Erythromycin

What is Erythromycin?

Erythromycin is a macrolide antibiotic derived from Saccharopolyspora erythraea and was one of the first macrolide antibiotics introduced into clinical practice. It remains an important therapeutic option in sexual health, particularly as an alternative treatment for chlamydia and other bacterial STIs in patients who cannot take first-line agents.

Erythromycin is listed on the WHO Model List of Essential Medicines and has a well-established safety profile, including extensive data on use during pregnancy. While newer macrolides such as azithromycin have largely replaced erythromycin for many indications due to a more convenient dosing schedule and better gastrointestinal tolerability, erythromycin remains clinically relevant.

How does Erythromycin work?

Erythromycin inhibits bacterial protein synthesis by binding reversibly to the 50S ribosomal subunit of susceptible bacteria. Specifically, it prevents translocation — the movement of the ribosome along the messenger RNA strand — which halts the elongation of the polypeptide chain.

Key mechanisms:

• Bacteriostatic activity: At standard doses, erythromycin inhibits bacterial growth (bacteriostatic) by preventing protein synthesis, allowing the immune system to clear the infection
• Bactericidal at high concentrations: Against some organisms, erythromycin can kill bacteria at higher concentrations
• Intracellular penetration: Erythromycin concentrates within phagocytic cells (macrophages, neutrophils), making it effective against intracellular pathogens like Chlamydia trachomatis
• Spectrum: Active against gram-positive cocci, Chlamydia, Mycoplasma, Ureaplasma, and Treponema pallidum

Who is Erythromycin for?

In sexual health practice, erythromycin is used for:

• Chlamydia (alternative, particularly in pregnancy): WHO and European STI guidelines recommend erythromycin as an alternative for chlamydia treatment in pregnant women who cannot take azithromycin
• Non-gonococcal urethritis (NGU): Alternative when first-line agents are not appropriate
• Syphilis (penicillin allergy, pregnancy): Alternative to doxycycline in pregnant, penicillin-allergic patients (though efficacy data are limited — specialist guidance is required)
• Lymphogranuloma venereum (LGV): Second-line alternative to doxycycline

Erythromycin is not appropriate for:

• Patients with known macrolide allergy
• Concurrent use of certain QT-prolonging medicines (risk of serious cardiac arrhythmia)
• Concurrent use of ergotamine or dihydroergotamine (risk of ergot toxicity)
• Patients taking pimozide, terfenadine, or astemizole

Dosage

Indication	Dose	Duration
Chlamydia (pregnancy alternative)	500 mg four times daily	7 days
Chlamydia (alternative, non-pregnant)	500 mg twice daily (erythromycin ethylsuccinate)	14 days
NGU (alternative)	500 mg four times daily	7 days
Syphilis (penicillin allergy — specialist guidance)	500 mg four times daily	14 days (early)

Erythromycin is available as tablets, capsules, and oral suspension. It should be taken with food to reduce gastrointestinal side effects. Different salt forms (erythromycin base, stearate, ethylsuccinate) may have slightly different dosing — the prescribing doctor will specify the appropriate form and dose.

Side effects

Gastrointestinal side effects are the most significant limitation of erythromycin and affect a large proportion of patients.

Very common side effects (affecting more than 1 in 10 people)

• Nausea
• Vomiting
• Abdominal cramping and pain
• Diarrhoea

Common side effects

• Loss of appetite
• Flatulence
• Headache

Less common but clinically significant

• Cholestatic jaundice (hepatotoxicity): Particularly with the estolate form; presents as jaundice, abdominal pain, and elevated liver enzymes — usually reversible on stopping
• QT interval prolongation: Can lead to potentially life-threatening cardiac arrhythmias (torsades de pointes), especially in combination with other QT-prolonging drugs
• Ototoxicity: Hearing loss and tinnitus, particularly with high doses and prolonged use — usually reversible

Rare but serious

• Severe allergic reactions (anaphylaxis)
• Clostridioides difficile colitis

Erythromycin vs alternative antibiotics for chlamydia

Antibiotic	Pregnancy Safe	Dose	Duration	GI Tolerability
Doxycycline (1st line)	No	100 mg twice daily	7 days	Moderate
Azithromycin (Zithromax)	Generally yes	1 g single dose	1 day	Good
Erythromycin	Yes	500 mg four times daily	7 days	Poor
Amoxicillin	Yes	500 mg three times daily	7 days	Good

Why erythromycin is still used: Despite its gastrointestinal side effect profile, erythromycin has an extensive safety record in pregnancy and remains an established alternative when other agents are contraindicated. In non-pregnant patients, azithromycin is generally preferred as an alternative macrolide due to its single-dose convenience and better tolerability.

How to access Erythromycin online in Europe

Erythromycin is a prescription-only medicine (POM) in all EU member states. It cannot legally be dispensed without a valid prescription from a licensed medical professional.

Prescrivia operates as a technology intermediary: we do not prescribe medicines, employ doctors, or sell medicines. Our platform connects patients with independent EU-registered doctors who can conduct confidential online health assessments.

The process:

1. Complete a health assessment: Describe your symptoms, pregnancy status, allergies, and current medications.
2. Doctor review: An independent EU-registered doctor reviews your assessment. If erythromycin is clinically appropriate, they may issue a prescription.
3. Pharmacy fulfilment: The prescription is sent to a licensed EU pharmacy partner for dispensing and direct delivery.

Important: Prescrivia does not guarantee that a prescription will be issued. All prescribing decisions are made independently by qualified medical professionals.

Important safety information

Do not take Erythromycin if you:

• Are allergic to erythromycin or other macrolide antibiotics
• Are taking pimozide, terfenadine, astemizole, cisapride, ergotamine, or dihydroergotamine (potentially fatal interactions)
• Have a history of QT prolongation or are taking other QT-prolonging medicines without specialist assessment

Tell your doctor before taking Erythromycin if you:

• Have liver disease
• Have myasthenia gravis (erythromycin may worsen muscle weakness)
• Are taking statins (particularly atorvastatin, simvastatin, lovastatin — risk of myopathy increases)
• Are taking warfarin (monitor INR closely)
• Are taking ciclosporin or tacrolimus (erythromycin raises their blood levels)

Drug interactions: Erythromycin is a significant CYP3A4 inhibitor. The number of potentially affected medicines is large — always provide a complete medication list to your doctor during assessment.

Sources

Medical information on this page is based on the following sources:

• World Health Organization (WHO). WHO Guidelines for the Treatment of Chlamydia trachomatis. 2016. who.int
• World Health Organization (WHO). WHO Model List of Essential Medicines. 22nd edition, 2021.
• European Medicines Agency (EMA). Erythromycin — Summary of Product Characteristics. Available at: ema.europa.eu
• European STI Guidelines Editorial Board. 2015 European guideline on the management of Chlamydia trachomatis infections. Journal of the European Academy of Dermatology and Venereology.

This content is reviewed periodically to reflect updated clinical guidance. It is provided for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for advice relevant to your individual health circumstances.