Erythromycin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	10-20 mg/kg	q8h	Duration: 5-7 days or as directed Notes: Use enteric-coated tablets or estolate/ethylsuccinate forms to improve absorption. Administer on an empty stomach if possible.
Cat	PO	10-20 mg/kg	q8h	Duration: 5-7 days or as directed Notes: Cats may be more sensitive to GI upset; consider administering with a small amount of food if vomiting occurs.
Horse	IV	5-10 mg/kg	q6-12h	Duration: 5-7 days or as directed Notes: Oral absorption is poor; IV administration is preferred. For Rhodococcus equi, use in combination with rifampin (10-20 mg/kg PO q12h).
Cattle	IM	2.5-5 mg/kg	q24h	Duration: 3-5 days Notes: Use only for susceptible infections. Observe withdrawal times. Not for use in lactating dairy cows if milk withdrawal is not observed.
Sheep/Goats	IM	2.5-5 mg/kg	q24h	Duration: 3-5 days Notes: Extra-label use; observe withdrawal times.
Rabbit	PO	10-20 mg/kg	q12h	Duration: 5-7 days Notes: Use with caution in rabbits due to risk of GI dysbiosis; monitor for diarrhea.
Poultry (chickens, turkeys)	PO (water medication)	10-20 mg/kg body weight per day	q24h	Duration: 3-5 days Notes: Administer in drinking water. Ensure adequate water intake. Observe withdrawal times for eggs and meat.

Clinical Indications & Species Uses

General Indications

Treatment of bacterial infections caused by susceptible gram-positive organisms and mycoplasmas
Alternative to penicillins in penicillin-allergic patients

Dog (Canine)

Treatment of bacterial infections caused by susceptible gram-positive organisms, including skin and soft tissue infections, respiratory tract infections, and urinary tract infections
Treatment of Campylobacteriosis
Treatment of Bordetella bronchiseptica infections (kennel cough)
Treatment of Mycoplasma infections
Alternative for penicillin-allergic patients

Cat (Feline)

Treatment of bacterial infections caused by susceptible organisms, including respiratory infections, skin infections, and wound infections
Treatment of Mycoplasma infections
Treatment of Chlamydophila felis conjunctivitis
Treatment of Bordetella bronchiseptica infections

Horse (Equine)

Treatment of respiratory tract infections caused by susceptible organisms, including Rhodococcus equi (in combination with rifampin)
Treatment of Mycoplasma infections
Treatment of contagious equine metritis (Taylorella equigenitalis)

Cattle (Bovine)

Treatment of respiratory disease (pneumonia) caused by susceptible organisms, including Pasteurella multocida and Mycoplasma bovis
Treatment of foot rot (interdigital necrobacillosis)
Treatment of metritis
Treatment of mastitis (though not first-line)

Small Ruminants (Sheep / Goat)

Treatment of respiratory infections
Treatment of enteritis caused by Campylobacter
Treatment of mastitis (though not first-line)

Rabbit & Small Mammals

Treatment of respiratory infections (e.g., Pasteurella multocida)
Treatment of enteritis (e.g., Clostridium spp.)
Treatment of abscesses (in combination with surgical drainage)

Avian & Poultry

Treatment of mycoplasmosis (Mycoplasma gallisepticum, M. synoviae)
Treatment of infectious coryza
Treatment of chronic respiratory disease (CRD) in chickens and turkeys

Exotic & Other Species

Reptiles: treatment of bacterial infections (e.g., abscesses, respiratory infections)
Small mammals (e.g., ferrets, guinea pigs): treatment of respiratory and skin infections

Pharmacology & Mechanism of Action

Drug Class: Macrolide antibiotic | Pharmacological Group: Antibacterial

Mechanism of Action: Erythromycin is a macrolide antibiotic that inhibits bacterial protein synthesis by binding reversibly to the 50S ribosomal subunit, specifically to the 23S rRNA of the 50S subunit. This binding blocks the translocation step of peptide chain elongation, thereby preventing the growth and replication of susceptible bacteria. It is generally bacteriostatic, but may be bactericidal at high concentrations or against highly susceptible organisms. It exerts its effect on a wide range of gram-positive bacteria, some gram-negative bacteria, and certain mycoplasmas, chlamydiae, and rickettsiae.

