Drug Interactions and Contraindications Flashcards

Per FDA labeling, what is the maximum daily simvastatin dose for a patient also taking amlodipine?

Answer: 20 mg

Amlodipine weakly inhibits CYP3A4 and raises simvastatin exposure, so simvastatin should not exceed 20 mg daily to limit myopathy risk.

A heart failure patient on lisinopril and spironolactone is also taking an over-the-counter salt substitute. Which lab abnormality is most likely?

Answer: Hyperkalemia

ACE inhibitors and spironolactone both reduce potassium excretion, and potassium chloride salt substitutes add a potassium load, causing hyperkalemia.

How should oral ciprofloxacin be timed relative to a calcium carbonate antacid?

Answer: Take ciprofloxacin at least 2 hours before or 6 hours after the antacid

Polyvalent cations such as calcium, magnesium, aluminum, iron, and zinc chelate fluoroquinolones and markedly reduce their absorption.

Which drug combination is contraindicated because of potent CYP1A2 inhibition leading to severe hypotension and sedation?

Answer: Tizanidine and ciprofloxacin

Ciprofloxacin and fluvoxamine strongly inhibit CYP1A2, raising tizanidine exposure several-fold, so the combination is contraindicated.

A patient on metformin with an eGFR of 40 mL/min/1.73 m2 is scheduled for a CT scan with iodinated contrast. What is recommended?

Answer: Hold metformin at the time of the procedure and restart 48 hours later after confirming stable renal function

Contrast can cause acute kidney injury, leading to metformin accumulation and lactic acidosis, so it is held in patients with eGFR 30-60 and resumed after renal function is rechecked.

A patient on levothyroxine starts calcium carbonate supplements and develops rising TSH. What is the best advice?

Answer: Separate levothyroxine and calcium by at least 4 hours

Calcium binds levothyroxine in the GI tract and reduces absorption, so doses should be separated by about 4 hours.

A patient on digoxin 0.25 mg daily is started on amiodarone. What adjustment is generally recommended?

Answer: Reduce digoxin dose by about 50% and monitor levels

Amiodarone inhibits P-glycoprotein and reduces digoxin clearance, roughly doubling digoxin concentrations.