Macrolides are the go-to when a patient is penicillin-allergic and needs coverage for respiratory or atypical infections — but their cardiac and GI risks catch students off guard.
Macrolides — azithromycin, erythromycin, clarithromycin — work by binding the 50S ribosomal subunit of bacteria, blocking protein synthesis. This makes them bacteriostatic at standard doses. They cover gram-positive cocci, atypical organisms (Mycoplasma pneumoniae, Chlamydia, Legionella), and some gram-negatives like H. influenzae. Azithromycin is the most commonly prescribed; its long tissue half-life allows the classic Z-pack (5-day course). Erythromycin is used in neonatal eye prophylaxis (0.5% ophthalmic ointment applied within 1 hour of birth to prevent gonococcal ophthalmia neonatorum). A critical adverse effect is QT prolongation, which increases the risk of torsades de pointes — always assess the patient's cardiac history and concurrent QT-prolonging medications before administration. GI distress (nausea, abdominal cramping, diarrhea) is the most common side effect, especially with erythromycin, because macrolides stimulate motilin receptors in the gut. Hepatotoxicity can occur; monitor for jaundice, dark urine, and elevated liver enzymes. Erythromycin and clarithromycin are potent CYP3A4 inhibitors, creating dangerous drug interactions — particularly with warfarin, statins, and carbamazepine (azithromycin has minimal CYP3A4 activity). Teach the patient to report palpitations, syncope, or severe diarrhea immediately.
Key Distinctions
Don't confuse macrolides (50S inhibitors, bacteriostatic) with tetracyclines (30S inhibitors). Students mix up erythromycin eye ointment for neonates (prevents gonococcal infection) with silver nitrate (historically used, now outdated). QT prolongation is a macrolide risk — penicillins do not carry this cardiac concern, so switching a penicillin-allergic patient to azithromycin introduces a new safety consideration.
Clinical Pearl
Think 'Macro-LONG QT' — macrolides lengthen the QT interval. Always check the ECG history and med list before hanging that Z-pack.