Tetracycline Hydrochloride

Tetracycline Antibiotic Polyketide Antibiotics

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 15-25 mg/kg q8h Duration: 5-14 days depending on infection
Notes: Administer on an empty stomach, 1 hour before or 2 hours after feeding. Avoid concurrent administration with dairy products, antacids, or iron supplements.
Dog IV 5-10 mg/kg q12h Duration: As per clinical response
Notes: Administer slowly over 5 minutes. Not commonly used IV due to availability of other tetracyclines.
Cat PO 15-25 mg/kg q8h Duration: 5-14 days depending on infection
Notes: Administer on an empty stomach. Use with caution in cats due to potential gastrointestinal upset.
Cat IV 5-10 mg/kg q12h Duration: As per clinical response
Notes: Administer slowly. Not commonly used IV.
Horse PO 10-20 mg/kg q12h Duration: 5-10 days
Notes: Oral absorption is variable; use with caution. May cause gastrointestinal disturbances.
Horse IV 5-10 mg/kg q12h Duration: 5-10 days
Notes: Administer slowly. Not commonly used due to availability of other tetracyclines.
Cattle PO 10-20 mg/kg q12h Duration: 3-5 days
Notes: Oral use is limited due to ruminal degradation. Not recommended for adult ruminants.
Cattle IV 5-10 mg/kg q12h Duration: 3-5 days
Notes: Administer slowly. Use with caution in neonates.
Cattle IM 5-10 mg/kg q24h Duration: 3-5 days
Notes: May cause injection site reactions. Not recommended for lactating dairy cattle due to milk residues.
Small Ruminants (Sheep/Goats) PO 10-20 mg/kg q12h Duration: 3-5 days
Notes: Oral use is limited in ruminants. Use with caution.
Small Ruminants (Sheep/Goats) IV 5-10 mg/kg q12h Duration: 3-5 days
Notes: Administer slowly.
Rabbit PO 20-50 mg/kg q8-12h Duration: 5-7 days
Notes: Use with caution due to risk of gastrointestinal dysbiosis. Monitor for diarrhea.
Rabbit SC 10-20 mg/kg q12h Duration: 5-7 days
Notes: May cause tissue irritation.
Birds (Poultry) PO (drinking water) 10-20 mg/kg q24h Duration: 3-5 days
Notes: Ensure fresh solution daily. Withdrawal times must be observed.
Birds (Companion) PO 25-50 mg/kg q12h Duration: 5-7 days
Notes: May be given via crop gavage.
Reptiles PO 10-20 mg/kg q24h Duration: 7-14 days
Notes: Use with caution in herbivorous reptiles due to gut flora disruption.

Clinical Indications & Species Uses

General Indications
  • Treatment of infections caused by susceptible gram-positive and gram-negative bacteria
  • Rickettsial infections (e.g., Rocky Mountain spotted fever, ehrlichiosis, anaplasmosis)
  • Chlamydial infections
  • Mycoplasma infections
  • Leptospirosis
  • Brucellosis (in combination)
  • Some protozoal infections (e.g., balantidiasis)
Dog (Canine)
  • Treatment of infections caused by susceptible gram-positive and gram-negative bacteria
  • Rickettsial infections (e.g., Rocky Mountain spotted fever, ehrlichiosis)
  • Chlamydial infections
  • Mycoplasma infections
  • Leptospirosis (as an alternative)
  • Brucellosis (in combination with other antibiotics)
  • Periodontal disease (local delivery)
  • Urinary tract infections (when susceptible)
Cat (Feline)
  • Treatment of infections caused by susceptible bacteria
  • Chlamydophila felis infection (feline chlamydiosis)
  • Mycoplasma infections (e.g., hemotropic mycoplasmosis)
  • Rickettsial infections (e.g., ehrlichiosis)
  • Bartonellosis (in combination with other agents)
  • Upper respiratory infections (as part of a protocol)
Horse (Equine)
  • Treatment of susceptible bacterial infections
  • Respiratory tract infections (e.g., pneumonia, pleuropneumonia)
  • Joint infections (septic arthritis) when susceptible
  • Skin and soft tissue infections
  • Leptospirosis (in combination with other antibiotics)
  • Potomac horse fever (Neorickettsia risticii) - though oxytetracycline is preferred
Cattle (Bovine)
  • Treatment of bacterial infections caused by susceptible organisms
  • Respiratory disease (e.g., pneumonia, shipping fever)
  • Foot rot (interdigital necrobacillosis)
  • Metritis
  • Leptospirosis
  • Anaplasmosis (in some regions)
  • Chlamydial infections
Small Ruminants (Sheep / Goat)
  • Treatment of susceptible bacterial infections
  • Respiratory infections
  • Enteritis (e.g., due to E. coli, Salmonella)
  • Foot rot
  • Chlamydial infections (e.g., enzootic abortion in ewes)
  • Mycoplasma infections
Rabbit & Small Mammals
  • Treatment of susceptible bacterial infections
  • Respiratory infections (e.g., Pasteurella multocida)
  • Urinary tract infections
  • Mycoplasma infections
  • Gastrointestinal infections (with caution due to dysbiosis risk)
Avian & Poultry
  • Treatment of susceptible bacterial infections
  • Respiratory infections (e.g., Mycoplasma gallisepticum)
  • Chlamydiosis (psittacosis)
  • Colibacillosis
  • Salmonellosis
  • Ornithobacteriosis
Exotic & Other Species
  • Reptiles: treatment of susceptible bacterial infections (e.g., respiratory, skin)
  • Small mammals (e.g., ferrets, guinea pigs): treatment of respiratory and gastrointestinal infections
  • Fish: treatment of bacterial infections (e.g., furunculosis, columnaris) - though other tetracyclines are more common

