Erythromycin

• Respiratory Tract Infections
• Skin and Soft Tissue Infections
• Gastroparesis (prokinetic)
• Pertussis (Whooping Cough)

Erythromycin is a macrolide antibiotic used to treat various bacterial infections and as a prokinetic agent for gastrointestinal motility disorders. This versatile antibiotic has been in clinical use for decades and remains important for patients with penicillin allergies and for specific indications.

Mechanism of Action

Erythromycin exerts its antibacterial effect by binding to the 50S ribosomal subunit of susceptible bacteria, specifically to the 23S rRNA component. This binding blocks the translocation step of protein synthesis, preventing the growing polypeptide chain from moving from the A-site to the P-site of the ribosome. The effect is primarily bacteriostatic, though it can be bactericidal at high concentrations against highly susceptible organisms. Erythromycin also acts as a motilin receptor agonist in the gastrointestinal tract, stimulating gastric motility and accelerating gastric emptying through its prokinetic effects.

Available Formulations

Erythromycin is available in multiple formulations: erythromycin base (tablets, capsules), erythromycin stearate (tablets), erythromycin ethylsuccinate (oral suspension, tablets, chewable tablets), and erythromycin lactobionate (injection). Topical and ophthalmic formulations are also available. The base and stearate forms should be taken on an empty stomach, while ethylsuccinate can be taken with food to minimize GI upset.

Medical Uses

Erythromycin is FDA-approved for upper and lower respiratory tract infections (pneumonia, bronchitis, pertussis), skin and soft tissue infections, diphtheria, intestinal amebiasis, acute pelvic inflammatory disease (IV), syphilis in penicillin-allergic patients, Legionnaire's disease, chlamydial infections, and prophylaxis of rheumatic fever. Off-label uses include diabetic gastroparesis (prokinetic), acne vulgaris (topical), and ophthalmic infections. It remains important for patients allergic to penicillin.

Dosing Guidelines

For oral administration, typical adult doses range from 250-500 mg every 6 hours or 400-800 mg (ethylsuccinate) every 6-12 hours. For severe infections, up to 4 g daily may be used. For gastroparesis, 250 mg three times daily before meals is typical. Duration varies by indication. The base and stearate should be taken 1 hour before or 2 hours after meals; ethylsuccinate may be taken with food. Intravenous erythromycin is given by slow infusion due to venous irritation.

Important Safety Information

Erythromycin can cause QT prolongation and has been associated with potentially fatal cardiac arrhythmias, particularly torsades de pointes, especially when combined with other QT-prolonging drugs or in patients with electrolyte abnormalities. Hepatotoxicity, including cholestatic hepatitis, can occur, particularly with the estolate form. Infantile hypertrophic pyloric stenosis has been associated with erythromycin use in neonates. Clostridioides difficile-associated diarrhea may occur. The medication should be used with caution in patients with myasthenia gravis.

Drug Interactions

Erythromycin is a potent CYP3A4 inhibitor with numerous significant drug interactions. It increases levels of theophylline, carbamazepine, cyclosporine, tacrolimus, digoxin, midazolam, triazolam, lovastatin, simvastatin, and many others. Concurrent use with QT-prolonging drugs (Class IA and III antiarrhythmics, certain antipsychotics, fluoroquinolones) is contraindicated. Ergot derivatives are contraindicated due to ergotism risk. Colchicine toxicity can occur with concurrent use.

Special Populations

Erythromycin is pregnancy category B; it is one of the preferred macrolides during pregnancy when needed. The estolate form should be avoided in pregnancy due to hepatotoxicity risk. The medication is excreted in breast milk; use with caution during breastfeeding. Erythromycin has been associated with infantile hypertrophic pyloric stenosis when used in neonates under 2 weeks of age. Elderly patients may be more susceptible to QT prolongation and hepatotoxicity. No dose adjustment is needed for renal impairment. The medication is hepatically metabolized; use with caution in hepatic impairment.