APC APC Drug Interactions and Contraindications 2 — Questions and Answers
Question 1: Which combination poses the greatest risk of clinically significant QT prolongation?
- Azithromycin and haloperidol (Correct answer)
- Amoxicillin and paracetamol
- Lisinopril and metformin
- Omeprazole and atorvastatin
Correct answer: Azithromycin and haloperidol
Both azithromycin and haloperidol independently prolong the cardiac QT interval, and their combination greatly increases the risk of torsades de pointes.
Question 2: Lithium toxicity risk is significantly increased when a patient is also prescribed which drug?
- Amlodipine
- Hydrochlorothiazide (Correct answer)
- Atorvastatin
- Pantoprazole
Correct answer: Hydrochlorothiazide
Thiazide diuretics reduce renal sodium excretion, causing compensatory lithium reabsorption in the proximal tubule and elevated lithium plasma concentrations.
Question 3: Which drug interaction requires the pharmacist to counsel the patient to avoid grapefruit juice?
- Atorvastatin therapy (Correct answer)
- Metformin therapy
- Amoxicillin therapy
- Folic acid supplementation
Correct answer: Atorvastatin therapy
Grapefruit juice inhibits intestinal CYP3A4, increasing atorvastatin bioavailability and raising the risk of myopathy and rhabdomyolysis.
Question 4: A patient on warfarin starts a course of broad-spectrum antibiotics. The pharmacist should counsel them to watch for signs of:
- Reduced anticoagulation effect
- Enhanced anticoagulation and increased bleeding risk (Correct answer)
- Skin photosensitivity
- Elevated blood sugar
Correct answer: Enhanced anticoagulation and increased bleeding risk
Broad-spectrum antibiotics reduce gut flora that produce vitamin K, leading to decreased vitamin K levels and enhanced warfarin anticoagulation effect.
Question 5: Clopidogrel's antiplatelet efficacy is significantly reduced when combined with which drug class?
- Beta-blockers
- Proton pump inhibitors (PPIs) (Correct answer)
- ACE inhibitors
- Loop diuretics
Correct answer: Proton pump inhibitors (PPIs)
Many PPIs, especially omeprazole, inhibit CYP2C19 which is required to convert clopidogrel to its active metabolite, reducing its antiplatelet effect.
Question 6: When counseling a patient about the interaction between tetracyclines and dairy products, the pharmacist explains that dairy:
- Increases tetracycline bioavailability
- Has no effect on tetracycline absorption
- Chelates tetracycline reducing its absorption (Correct answer)
- Speeds up tetracycline renal elimination
Correct answer: Chelates tetracycline reducing its absorption
Calcium in dairy products forms insoluble chelate complexes with tetracyclines in the GI tract, significantly reducing their oral absorption.
APC APC Drug Interactions and Contraindications 2
Which combination poses the greatest risk of clinically significant QT prolongation?