Sulfadimethoxine

Sulfonamide antibiotic Sulfonamides

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 55 mg/kg q24h Duration: 5-7 days
Notes: For coccidiosis, give 55 mg/kg on day 1, then 27.5 mg/kg q24h for 5-9 days.
Cat PO 55 mg/kg q24h Duration: 5-7 days
Notes: For coccidiosis, give 55 mg/kg on day 1, then 27.5 mg/kg q24h for 5-9 days.
Horse IV 25 mg/kg q24h Duration: 3-5 days
Notes: For severe infections, may be given IV initially, then switch to oral.
Cattle PO 25 mg/kg q24h Duration: 3-5 days
Notes: For coccidiosis, give 25 mg/kg on day 1, then 12.5 mg/kg q24h for 4 days.
Sheep/Goat PO 25 mg/kg q24h Duration: 3-5 days
Notes: Use with caution in lactating animals; observe withdrawal times.
Rabbit PO 50 mg/kg q24h Duration: 5 days
Notes: For coccidiosis, may be given in drinking water.
Poultry PO (drinking water) 0.05% solution Continuous Duration: 5-7 days
Notes: For coccidiosis, use 0.05% in drinking water for 5-7 days.

Clinical Indications & Species Uses

General Indications
  • Treatment of susceptible bacterial infections
  • Coccidiosis
Dog (Canine)
  • Treatment of bacterial infections of the respiratory tract, urinary tract, skin, and soft tissue
  • Coccidiosis
  • Toxoplasmosis (in combination with pyrimethamine)
Cat (Feline)
  • Treatment of bacterial infections of the respiratory tract, urinary tract, skin, and soft tissue
  • Coccidiosis
  • Toxoplasmosis (in combination with pyrimethamine)
Horse (Equine)
  • Treatment of respiratory tract infections
  • Strangles (Streptococcus equi)
  • Coccidiosis
Cattle (Bovine)
  • Treatment of bacterial pneumonia
  • Coccidiosis
  • Shipping fever complex
  • Foot rot
Small Ruminants (Sheep / Goat)
  • Treatment of bacterial pneumonia
  • Coccidiosis
  • Enteritis
Rabbit & Small Mammals
  • Treatment of coccidiosis
  • Bacterial enteritis
Avian & Poultry
  • Treatment of coccidiosis
  • Fowl cholera
  • Infectious coryza
Exotic & Other Species
  • Treatment of coccidiosis in reptiles
  • Bacterial infections in small mammals (e.g., ferrets, guinea pigs)

Pharmacology & Mechanism of Action

Drug Class: Sulfonamide antibiotic | Pharmacological Group: Sulfonamides

Mechanism of Action: Sulfadimethoxine is a competitive antagonist of para-aminobenzoic acid (PABA), an essential substrate for the bacterial synthesis of dihydrofolic acid. By inhibiting dihydropteroate synthase, it blocks the production of folate, which is required for nucleic acid synthesis and bacterial growth. This bacteriostatic action is effective against a broad range of gram-positive and gram-negative organisms, as well as some protozoa such as coccidia.

Pharmacodynamics: Sulfadimethoxine exhibits a prolonged half-life in many species due to its high lipid solubility and extensive tubular reabsorption in the kidney. It is effective against susceptible strains of Streptococcus, Staphylococcus, Escherichia coli, Salmonella, Pasteurella, and others. It is often used in combination with ormetoprim or trimethoprim to provide synergistic bactericidal activity by inhibiting sequential steps in folate synthesis.

⚑ Pharmacokinetics Summary

Absorption: Sulfadimethoxine is rapidly and almost completely absorbed from the gastrointestinal tract in most species. Peak plasma concentrations are achieved within 2-6 hours after oral administration. In cattle, oral absorption is slower and less complete, but still effective.
Distribution: It is widely distributed throughout the body, including into cerebrospinal fluid, pleural fluid, and synovial fluid. It crosses the placenta and is distributed into milk. Protein binding is moderate to high, ranging from 60-90% depending on the species.
Metabolism: Sulfadimethoxine is metabolized in the liver primarily by acetylation and glucuronide conjugation. The extent of metabolism varies by species; in dogs and cats, acetylation is minimal, while in ruminants, it is more extensive.
Excretion: Excretion is primarily renal, with both glomerular filtration and tubular secretion. Because of its high lipid solubility, tubular reabsorption is extensive, leading to a long elimination half-life. A small amount is excreted in feces.
Half-Life: Dog: 13-25 hours; Cat: 10-20 hours; Horse: 10-15 hours; Cattle: 10-20 hours; Poultry: 10-15 hours
Bioavailability: Oral bioavailability is high (70-100%) in most species, but may be reduced in ruminants due to ruminal degradation.
Protein Binding: 60-90% depending on species

