Doxycycline

Tetracycline antibiotic Antibacterial

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 5-10 mg/kg q12h or q24h Duration: 7-14 days (up to 28 days for rickettsial infections)
Notes: Administer with food to reduce GI upset. For Ehrlichia, treat for 28 days. For Lyme disease, treat for 14-28 days.
Cat PO 5-10 mg/kg q12h or q24h Duration: 7-14 days (up to 28 days for Mycoplasma haemofelis)
Notes: Use with caution in cats due to risk of esophagitis; administer with water or food. For Chlamydia, treat for 14 days.
Horse IV 10 mg/kg q12h Duration: 5-14 days
Notes: IV administration preferred due to variable oral absorption. For Potomac horse fever, treat for 5-7 days.
Horse PO 10-20 mg/kg q12h Duration: 5-14 days
Notes: Oral absorption is variable; monitor response. May cause diarrhea.
Cattle IV 10 mg/kg q24h Duration: 3-5 days
Notes: Extra-label use; observe withdrawal times. For anaplasmosis, treat for 5-7 days.
Cattle PO 10 mg/kg q12h Duration: 3-5 days
Notes: Oral administration may be less reliable; consider IV for severe infections.
Sheep/Goat PO 5-10 mg/kg q12h Duration: 5-7 days
Notes: Extra-label use; ensure adequate hydration.
Rabbit PO 2.5-5 mg/kg q12h Duration: 7-14 days
Notes: Administer with water to prevent esophageal irritation. Monitor for GI stasis.
Bird (Psittacines) PO 25-50 mg/kg q12h Duration: 7-14 days
Notes: Doses vary by species; use lower end for small birds. For Chlamydia, treat for 45 days.
Reptiles PO 5-10 mg/kg q24h Duration: 7-14 days
Notes: Administer with food or water. Adjust based on species and temperature.

Clinical Indications & Species Uses

General Indications
  • Broad-spectrum antibiotic for susceptible bacterial infections.
  • Treatment of rickettsial, chlamydial, and mycoplasmal infections.
  • Adjunct therapy for inflammatory conditions (e.g., periodontal disease, immune-mediated diseases).
Dog (Canine)
  • Treatment of infections caused by susceptible Gram-positive and Gram-negative bacteria, including Rickettsia rickettsii (Rocky Mountain spotted fever), Ehrlichia canis, Anaplasma phagocytophilum, Borrelia burgdorferi (Lyme disease), Leptospira, Chlamydia, Mycoplasma, and Bartonella.
  • Canine periodontal disease (as adjunct to dental cleaning and scaling).
  • Upper respiratory tract infections (e.g., Bordetella bronchiseptica).
  • Urinary tract infections (when susceptible).
  • Prostatitis (due to good tissue penetration).
Cat (Feline)
  • Treatment of feline upper respiratory infections (e.g., Chlamydia felis, Mycoplasma spp., Bordetella bronchiseptica).
  • Hemotropic mycoplasmosis (Mycoplasma haemofelis).
  • Bartonellosis (Bartonella henselae).
  • Ehrlichiosis and anaplasmosis.
  • Infectious anemia (as part of combination therapy).
Horse (Equine)
  • Treatment of infections caused by susceptible organisms, including Ehrlichia rickettsii (Potomac horse fever), Anaplasma phagocytophilum, Borrelia burgdorferi (Lyme disease), and Lawsonia intracellularis (equine proliferative enteropathy).
  • Respiratory tract infections (e.g., Rhodococcus equi, but not first-line).
  • Skin and soft tissue infections.
Cattle (Bovine)
  • Treatment of respiratory disease (e.g., Pasteurella multocida, Mannheimia haemolytica) when susceptible.
  • Anaplasmosis (Anaplasma marginale).
  • Chlamydial infections.
  • Foot rot (in combination with other therapy).
Small Ruminants (Sheep / Goat)
  • Treatment of infections caused by susceptible organisms, including Chlamydia, Mycoplasma, and Rickettsia.
  • Respiratory infections.
  • Enteric infections (e.g., Lawsonia intracellularis in pigs, but off-label in sheep/goats).
Rabbit & Small Mammals
  • Treatment of respiratory infections (e.g., Pasteurella multocida, Bordetella bronchiseptica).
  • Mycoplasmosis.
  • Dental infections (as adjunct).
Avian & Poultry
  • Treatment of infections caused by Mycoplasma, Chlamydia (psittacosis), and other susceptible bacteria.
  • Respiratory infections.
  • Enteric infections.
Exotic & Other Species
  • Reptiles: treatment of respiratory infections (e.g., Mycoplasma), abscesses, and septicemia.
  • Small mammals (ferrets, guinea pigs): respiratory infections, Mycoplasma, and Chlamydia.
  • Fish: treatment of bacterial infections (e.g., Aeromonas, Pseudomonas) in ornamental fish.

