Drug Interactions and Contraindications Flashcards

A patient stabilised on lithium for bipolar disorder is prescribed naproxen for arthritis. What is the most likely outcome?

Answer: Lithium levels increase due to reduced renal prostaglandin-mediated excretion

NSAIDs reduce renal prostaglandin synthesis, decreasing renal blood flow and lithium excretion, which can increase lithium plasma levels into the toxic range.

Beta-blockers are contraindicated in patients with which of the following respiratory conditions?

Answer: Reactive airway disease / bronchial asthma

Non-selective beta-blockers block beta-2 receptors in bronchial smooth muscle, causing bronchoconstriction and potentially fatal bronchospasm in asthmatic patients.

Concomitant use of clopidogrel and omeprazole is clinically significant because:

Answer: Omeprazole inhibits CYP2C19, reducing conversion of clopidogrel to its active metabolite

Clopidogrel is a prodrug requiring CYP2C19 for activation; omeprazole's CYP2C19 inhibition reduces formation of the active thiol metabolite, diminishing antiplatelet efficacy.

Oral tetracyclines should not be taken with dairy products because calcium:

Answer: Forms insoluble chelate complexes with tetracyclines, reducing absorption

Calcium (and other divalent/trivalent cations) chelates tetracyclines to form insoluble complexes in the gut, significantly reducing oral bioavailability by up to 50–80%.

Oral fluoroquinolones (e.g., ciprofloxacin) should be administered at least 2 hours before or 6 hours after antacids primarily because antacids:

Answer: Chelate fluoroquinolones via magnesium/aluminium ions, reducing absorption

Magnesium and aluminium in antacids form insoluble chelate complexes with fluoroquinolones in the gastrointestinal tract, markedly reducing their oral absorption.

Co-prescribing methotrexate with NSAIDs increases the risk of methotrexate toxicity primarily because:

Answer: NSAIDs reduce renal clearance and tubular secretion of methotrexate, increasing plasma levels

NSAIDs reduce renal prostaglandin synthesis, decreasing renal blood flow and inhibiting organic anion transporters involved in tubular secretion of methotrexate, leading to dangerous accumulation.

Ciprofloxacin co-administered with theophylline is likely to cause which outcome?

Answer: Theophylline toxicity due to CYP1A2 inhibition by ciprofloxacin

Ciprofloxacin is a potent CYP1A2 inhibitor; since theophylline is primarily metabolised by CYP1A2, co-administration can reduce theophylline clearance by up to 30–50%, causing toxicity (seizures, arrhythmias).