Pharmacodynamics: Erythromycin exhibits time-dependent killing and has a post-antibiotic effect (PAE) against susceptible organisms. Its activity is enhanced at alkaline pH. It is effective against many aerobic and anaerobic gram-positive bacteria, including Staphylococcus spp., Streptococcus spp., and some gram-negative organisms such as Pasteurella multocida, Bordetella bronchiseptica, and Campylobacter spp. It is also active against Mycoplasma, Chlamydia, and Rickettsia. Resistance can develop via target site modification (erm genes), efflux pumps, or enzymatic inactivation.

⚡ Pharmacokinetics Summary

Absorption: Erythromycin base is acid-labile and is usually administered as enteric-coated tablets or as esters (e.g., estolate, ethylsuccinate) to improve oral absorption. Absorption occurs primarily in the upper small intestine. Food may delay absorption but does not significantly reduce total bioavailability. In dogs, oral absorption is variable; the base has a bioavailability of about 50-70% when given as enteric-coated tablets. In horses, oral absorption is poor and unreliable; therefore, intravenous administration is preferred. In ruminants, oral administration is not recommended due to ruminal degradation.

Distribution: Erythromycin is widely distributed throughout the body, including into tissues and fluids such as lung, liver, kidney, and prostate. It penetrates well into intracellular sites and is concentrated in phagocytes. It crosses the placenta and is excreted in milk. It does not penetrate the cerebrospinal fluid well unless the meninges are inflamed. Protein binding is approximately 70-80% in dogs and cats, and about 65-75% in horses.

Metabolism: Erythromycin is metabolized primarily in the liver via demethylation and hydroxylation, involving cytochrome P450 enzymes (CYP3A4 in humans; similar enzymes in animals). It can inhibit its own metabolism and that of other drugs. The metabolites are microbiologically inactive.

Excretion: Erythromycin is primarily excreted in bile and feces, with only about 5-15% excreted unchanged in urine. Enterohepatic recirculation occurs. In animals with hepatic dysfunction, elimination may be prolonged.

Half-Life: Dogs: approximately 1-2 hours; Cats: approximately 1-2 hours; Horses: approximately 1-2 hours; Cattle: approximately 2-3 hours; Rabbits: approximately 1-2 hours.

Bioavailability: Oral: variable; dogs 50-70% (enteric-coated base); horses poor and unreliable; cattle poor due to rumen degradation. Injectable forms are 100% bioavailable.

Protein Binding: Dogs: 70-80%; Cats: 70-80%; Horses: 65-75%; Cattle: 70-80%.

Available Formulations & Strengths

Oral Tablet (enteric-coated) 250 mg, 500 mg (PO)

Oral Suspension (as estolate or ethylsuccinate) 125 mg/5 mL, 250 mg/5 mL (PO)

Injectable Solution (as lactobionate or gluceptate) 50 mg/mL (as lactobionate) (IV)

Injectable Solution (as phosphate) 100 mg/mL (IM)

Ophthalmic Ointment 5 mg/g (0.5%) (Ophthalmic)

Topical Ointment 1% or 2% (Topical)

Contraindications & Clinical Warnings

Contraindications:

Known hypersensitivity to erythromycin or other macrolides
Severe hepatic impairment
Concurrent use with drugs that prolong QT interval (e.g., terfenadine, astemizole, cisapride) due to risk of fatal arrhythmias
Use in horses with diarrhea or suspected gastrointestinal stasis (may exacerbate)
Use in rabbits and other hindgut fermenters due to risk of enterotoxemia (unless specifically indicated and with caution)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic disease.
Use with caution in animals with myasthenia gravis (may exacerbate weakness).
In horses, oral administration may cause severe diarrhea and colitis; use IV route preferentially.
In ruminants, oral administration may disrupt ruminal flora; use injectable forms.
In rabbits and rodents, erythromycin can cause fatal enterotoxemia due to overgrowth of Clostridium difficile; use only when no alternative and with careful monitoring.
May cause gastrointestinal upset (vomiting, diarrhea) in dogs and cats; administer with food if GI upset occurs.
Prolonged use may result in superinfection with resistant organisms.
Use in food animals must comply with withdrawal times and extra-label drug use regulations.

