A 35-year-old woman with community-acquired pneumonia caused by *Streptococcus pneumoniae* is prescribed azithromycin. Regarding macrolide adverse effects and drug interactions, all of the following are true EXCEPT:

Explanation

## Macrolide Adverse Effects and Drug Interactions ### Correct Statements **High-Yield:** Macrolides, particularly erythromycin and azithromycin, can prolong the QT interval by blocking cardiac potassium channels. This risk is increased in patients with electrolyte abnormalities (hypokalemia, hypomagnesemia) or concurrent use of other QT-prolonging agents (antiarrhythmics, antipsychotics, fluoroquinolones). Torsades de pointes is a serious but rare complication. **Key Point:** Erythromycin is a potent CYP3A4 inhibitor and significantly increases plasma concentrations of drugs metabolized by this enzyme, including theophylline (narrow therapeutic index), warfarin (increased bleeding risk), cyclosporine (nephrotoxicity), and statins (myopathy risk). Azithromycin is a weaker CYP3A4 inhibitor; clarithromycin is intermediate. **Clinical Pearl:** Macrolides act as motilin receptor agonists in the GI tract, enhancing gastric and small intestinal motility. This explains their prokinetic effects but also contributes to GI side effects (nausea, vomiting, diarrhea, abdominal cramping). Azithromycin is sometimes used off-label as a prokinetic agent in gastroparesis. ### Why Macrolides Are CONTRAINDICATED in Myasthenia Gravis **Warning:** Macrolides are NOT safe in myasthenia gravis. They can worsen neuromuscular blockade and precipitate myasthenic crisis. This is a critical safety point. **Mechanism:** Macrolides inhibit acetylcholine release from the presynaptic terminal at the neuromuscular junction, compounding the defect in acetylcholine signaling already present in myasthenia gravis. They may also have direct effects on muscle contractility. **High-Yield:** Macrolides are among the drugs that can unmask or exacerbate myasthenia gravis — a frequently tested contraindication in NEET PG. ### Summary Table: Macrolide Safety Concerns | Adverse Effect / Interaction | Mechanism | Clinical Significance | |-----|-----------|----------| | QT prolongation | Cardiac K^+^ channel blockade | Risk of torsades de pointes; avoid in hypokalemia | | CYP3A4 inhibition (erythromycin > clarithromycin > azithromycin) | Hepatic enzyme inhibition | Increased levels of theophylline, warfarin, cyclosporine, statins | | GI side effects | Motilin receptor agonism | Nausea, vomiting, diarrhea, abdominal cramping | | **Myasthenia gravis exacerbation** | **Decreased acetylcholine release at NMJ** | **Contraindicated; risk of myasthenic crisis** | | Hepatotoxicity (rare) | Cholestasis | Jaundice, elevated LFTs | | Ototoxicity (rare) | Direct 8th nerve toxicity | Hearing loss, tinnitus; higher risk with high doses | **Mnemonic:** **MACROLIDE CONTRAINDICATIONS** — **MG** (Myasthenia Gravis), **QT** (QT prolongation risk), **CYP** (CYP3A4 interactions).