Generic Name

Erythromycin

Mechanism

• Binding target: 50S ribosomal subunit (23S rRNA)
• Inhibition: Blocks the peptidyl‑transferase reaction and the exit tunnel for the nascent peptide chain, preventing elongation of the polypeptide.
• Effect: Bacteriostatic against most gram‑positive cocci (e.g., *Streptococcus pyogenes*) and some gram‑negative organisms (e.g., *Haemophilus influenzae*).
• Anti‑inflammatory: Reduces the release of pro‑inflammatory cytokines (IL‑6, TNF‑α), useful in allergic bronchopulmonary aspergillosis and chronic obstructive pulmonary disease.

Pharmacokinetics

• Absorption: Oral bioavailability ~15–30 % (rapid first‑pass hepatic metabolism).
• Distribution: High protein binding (~75 %); penetrates well into tissues and secretions (pulmonary, bronchial, genital).
• Metabolism: Extensive first‑pass glucuronidation and CYP3A4 metabolism in the liver.
• Elimination: Primarily fecal via biliary excretion; renal clearance of ~10 % of the dose.
• Half‑life: 1–2 h (oral) and 3–6 h (IV), requiring frequent dosing.

Indications

• Respiratory tract infections:
• *Mycoplasma pneumoniae* (macrolide‑resistant strains, use with caution)
• *Chlamydia pneumoniae*
• Skin and soft‑tissue infections:
• Mild to moderate infections of *Staphylococcus aureus* (methicillin‑susceptible)
• Sexually transmitted infections:
• *Chlamydia trachomatis* (single dose 1 g oral)
• *Ureaplasma urealyticum* (contextual use)
• Prophylaxis:
• Beta‑hemolytic streptococcal sore throat to prevent rheumatic fever.
• Adjunctive therapy:
• Allergic asthma or bronchopulmonary aspergillosis: reduces bronchial inflammation.

Contraindications

• Contraindications
• Hypersensitivity to macrolides or the specific formulation.
• Severe hepatic impairment (ALT > 3× ULN).
• QT prolongation or congenital long QT syndrome.
• Warnings
• Drug interactions:
• CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir) ↑ erythromycin levels.
• CYP3A4 inducers (rifampin, phenytoin, carbamazepine) ↓ efficacy.
• Vitamin K antagonists (warfarin) ↑ bleeding risk.
• Hepatotoxicity: Monitor LFTs; liver injury most frequent with IV preparations.
• Gastro‑intestinal toxicity: Diarrhea, nausea, vomiting; particularly due to stimulation of intestinal motility.
• Metabolic adaptation: In patients on pro‑drugs requiring hepatic activation (e.g., morphine to morphine‑3‑glucuronide), erythromycin can inhibit conversion, worsening analgesia.

Dosing

• IV infusion rate: 30 mg/min (max 30 mg/min) to prevent hypersensitivity reactions.
• Oral formulation: Use with food to maximize absorption but can increase GI upset.

Adverse Effects

• Common (≤5 %)
• Nausea, vomiting, abdominal pain, diarrhea
• Fluorescent “flu” (red‑shifted urine with UV light) – benign
• Headache, dizziness
• Serious (≤1 %)
• Hepatotoxicity: cholestatic hepatitis, ↑ bilirubin
• QT prolongation → torsades de pointes
• Intracranial hypertension (rare, with high doses)
• Severe allergic reactions (anaphylaxis)
• Superinfection with resistant organisms (e.g., *Pseudomonas*, *Enterococcus*)

Monitoring

• Baseline: LFTs, renal panel, serum electrolytes, ECG if QT risk.
• Follow‑up:
• After 48–72 h of IV therapy: repeat LFTs, electrolytes, ECG.
• For extended courses: Check LFTs weekly and ECG monthly.
• Drug Interactions: Review all concomitant medications; adjust dose or monitor therapeutic levels.
• Therapeutic Drug Monitoring: Only in severe infections or for IV formulations to achieve target serum trough ≥ 4 µg/mL.

Clinical Pearls

• Use as an alternative to penicillin in mild streptococcal pharyngitis; however, avoid in severe penicillin allergy because cross‑reactivity to other β‑lactams is rare.
• Low bioavailability and extensive hepatic metabolism together make oral erythromycin susceptible to drug‑drug interactions; choose clarithromycin cautiously when CYP3A4 modulation is a concern.
• Fluorescence of urine under UV is diagnostic of successful ingestion—useful bedside test for compliance in pediatric patients.
• Avoid IV erythromycin in patients on fluoroquinolones; both can prolong QT, dramatically increasing arrhythmia risk.
• Metabolite accumulation in hepatic failure: many adverse events stem from the active metabolites (erythromycin estolate); switch to an alternative macrolide (azithromycin) when liver function impaired.
• Dose adjustment not required for renal failure but monitor for GI complications – consider adding prokinetic (prochlorperazine) if tolerance is an issue.
• Optimize absorption: administer with a high‑fat meal or a small snack; however, be mindful that food can increase GI side effects.
• Clinical evidence: Erythromycin remains less effective against *Mycoplasma* due to widespread macrolide resistance; use with microbiologic data when possible.

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• Quick Ref:
• Class: Macrolide
• Action: 50S ribosomal inhibition
• Key interactions: CYP3A4 inhibitors ↑, CYP3A4 inducers ↓
• Common serious side effect: QT prolongation → torsades.

Use this card as a snapshot for exam prep or bedside quick‑reference.