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Sunday, July 27, 2025

Erythromycin


Erythromycin is a macrolide antibiotic used to treat a broad range of bacterial infections, especially in patients who are allergic to penicillin. It was one of the first macrolides introduced and remains clinically valuable due to its spectrum of activity, pharmacokinetics, and availability in multiple formulations. It also exhibits prokinetic effects, making it useful in certain gastrointestinal conditions.

Pharmacological Classification

  • Therapeutic class: Antibiotic

  • Pharmacologic class: Macrolide

  • ATC code: J01FA01

  • Chemical class: 14-membered macrolide lactone ring

  • Brand names: Erythrocin, Erythroped, Erymax, Ilosone, Ery-tab

  • Legal status: Prescription-only (Rx)

  • Formulations: Oral tablets, capsules, oral suspension, eye ointment, topical gel/lotion, intravenous solution

Mechanism of Action

Erythromycin exerts its antibacterial effect by inhibiting bacterial protein synthesis. It binds reversibly to the 50S ribosomal subunit of susceptible bacteria, thereby inhibiting translocation steps in protein elongation. This action is bacteriostatic, but at higher concentrations, it can be bactericidal against highly susceptible organisms.

It is especially effective against gram-positive cocci, some gram-negative organisms, and atypical pathogens like Mycoplasma pneumoniae, Chlamydia trachomatis, and Legionella pneumophila.

Antibacterial Spectrum

Gram-positive aerobes:

  • Streptococcus pyogenes

  • Streptococcus pneumoniae

  • Staphylococcus aureus (excluding most MRSA strains)

Gram-negative aerobes:

  • Bordetella pertussis

  • Campylobacter jejuni

  • Neisseria gonorrhoeae (some strains)

  • Haemophilus influenzae (less active unless combined with sulfonamides)

Atypical bacteria:

  • Mycoplasma pneumoniae

  • Chlamydia pneumoniae

  • Legionella pneumophila

Anaerobes: Limited activity

Therapeutic Indications

Infectious Disease Use

  • Respiratory tract infections (e.g., pharyngitis, sinusitis, bronchitis, pneumonia)

  • Skin and soft tissue infections (e.g., impetigo, acne, erysipelas, cellulitis)

  • Ocular infections (e.g., bacterial conjunctivitis via erythromycin eye ointment)

  • Chlamydial infections, particularly in pregnant women as an alternative to tetracyclines

  • Syphilis, gonorrhea, and pelvic inflammatory disease (when penicillin is contraindicated)

  • Whooping cough (pertussis) treatment and prophylaxis

  • Campylobacter enteritis

  • Diphtheria prophylaxis (carriers and close contacts)

Gastrointestinal Use

  • Gastroparesis and diabetic gastropathy (off-label): Erythromycin acts as a motilin receptor agonist, enhancing gastric emptying

Dermatologic Use

  • Topical erythromycin gel/lotion is widely used for mild to moderate acne vulgaris, often in combination with benzoyl peroxide or clindamycin

Ophthalmologic Use

  • Neonatal conjunctivitis prophylaxis: Erythromycin ophthalmic ointment is used shortly after birth to prevent Neisseria gonorrhoeae or Chlamydia trachomatis transmission

Dosage and Administration

Oral dosage (adults)

  • Typical dose: 250–500 mg every 6 hours or 500 mg every 12 hours

  • Maximum: 2–4 g/day depending on severity

Oral dosage (children)

  • 30–50 mg/kg/day in divided doses

  • Adjusted by weight and infection type

Topical use (acne)

  • Apply thin film once or twice daily

  • Often used in fixed combination products

IV formulation

  • Reserved for serious infections or patients unable to take oral medications

  • Dose: 15–20 mg/kg/day in divided doses

  • Must be diluted and infused slowly to reduce the risk of phlebitis

Ophthalmic use

  • One application to each eye within one hour of birth in neonates

  • In bacterial conjunctivitis: apply every 4–12 hours

Pharmacokinetics

  • Absorption: Oral bioavailability ~30–65%

  • Distribution: Wide distribution; penetrates most tissues but poor CSF penetration unless meninges are inflamed

  • Plasma protein binding: ~70–90%

  • Metabolism: Hepatic (CYP3A4 substrate)

