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 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?

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?