Drug Interactions and Adverse Effects Flashcards

Which adverse effect is specifically associated with fluoroquinolone antibiotics (e.g., ciprofloxacin, levofloxacin)?

Answer: Tendinopathy and tendon rupture (especially Achilles), QT prolongation, and CNS effects

Fluoroquinolones carry black-box warnings for: tendinopathy/tendon rupture (especially Achilles, risk increased with corticosteroids and age >60), peripheral neuropathy, CNS effects, QT prolongation (especially moxifloxacin), and aortic aneurysm. The FDA/Saudi MOH restrict their use to situations where no alternatives exist.

A patient on azathioprine for organ transplant is prescribed allopurinol for gout. What is the critical interaction?

Answer: Allopurinol inhibits xanthine oxidase, dramatically increasing azathioprine/6-mercaptopurine levels causing fatal myelosuppression

Azathioprine is metabolized by xanthine oxidase (XO) to inactive metabolites. Allopurinol inhibits XO, causing dramatic 4-fold increases in azathioprine/6-mercaptopurine levels, causing fatal myelosuppression. The azathioprine dose must be reduced by 75% if allopurinol is added, or febuxostat used instead.

A patient develops a severe skin reaction with blistering, mucosal involvement, and skin detachment covering >30% of body surface area after starting carbamazepine. What syndrome is this?

Answer: Toxic epidermal necrolysis (TEN) — a life-threatening drug reaction

TEN involves >30% body surface area detachment of the epidermis (SJS = <10%, SJS/TEN overlap = 10–30%). Caused most often by carbamazepine, allopurinol, sulfonamides, lamotrigine, and phenytoin. HLA-B*1502 allele increases TEN risk with carbamazepine in Han Chinese/Southeast Asian patients.

Which adverse effect requires immediate discontinuation of clozapine?

Answer: Absolute neutrophil count (ANC) below 500/mm3 — agranulocytosis

Clozapine's most dangerous adverse effect is agranulocytosis. An ANC below 500/mm3 requires immediate, permanent discontinuation of clozapine. The REMS program mandates weekly CBC for 6 months (then biweekly for 6 months, then monthly) to detect early neutropenia before fatal infection occurs.

A patient on simvastatin 80 mg is started on amlodipine. What is the interaction concern?

Answer: Amlodipine inhibits CYP3A4 mildly, potentially raising simvastatin levels; however the main concern is the FDA dose limit for simvastatin 80 mg

Simvastatin is metabolized by CYP3A4. Amlodipine is a mild CYP3A4 inhibitor. However, the primary concern with simvastatin 80 mg is that the FDA has restricted this dose due to high myopathy risk. Strong CYP3A4 inhibitors (azole antifungals, certain macrolides) are absolute contraindications with simvastatin.

A patient develops pulmonary fibrosis after long-term amiodarone therapy. What monitoring is required to detect this complication?

Answer: Annual pulmonary function tests (PFTs) and chest X-ray (or HRCT if symptoms occur)

Amiodarone can cause pulmonary toxicity (hypersensitivity pneumonitis or pulmonary fibrosis) in 5–10% of patients on long-term therapy. Monitoring includes annual chest X-ray, PFTs, and evaluation of any new respiratory symptoms. Other amiodarone toxicities require thyroid function, LFTs, and ophthalmology (corneal microdeposits, optic neuropathy).