Pharmacology & Mechanism of Action

Drug Class: Tetracycline antibiotic | Pharmacological Group: Antibacterial

Mechanism of Action: Doxycycline is a bacteriostatic antibiotic that inhibits bacterial protein synthesis by binding reversibly to the 30S ribosomal subunit, specifically to the 16S rRNA, preventing the attachment of aminoacyl-tRNA to the mRNA-ribosome complex. This action blocks peptide chain elongation, thereby inhibiting bacterial growth. It is a lipophilic tetracycline that penetrates cell membranes and tissues effectively, and it also has anti-inflammatory and immunomodulatory properties, including inhibition of matrix metalloproteinases (MMPs) and reduction of pro-inflammatory cytokines, which contribute to its efficacy in non-infectious conditions.

Pharmacodynamics: Doxycycline exhibits broad-spectrum activity against many Gram-positive and Gram-negative bacteria, including Rickettsia, Chlamydia, Mycoplasma, and spirochetes. It is generally bacteriostatic but may be bactericidal at high concentrations. Its lipophilicity enhances tissue penetration, including into the prostate, central nervous system, and ocular tissues. It also has anti-inflammatory effects, such as reducing neutrophil chemotaxis, inhibiting phospholipase A2, and scavenging reactive oxygen species. These properties make it useful in treating inflammatory conditions like canine periodontal disease and feline upper respiratory infections.

⚡ Pharmacokinetics Summary

Absorption: Doxycycline is well absorbed after oral administration in most species, with bioavailability ranging from 60-100% in dogs and cats. Absorption is slightly reduced by food, but unlike other tetracyclines, it is not significantly affected by divalent cations (calcium, magnesium, iron) due to its high lipophilicity. In horses, oral absorption is variable and may be incomplete; intravenous administration is often preferred for systemic infections.
Distribution: Doxycycline is widely distributed throughout the body, with a volume of distribution of approximately 0.7-1.5 L/kg in dogs and cats. It penetrates well into tissues and fluids, including the prostate, lung, liver, kidney, and cerebrospinal fluid (CSF) (about 20-30% of plasma levels). It crosses the placenta and is excreted in milk. Protein binding is moderate to high, ranging from 80-90% in dogs and cats.
Metabolism: Doxycycline is metabolized in the liver to a limited extent, with the majority of the drug excreted unchanged. It undergoes enterohepatic recirculation, which contributes to its prolonged half-life. Unlike other tetracyclines, it does not accumulate significantly in renal failure because it is primarily eliminated via the feces.
Excretion: Doxycycline is excreted primarily in the feces (via bile and intestinal secretion) as active drug, with approximately 20-30% excreted in the urine. In dogs, about 40% is excreted in feces and 20% in urine. The elimination half-life is approximately 10-12 hours in dogs, 8-10 hours in cats, and 10-12 hours in horses.
Half-Life: Dogs: 10-12 hours; Cats: 8-10 hours; Horses: 10-12 hours; Cattle: 10-12 hours (after IV); Rabbits: 8-10 hours; Birds: 6-8 hours (variable).
Bioavailability: Dogs: 60-100% (oral); Cats: 60-100% (oral); Horses: 20-40% (oral, variable); Cattle: 60-80% (oral); Rabbits: 60-80% (oral); Birds: 50-70% (oral).
Protein Binding: Dogs: 80-90%; Cats: 80-90%; Horses: 80-90%; Cattle: 80-90%; Rabbits: 80-90%.

Available Formulations & Strengths

Oral Tablet 20 mg, 50 mg, 100 mg (PO)
Oral Capsule 50 mg, 100 mg (PO)
Oral Suspension 25 mg/5 mL, 50 mg/5 mL (PO)
Injectable Solution 100 mg/mL (veterinary), 10 mg/mL (human) (IV)
Powder for Oral Solution Various (e.g., 100 g per bottle) (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to doxycycline or other tetracyclines.
  • Severe hepatic impairment (use with caution).
  • Concurrent use with isotretinoin (risk of pseudotumor cerebri).
  • Use in young animals (under 6 months of age) due to risk of tooth discoloration and bone growth retardation, unless benefit outweighs risk.
  • Use in pregnant or lactating animals (may affect fetal bone and teeth development).
  • Do not use in horses intended for human consumption (withdrawal times must be observed).
Warnings & Clinical Precautions:
  • Administer oral doxycycline with a full glass of water or food to reduce esophageal irritation and ulceration, especially in cats and horses.
  • Use with caution in animals with renal or hepatic disease; dose adjustment may be necessary.
  • May cause photosensitivity; avoid excessive sunlight exposure during treatment.
  • In horses, oral administration may cause diarrhea and colitis; monitor closely.
  • In ruminants, oral administration may disrupt rumen flora; use with caution.
  • Do not use in animals with a history of myasthenia gravis (may exacerbate weakness).
  • Use in young animals may cause permanent tooth discoloration (yellow-gray-brown) and enamel hypoplasia.
  • Concurrent use with antacids, iron, calcium, magnesium, or zinc supplements may reduce absorption; separate administration by 2-3 hours.
  • In cats, doxycycline tablets should be followed by water or a treat to prevent esophagitis.
  • For food animals, observe withdrawal times to avoid drug residues.