  • Elimination: Primarily biliary; small renal component

  • Half-life: ~1.5–2.5 hours; prolonged in hepatic impairment

Contraindications

  • Hypersensitivity to erythromycin or other macrolides

  • Concomitant use with drugs metabolized by CYP3A4 known to cause QT prolongation (e.g., cisapride, pimozide, ergotamine)

  • Pre-existing liver disease with cholestasis or hepatic dysfunction

Warnings and Precautions

QT Prolongation and Torsades de Pointes

  • Erythromycin can prolong the QT interval, increasing the risk of torsades de pointes, especially in:

    • Patients with electrolyte imbalances (hypokalemia, hypomagnesemia)

    • Concomitant use of other QT-prolonging drugs

    • Hepatic impairment

Hepatotoxicity

  • Associated with cholestatic hepatitis, usually reversible after discontinuation

  • Monitor liver enzymes in prolonged therapy

Pseudomembranous Colitis

  • May result from Clostridioides difficile overgrowth

  • Consider in patients with severe or persistent diarrhea

Superinfections

  • Long-term use may result in fungal or resistant bacterial overgrowth

Adverse Effects

Common

  • Gastrointestinal: Nausea, vomiting, diarrhea, abdominal pain

  • Taste disturbances (metallic taste)

  • Injection site reactions (IV): Phlebitis, pain

Uncommon to Rare

  • Hepatotoxicity: Cholestatic jaundice

  • Ototoxicity: Reversible hearing loss with high doses or IV use

  • Allergic reactions: Rash, urticaria, anaphylaxis

  • QT prolongation, arrhythmias

  • Pancreatitis (rare)

Drug Interactions

CYP3A4 Inhibition

Erythromycin is a potent inhibitor of CYP3A4, and it may increase the serum levels of many drugs, including:

  • Statins (e.g., simvastatin, atorvastatin): Increased risk of rhabdomyolysis

  • Carbamazepine: Risk of toxicity

  • Theophylline: Increased levels and side effects

  • Warfarin: Enhanced anticoagulant effect

  • Cisapride, pimozide, astemizole: Risk of fatal arrhythmias; contraindicated

  • Digoxin: Increased absorption due to suppression of gut flora

Other Interactions

  • Clindamycin and chloramphenicol: Antagonistic interaction due to similar ribosomal binding site

  • Drugs causing QT prolongation: Additive risk when combined

Pregnancy and Lactation

Pregnancy

  • Category B (U.S.): No evidence of fetal harm in animal studies; used safely in humans for decades

  • Preferred macrolide in pregnancy when an alternative to penicillin is needed

Lactation

  • Erythromycin is excreted in breast milk

  • Generally considered safe; monitor for GI side effects (e.g., diarrhea, candidiasis) in infants

Use in Special Populations

  • Hepatic impairment: Use with caution; dose adjustments may be needed

  • Renal impairment: No major adjustment required unless severe

  • Elderly: Monitor for cardiac effects and hepatotoxicity

  • Pediatrics: Widely used in children; dosing based on body weight

Clinical Comparisons

Vs. Clarithromycin

  • Clarithromycin has improved acid stability, longer half-life, and better tissue penetration

  • Less GI irritation than erythromycin

  • More effective against H. influenzae

Vs. Azithromycin

  • Azithromycin has fewer GI side effects, once-daily dosing, and a longer half-life

  • Better activity against gram-negative organisms, but less active than erythromycin against S. pyogenes

Vs. Penicillins

  • Erythromycin is used as an alternative in penicillin-allergic patients

  • Effective against many of the same gram-positive organisms, though increasing resistance is a concern

Patient Counseling Points

  • Take with food if stomach upset occurs, but absorption may be slightly reduced

  • Do not take with aluminum or magnesium-containing antacids

  • Report symptoms of liver dysfunction: yellowing of skin/eyes, dark urine, fatigue

  • Watch for signs of arrhythmias or dizziness

  • For acne: Apply a thin layer to clean, dry skin; avoid eyes and mucous membranes

  • Complete the entire prescribed course, even if symptoms improve

  • Use non-hormonal contraception if using erythromycin with oral contraceptives, due to reduced efficacy via enzyme induction in some cases




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