Adverse Effects & Reactions

Common:

Gastrointestinal disturbances (vomiting, diarrhea, anorexia)
Injection site reactions (pain, swelling, sterile abscesses) with IM administration
Hypersensitivity reactions (rash, urticaria)

Serious / Severe:

Hepatotoxicity (elevated liver enzymes, jaundice)
Ototoxicity (especially with high doses or renal impairment)
Cardiac arrhythmias (QT prolongation, torsades de pointes)
Severe diarrhea/colitis in horses and rabbits
Anaphylaxis (rare)

Rare:

Blood dyscrasias (eosinophilia, leukopenia)
Stevens-Johnson syndrome
Pseudomembranous colitis

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Theophylline	Erythromycin inhibits CYP450 metabolism of theophylline, increasing theophylline levels and risk of toxicity (seizures, arrhythmias).	High
Digoxin	Erythromycin may increase digoxin absorption and levels, leading to digoxin toxicity.	Moderate
Warfarin	Erythromycin may potentiate the anticoagulant effect of warfarin by inhibiting its metabolism, increasing bleeding risk.	High
Corticosteroids (e.g., methylprednisolone)	Erythromycin may increase corticosteroid levels and effects.	Moderate
Antacids	Antacids may decrease the absorption of oral erythromycin; separate administration by at least 2 hours.	Mild
Clindamycin	May have antagonistic effects; avoid concurrent use.	Moderate
Drugs that prolong QT interval (e.g., fluoroquinolones, azole antifungals)	Additive QT prolongation, increasing risk of ventricular arrhythmias.	High

Overdose & Toxicity Management

Signs of Toxicity:

Severe vomiting and diarrhea
Dehydration
Hearing loss (especially with high doses)
Hepatotoxicity
Cardiac arrhythmias (QT prolongation)
Respiratory depression (in severe cases)

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Induce vomiting if recent oral ingestion and animal is conscious (in dogs and cats). Administer activated charcoal to reduce absorption. Provide intravenous fluids for dehydration and electrolyte imbalances. Monitor liver enzymes, cardiac function (ECG), and hearing. In severe cases, consider hemodialysis (though erythromycin is highly protein-bound, so dialysis is not very effective). Discontinue the drug immediately.

Food Animal Withdrawal Times

🥩 Meat: 14 days🥛 Milk: 72 days🥚 Eggs: 7 days

Withdrawal times vary by formulation and country. For cattle, meat withdrawal is typically 14 days and milk 72 hours (3 days) for injectable forms. For poultry, meat withdrawal is 7 days and eggs 7 days. Always consult the label and local regulations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), with excursions permitted between 15-30°C (59-86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep in tightly closed container. Do not freeze oral suspensions. Injectable solutions should be stored according to manufacturer's instructions.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Erythromycin is approved for use in dogs, cats, horses, cattle, and poultry in various formulations. However, many uses are extra-label. Check current FDA approvals for specific products.

Extra-Label (Off-Label) Use: In the US, extra-label use of erythromycin in food animals is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) only with a valid veterinary-client-patient relationship and a written prescription. It is prohibited to use erythromycin in an extra-label manner in feed for food animals. Withdrawal times must be extended as determined by the veterinarian.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Erythromycin is a valuable antibiotic for treating infections caused by gram-positive bacteria and mycoplasmas, especially in cases of penicillin allergy. However, its use has declined due to the availability of newer macrolides (e.g., azithromycin, clarithromycin) with better pharmacokinetics and fewer gastrointestinal side effects. In veterinary medicine, it is still used for specific conditions such as Rhodococcus equi pneumonia in foals (in combination with rifampin), Campylobacteriosis in dogs, and Mycoplasma infections in various species. When using erythromycin, consider the potential for drug interactions, especially with drugs metabolized by CYP450 enzymes. In food animals, strict adherence to withdrawal times is essential. For horses, oral administration is unreliable and may cause severe diarrhea; IV administration is preferred. In rabbits and rodents, use with extreme caution due to the risk of enterotoxemia. Always perform culture and sensitivity testing when possible to ensure susceptibility.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)