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea, anorexia)
  • Esophageal irritation or stricture (especially in cats and horses)
  • Photosensitivity (skin rash on sun-exposed areas)
  • Tooth discoloration in young animals
  • Injection site pain or phlebitis (IV administration)

Serious / Severe:

  • Esophageal ulceration or stricture (cats, horses)
  • Hepatotoxicity (rare but possible)
  • Nephrotoxicity (rare, more common with outdated tetracyclines)
  • Blood dyscrasias (neutropenia, thrombocytopenia)
  • Pseudotumor cerebri (increased intracranial pressure)
  • Severe diarrhea or colitis (especially in horses)
  • Anaphylaxis (rare)

Rare:

  • Superinfection with resistant organisms (e.g., Candida)
  • Hypersensitivity reactions (urticaria, angioedema)
  • Hemolytic anemia
  • Thrombophlebitis at injection site
  • Alopecia (in some species)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Antacids (aluminum, calcium, magnesium) Reduced absorption of doxycycline due to chelation; separate administration by 2-3 hours. High
Iron, zinc, calcium supplements Reduced absorption; separate administration by 2-3 hours. High
Warfarin and other anticoagulants Doxycycline may enhance anticoagulant effect; monitor coagulation parameters. Moderate
Barbiturates (e.g., phenobarbital) Increased metabolism of doxycycline, reducing its half-life and efficacy. Moderate
Carbamazepine, phenytoin Increased metabolism of doxycycline, reducing its efficacy. Moderate
Oral contraceptives (in humans) May reduce efficacy of oral contraceptives; not relevant in veterinary medicine. Mild
Penicillins (e.g., amoxicillin) Antagonistic effect (bacteriostatic vs. bactericidal); avoid concurrent use. Moderate
Isotretinoin Increased risk of pseudotumor cerebri; avoid concurrent use. High

Overdose & Toxicity Management

Signs of Toxicity:

  • Gastrointestinal signs (vomiting, diarrhea, anorexia)
  • Esophageal irritation or ulceration
  • Hepatotoxicity (elevated liver enzymes)
  • Renal toxicity (rare)
  • Neurological signs (ataxia, seizures) in severe cases
  • Photosensitivity

Emergency Treatment Protocol: Treatment is primarily supportive. Induce vomiting if recent ingestion and animal is stable (do not induce in cats due to risk of esophageal injury). Administer activated charcoal to reduce absorption. Provide fluid therapy to maintain hydration and promote excretion. Monitor liver and kidney function. For severe GI signs, use antiemetics and gastroprotectants (e.g., sucralfate). In cases of esophageal irritation, consider feeding soft food and using sucralfate. There is no specific antidote.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 5 days

Withdrawal times vary by country and formulation. In the US, for cattle, meat withdrawal is 7 days and milk withdrawal is 5 days when used according to label. For extra-label use, consult FARAD (Food Animal Residue Avoidance Databank). For poultry, withdrawal times may be longer; consult label. For horses, doxycycline is not approved for food animals; withdrawal times must be established based on extralabel use.

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 container tightly closed. Do not use if solution is discolored or contains precipitate. Oral suspensions should be shaken well before use and may require refrigeration; check label.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats (oral tablets) and for cattle (injectable). Not approved for horses, but commonly used extra-label.

Extra-Label (Off-Label) Use: In the US, doxycycline is approved for use in dogs and cats (oral tablets) and for certain indications in cattle (injectable). Extra-label use in other species or for other indications is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) provided a valid veterinarian-client-patient relationship exists, and withdrawal times are extended as needed. For food animals, extra-label use requires careful attention to withdrawal times and may require a withdrawal time extension.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Doxycycline is a valuable antibiotic in veterinary medicine due to its broad-spectrum activity, excellent tissue penetration, and anti-inflammatory properties. It is the drug of choice for many rickettsial infections (e.g., Ehrlichia, Anaplasma, Rocky Mountain spotted fever) and is commonly used for respiratory and soft tissue infections. Key clinical considerations: 1) Administer oral doxycycline with food or water to minimize esophageal irritation, especially in cats and horses. 2) In horses, IV administration is preferred for systemic infections due to variable oral absorption. 3) In food animals, adhere to withdrawal times to prevent residues. 4) Use with caution in young animals due to tooth discoloration. 5) Doxycycline is generally safe in renal impairment, but dose adjustment may be needed in severe hepatic disease. 6) Monitor for GI side effects, especially in horses and ruminants. 7) For chlamydial infections in birds, prolonged treatment (45 days) may be necessary. 8) Doxycycline has anti-inflammatory effects that may be beneficial in conditions like periodontal disease and immune-mediated diseases, but it should not be used as a sole therapy for these conditions without appropriate veterinary guidance.

References & Literature

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