Pharmacology & Mechanism of Action

Drug Class: Tetracycline Antibiotic | Pharmacological Group: Polyketide Antibiotics

Mechanism of Action: Tetracycline hydrochloride 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 ribosomal acceptor (A) site. This blocks the addition of new amino acids to the growing peptide chain, thereby inhibiting bacterial growth. It is effective against a wide range of gram-positive and gram-negative bacteria, as well as atypical organisms such as Mycoplasma, Chlamydia, Rickettsia, and some protozoa. Resistance can occur via efflux pumps, ribosomal protection proteins, or enzymatic inactivation.

Pharmacodynamics: Tetracycline exhibits time-dependent bactericidal or bacteriostatic activity depending on the concentration and organism. It has a broad spectrum of activity, but its effectiveness varies among species and pathogens. It is generally more active against gram-positive bacteria at lower concentrations, but gram-negative activity is also significant. It is particularly useful against intracellular pathogens due to its ability to penetrate cell membranes. The post-antibiotic effect is moderate, and the minimum inhibitory concentration (MIC) for susceptible organisms is typically ≤4 µg/mL.

⚡ Pharmacokinetics Summary

Absorption: Tetracycline is adequately absorbed from the gastrointestinal tract in monogastric animals, but absorption is variable and can be significantly reduced by the presence of food, especially dairy products, and by divalent or trivalent cations (calcium, magnesium, aluminum, iron) due to chelation. In ruminants, oral absorption is poor due to ruminal degradation. Bioavailability after oral administration is approximately 60-80% in dogs and cats when given on an empty stomach. Intramuscular and subcutaneous injections result in rapid absorption but may cause local irritation.
Distribution: Tetracycline is widely distributed throughout the body, including into pleural, peritoneal, synovial, and cerebrospinal fluids (though CSF levels are lower). It crosses the placenta and is excreted in milk. It has a volume of distribution of approximately 1-2 L/kg in dogs and cats. It accumulates in the reticuloendothelial system, liver, spleen, and bone, and is taken up by growing bones and teeth, leading to discoloration.
Metabolism: Tetracycline undergoes minimal hepatic metabolism. It is primarily excreted unchanged in the urine and feces. In the liver, a small fraction may be conjugated or degraded, but the majority is eliminated via glomerular filtration.
Excretion: Tetracycline is excreted primarily by the kidneys via glomerular filtration, with about 60% of an administered dose appearing in the urine within 24 hours. Biliary excretion also occurs, with enterohepatic recirculation, and the remainder is eliminated in feces. In animals with renal impairment, the half-life is prolonged, and dosage adjustment may be necessary.
Half-Life: Dogs: approximately 6-10 hours; Cats: approximately 6-8 hours; Horses: approximately 10-12 hours; Cattle: approximately 6-8 hours; Rabbits: approximately 2-4 hours; Birds: approximately 1-2 hours.
Bioavailability: Oral bioavailability is approximately 60-80% in dogs and cats when administered on an empty stomach, but can be reduced by up to 50% when given with food or milk. In ruminants, oral bioavailability is very low (<10%) due to ruminal degradation.
Protein Binding: Tetracycline is approximately 20-65% bound to plasma proteins, depending on the species.

