1. Clinical Overview
Erythromycin is a macrolide antibiotic derived from *Streptomyces erythreus*. It is a broad-spectrum bacteriostatic agent, primarily used for treating infections caused by Gram-positive bacteria, atypical pathogens, and some Gram-negative organisms. It is a first-line alternative for patients allergic to penicillins. In the Indian context, it is widely used for respiratory, skin, and soft tissue infections, and for prophylaxis in rheumatic fever.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are reached in 1-4 hours after oral administration. | Approximately 6-12 hours, depending on formulation and dose, necessitating multiple daily doses. | Approximately 30-65% for the base and stearate forms; enteric-coated formulations have variable absorption. Estolate salt has higher bioavailability. |
2. Mechanism of Action
Erythromycin reversibly binds to the 50S subunit of the bacterial ribosome, specifically at the peptidyl transferase center. This binding inhibits the translocation step of protein synthesis, where tRNA moves from the A-site to the P-site. This prevents the addition of new amino acids to the growing peptide chain, thereby inhibiting bacterial protein synthesis.
3. Indications & Uses
- Upper and lower respiratory tract infections (e.g., pharyngitis, tonsillitis, acute bronchitis, community-acquired pneumonia) caused by *Streptococcus pyogenes*, *Streptococcus pneumoniae*, *Mycoplasma pneumoniae*, *Legionella pneumophila*
- Skin and soft tissue infections (e.g., cellulitis, erysipelas, impetigo) caused by *Staphylococcus aureus* and *Streptococcus pyogenes*
- Pertussis (Whooping Cough) - for treatment and post-exposure prophylaxis
- Diphtheria - as adjunctive therapy to antitoxin and for eradication of carrier state
- Chlamydial infections (e.g., trachoma, conjunctivitis, urogenital infections in pregnancy where tetracyclines are contraindicated)
4. Dosage & Administration
Adult Dosage: 250-500 mg orally every 6 hours, OR 333 mg every 8 hours, OR 500 mg every 12 hours (for specific formulations). For severe infections: Up to 4 g/day in divided doses. Typical dose: 250 mg QID.
Administration: Oral tablet/capsule: Preferably taken on an empty stomach (1 hour before or 2 hours after meals) with a full glass of water to maximize absorption. Erythromycin ethylsuccinate and estolate may be taken with or without food. Do not crush or chew enteric-coated tablets. Complete the full prescribed course even if symptoms improve.
5. Side Effects
Common side effects may include:
- Nausea
- Vomiting
- Abdominal pain/cramping
- Diarrhea
- Anorexia
- Mild skin rash or itching
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Cisapride, Terfenadine, Astemizole | Increased risk of life-threatening ventricular arrhythmias (QT prolongation, Torsades de Pointes) | Contraindicated |
| Simvastatin, Lovastatin, Atorvastatin | Increased statin levels, significantly raising risk of myopathy and rhabdomyolysis | Major |
| Warfarin | Potentiates anticoagulant effect, increasing INR and risk of bleeding | Major |
| Carbamazepine | Increased carbamazepine levels leading to toxicity (dizziness, ataxia, nausea) | Major |
| Theophylline | Increased theophylline levels, risk of nausea, seizures, arrhythmias | Major |
| Digoxin | Increased digoxin bioavailability, risk of toxicity (nausea, bradycardia, arrhythmias) | Major |
| Cyclosporine, Tacrolimus | Increased immunosuppressant levels, risk of nephrotoxicity and neurotoxicity | Major |
| Midazolam, Triazolam | Increased sedation and prolonged effect | Major |
| Oral Contraceptives | Potential for reduced contraceptive efficacy; advise backup method | Moderate |
| Sildenafil | Increased sildenafil levels, risk of hypotension, visual disturbances | Major |
7. Patient Counselling
- DO complete the full course of therapy, even if you feel better.
- DO take the medication on an empty stomach (1 hr before or 2 hrs after meals) unless it is the estolate or ethylsuccinate salt/formulation.
- DO inform your doctor about all other medications, including OTC drugs, supplements, and herbal products.
- DO drink plenty of fluids.
- DONT skip doses. If you miss a dose, take it as soon as you remember, but skip if it's almost time for the next dose.
- DONT take antacids within 2 hours of taking erythromycin.
- DONT share your medication with others.
8. Toxicology & Storage
Overdose: Nausea, vomiting, diarrhea, epigastric distress, hearing loss (reversible or permanent), severe QT prolongation leading to ventricular arrhythmias.
Storage: Store at room temperature (15-25°C), in a cool, dry place, away from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack. For suspensions: reconstitute with cool water as directed, shake well before use, and store in refrigerator (2-8°C). Discard unused portion after 10-14 days as per manufacturer's instructions.