Drug Interactions

A patient on methotrexate is prescribed trimethoprim. Which type of drug interaction is most likely to occur, and what is the mechanism?

A 58-year-old man with atrial fibrillation on warfarin (INR 2.5) presents to the clinic. He was recently started on clarithromycin for community-acquired pneumonia. After 5 days of antibiotic therapy, he develops spontaneous bruising on his forearms and petechiae on his legs. His INR is now 8.2. Which mechanism best explains this drug interaction?

A 62-year-old woman on metformin (2 g/day) for type 2 diabetes is prescribed trimethoprim-sulfamethoxazole (TMP-SMX) for a urinary tract infection. After 3 days, she develops nausea, fatigue, and a serum creatinine rise from 0.9 to 1.4 mg/dL. Which investigation is most appropriate to identify the mechanism of renal dysfunction in 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?

A 48-year-old woman on warfarin for atrial fibrillation is started on fluconazole for oesophageal candidiasis. Her INR rises from 2.5 to 8.2 within 5 days. Which is the most common mechanism responsible for this warfarin–fluconazole interaction?