Available Formulations & Strengths

Oral Tablet 250 mg, 500 mg (PO)
Oral Capsule 250 mg, 500 mg (PO)
Oral Suspension 125 mg/5 mL (PO)
Injectable Solution 100 mg/mL, 200 mg/mL (IV, IM, SC)
Ophthalmic Ointment 1% (Ophthalmic)
Ophthalmic Solution 1% (Ophthalmic)
Topical Ointment 3% (Topical)
Powder for Oral Solution Various (e.g., 25 g, 100 g) (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to tetracyclines
  • Severe hepatic or renal impairment (use with caution, may require dose adjustment)
  • Pregnancy (especially during organogenesis) - may cause fetal skeletal and teeth discoloration
  • Lactating animals (excreted in milk, may affect nursing young)
  • Young animals (during tooth development, may cause permanent tooth discoloration and enamel hypoplasia)
  • Concurrent use with antacids, iron, calcium, magnesium, or aluminum-containing products (oral administration)
  • Use in horses with renal disease (may exacerbate)
  • Use in ruminants orally (ineffective due to ruminal degradation)
Warnings & Clinical Precautions:
  • Use with caution in animals with renal or hepatic impairment; adjust dose or extend interval.
  • Avoid use in pregnant animals unless the benefits outweigh risks; tetracyclines cross the placenta and can affect fetal bone and teeth development.
  • In young animals, tetracyclines can cause permanent discoloration of teeth and enamel hypoplasia; use only when no alternative is available.
  • Oral administration should be given on an empty stomach to enhance absorption; avoid concurrent administration with dairy products, antacids, or mineral supplements.
  • In herbivores (rabbits, ruminants), oral tetracyclines can disrupt normal gut flora, leading to dysbiosis and potentially fatal enteritis; use with caution and monitor.
  • Intramuscular and subcutaneous injections can cause pain, swelling, and tissue necrosis; use the IV route when possible for large doses.
  • Tetracyclines may cause photosensitivity; avoid excessive sunlight exposure during treatment.
  • In horses, intravenous administration should be slow to avoid cardiovascular collapse (rapid infusion may cause hypotension).
  • In food animals, observe withdrawal times to prevent residues in meat and milk.
  • Tetracycline may cause superinfections with resistant organisms (e.g., Candida, Clostridium difficile) due to disruption of normal flora.

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (nausea, vomiting, diarrhea)
  • Loss of appetite
  • Tooth discoloration in young animals
  • Injection site reactions (pain, swelling)
  • Photosensitivity (skin reactions on exposure to sunlight)

Serious / Severe:

  • Hepatotoxicity (rare but possible, especially with high doses or prolonged use)
  • Nephrotoxicity (especially in animals with pre-existing renal disease)
  • Esophageal ulceration or stricture (if oral tablets lodge in esophagus)
  • Superinfection with resistant organisms (e.g., Clostridium difficile-associated diarrhea)
  • Blood dyscrasias (thrombocytopenia, neutropenia, anemia)
  • Anaphylaxis (rare)

Rare:

