Drug Interactions

A 62-year-old woman on phenytoin for epilepsy, warfarin for atrial fibrillation, and oral contraceptives for hormone replacement is counselled about drug interactions. Which statement about phenytoin interactions is NOT correct?

A 62-year-old woman with type 2 diabetes mellitus on metformin 1000 mg BD and hypertension on lisinopril 10 mg daily presents with a 3-day history of acute gastroenteritis with persistent vomiting and diarrhoea. Her serum creatinine has risen from 0.8 mg/dL (baseline) to 1.6 mg/dL. She is haemodynamically stable but mildly dehydrated. What is the most appropriate immediate next step?

A 58-year-old man with atrial fibrillation on warfarin (INR target 2–3) presents to the clinic. He was recently started on fluconazole 400 mg daily for oral candidiasis. His INR measured today is 8.2 (previously 2.5 one week ago). He is asymptomatic with no signs of bleeding. What is the most appropriate immediate next step?

A 58-year-old man with atrial fibrillation on warfarin (INR 2.5) presents to the clinic. He was recently started on fluconazole 400 mg daily for oral candidiasis. Two weeks later, his INR is 4.8 and he reports spontaneous bruising on his arms. What is the mechanism of this drug interaction?

A 62-year-old woman with hypertension and depression is on lisinopril 10 mg daily and sertraline 100 mg daily. She develops a urinary tract infection and is prescribed trimethoprim-sulfamethoxazole (TMP-SMX) double-strength twice daily. After 5 days, she reports dizziness and fatigue. Serum potassium is 6.2 mEq/L (normal 3.5–5.0). What is the primary mechanism of hyperkalemia in this case?