Available Formulations & Strengths

Oral Tablet 125 mg, 250 mg, 500 mg (PO)
Oral Suspension 5% (50 mg/mL) (PO)
Injectable Solution 40% (400 mg/mL) (IV, SC)
Soluble Powder Various (PO (drinking water))

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to sulfonamides
  • Hepatic or renal insufficiency
  • Pregnancy (especially near term) due to risk of kernicterus in neonates
  • Use in animals with blood dyscrasias
  • Concurrent use with certain drugs that may potentiate toxicity (e.g., methotrexate)
Warnings & Clinical Precautions:
  • Ensure adequate water intake to prevent crystalluria
  • Use with caution in animals with liver or kidney disease
  • May cause photosensitivity in some species
  • In food animals, observe withdrawal times
  • Prolonged use may lead to superinfections
  • Do not use in animals with known sulfonamide allergy
  • In horses, IV injection should be slow to avoid adverse reactions

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea)
  • Loss of appetite
  • Crystalluria (if dehydrated)

Serious / Severe:

  • Blood dyscrasias (thrombocytopenia, leukopenia, agranulocytosis)
  • Hepatotoxicity
  • Renal toxicity (crystalluria, tubular necrosis)
  • Stevens-Johnson syndrome
  • Anaphylaxis

Rare:

  • Keratoconjunctivitis sicca (dry eye) in dogs
  • Polyarthritis in dogs
  • Folate deficiency leading to anemia

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Warfarin Sulfonamides may potentiate the anticoagulant effect by displacing warfarin from protein binding sites. High
Methotrexate Increased risk of methotrexate toxicity due to displacement from protein binding and reduced renal excretion. High
Phenytoin Sulfonamides may inhibit phenytoin metabolism, increasing phenytoin levels and toxicity. Moderate
Oral hypoglycemics (e.g., tolbutamide) Sulfonamides may enhance hypoglycemic effect. Moderate
Methenamine May increase risk of crystalluria due to acidification of urine. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Anorexia
  • Vomiting
  • Diarrhea
  • Crystalluria
  • Hematuria
  • Renal failure
  • CNS depression
  • Seizures

Emergency Treatment Protocol: Discontinue the drug immediately. Provide supportive care including fluid therapy to enhance renal excretion and prevent crystalluria. Alkalinization of urine with sodium bicarbonate may help reduce crystalluria. In severe cases, hemodialysis may be considered. Monitor renal function and blood counts.

Food Animal Withdrawal Times

πŸ₯© Meat: 7 daysπŸ₯› Milk: 4 daysπŸ₯š Eggs: 5 days

Withdrawal times may vary by country and formulation. Always follow label instructions and local regulations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25Β°C)

Light Sensitivity: Light-sensitive β€” protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs, cats, cattle, and poultry in the US. Some formulations may be approved for other species.

Extra-Label (Off-Label) Use: Extra-label use in food animals requires a valid veterinary-client-patient relationship and must comply with AMDUCA regulations. Withdrawal times must be extended if used extra-label.

Clinical Pearls & Practice Notes

πŸ’‘ Clinical Insights: Sulfadimethoxine is a valuable antibiotic for treating coccidiosis and various bacterial infections in veterinary medicine. Its long half-life allows once-daily dosing, improving owner compliance. However, it is bacteriostatic, so it is important to use an appropriate dose and duration to ensure efficacy. Adequate hydration is essential to prevent crystalluria. In food animals, strict adherence to withdrawal times is critical. Combination with ormetoprim (e.g., Primor) enhances its spectrum and bactericidal activity. Monitor for adverse effects, especially in animals with pre-existing renal or hepatic disease.

References & Literature

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