  • Intracranial hypertension (pseudotumor cerebri)
  • Pancreatitis
  • Stevens-Johnson syndrome
  • Exfoliative dermatitis
  • Hemolytic anemia
  • Eosinophilia
  • Vestibular toxicity (dizziness, ataxia)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Antacids (aluminum, calcium, magnesium) Reduced oral absorption of tetracycline due to chelation; separate administration by at least 2-3 hours. High
Iron supplements Reduced oral absorption of tetracycline; separate administration by at least 2-3 hours. High
Dairy products (milk, cheese) Reduced oral absorption due to calcium chelation; avoid concurrent administration. High
Bismuth subsalicylate Reduced absorption of tetracycline; separate administration. Moderate
Warfarin and other anticoagulants Tetracyclines may enhance the anticoagulant effect by reducing vitamin K-producing gut flora; monitor coagulation parameters. Moderate
Oral contraceptives (in humans, but relevant in research) Tetracyclines may reduce the efficacy of oral contraceptives; not directly applicable in veterinary medicine. Mild
Penicillins (e.g., amoxicillin) Bacteriostatic drugs may antagonize the bactericidal effect of penicillins; avoid concurrent use. Moderate
Cephalosporins Potential antagonism; avoid concurrent use. Moderate
Diuretics (e.g., furosemide) May increase the risk of nephrotoxicity; monitor renal function. Moderate
Methoxyflurane (anesthetic) Increased risk of nephrotoxicity; avoid concurrent use. High

Overdose & Toxicity Management

Signs of Toxicity:

  • Gastrointestinal signs: vomiting, diarrhea, anorexia
  • Hepatotoxicity: elevated liver enzymes, jaundice
  • Nephrotoxicity: increased BUN and creatinine, acute renal failure
  • Neurological signs: ataxia, depression, seizures (in severe cases)
  • Metabolic acidosis
  • Hypocalcemia (due to chelation)

Emergency Treatment Protocol: Treatment is primarily supportive. Discontinue the drug immediately. Induce vomiting if recent oral ingestion and the animal is conscious (in dogs and cats). Administer activated charcoal to reduce absorption. Provide intravenous fluids to maintain hydration and promote renal excretion. Monitor liver and kidney function. In severe cases, consider hemodialysis or peritoneal dialysis to enhance elimination. There is no specific antidote. Symptomatic treatment for gastrointestinal signs may include antiemetics and gastrointestinal protectants.

Food Animal Withdrawal Times

🥩 Meat: 4 days🥛 Milk: 4 days🥚 Eggs: 4 days

Withdrawal times vary by country and formulation. For tetracycline hydrochloride, typical withdrawal periods are: cattle: meat 4 days, milk 4 days; swine: meat 4 days; poultry: meat 4 days, eggs 4 days. Always follow label instructions and local regulations. Extra-label use may require extended withdrawal times.

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. Oral suspensions should be refrigerated after reconstitution and used within the specified time (usually 14 days). Injectable solutions should be stored at room temperature and protected from freezing.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Tetracycline hydrochloride is FDA-approved for use in veterinary medicine for various species, including cattle, swine, sheep, poultry, dogs, and cats. However, many products are marketed under approved new animal drug applications (NADAs) or abbreviated new animal drug applications (ANADAs).

Extra-Label (Off-Label) Use: In the United States, tetracycline is approved for use in certain food animals (cattle, swine, sheep, poultry) and non-food animals. Extra-label use in food animals is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) only with a valid veterinarian-client-patient relationship and a withdrawal time established by the veterinarian. Extra-label use in non-food animals is allowed. In the European Union, similar regulations apply under Regulation (EC) No 470/2009.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Tetracycline hydrochloride is a broad-spectrum antibiotic that is still used in veterinary medicine, though it has been largely replaced by newer tetracyclines such as doxycycline and minocycline due to better pharmacokinetics and fewer adverse effects. It is particularly useful for treating rickettsial infections, chlamydiosis, and mycoplasmosis. When using tetracycline, consider the following: (1) Administer orally on an empty stomach to maximize absorption; avoid concurrent administration with dairy products, antacids, or iron supplements. (2) In ruminants, oral administration is ineffective; use parenteral routes. (3) In young animals, avoid use if possible due to tooth discoloration. (4) Monitor renal and hepatic function during prolonged therapy. (5) In food animals, strictly observe withdrawal times. (6) Be aware of potential drug interactions, especially with penicillins and cephalosporins. (7) For local infections (e.g., periodontal disease), topical or local delivery may be beneficial. (8) Always perform culture and sensitivity testing when possible to ensure susceptibility. (9) In horses, intravenous administration should be slow to avoid adverse reactions. (10) In rabbits and other herbivores, oral tetracycline can cause fatal dysbiosis; use with extreme caution and consider alternative antibiotics.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)
  • 📚 Merck Veterinary Manual
  • 📚 FDA Approved Animal Drug Products (Green Book)