revealed right side subdural hemorrhage. The phenytoin level was 44.76 mg/dL, and INR was elevated to 5.22. Elevated INR caused by drug interactions associated with warfarin were suspected. We also discussed the possibility of phenytoin overdose induced ataxia. Clinical pharmacist did pharmacokinetic calculation and provided pharmacokinetic informations to neurosurgeon. After the patient was risk-free from hemorrhage, we suggested that neurosurgeon prescribe warfarin to the patient again and monitor INR periodically.