Sulfasalazine — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	10-25 mg/kg (up to 50 mg/kg in some cases)	q8h to q12h	Duration: 3-6 weeks for initial response; maintenance as needed Notes: Administer with food to reduce GI upset. Titrate to lowest effective dose. May be used with corticosteroids for refractory cases.
Cat	PO	10-20 mg/kg	q12h to q24h	Duration: 3-6 weeks for initial response; maintenance as needed Notes: Cats are more sensitive to sulfonamide adverse effects; use with caution and monitor for keratoconjunctivitis sicca (KCS).
Horse	PO	10-20 mg/kg	q12h	Duration: Variable; limited evidence Notes: Not routinely used; may be considered for inflammatory colitis under veterinary supervision.

Clinical Indications & Species Uses

General Indications

Inflammatory bowel disease (colitis, enteritis) in dogs and cats

Dog (Canine)

Inflammatory bowel disease (IBD)
Ulcerative colitis
Granulomatous colitis
Lymphocytic-plasmacytic enteritis
Steroid-responsive diarrhea (as adjunct)

Cat (Feline)

Inflammatory bowel disease (IBD)
Lymphocytic-plasmacytic colitis

Pharmacology & Mechanism of Action

Drug Class: 5-aminosalicylic acid derivative | Pharmacological Group: Anti-inflammatory / Disease-modifying antirheumatic drug (DMARD)

Mechanism of Action: Sulfasalazine is a prodrug that is cleaved by bacterial azoreductases in the colon to release 5-aminosalicylic acid (5-ASA, mesalamine) and sulfapyridine. The 5-ASA moiety is the active anti-inflammatory component, acting locally on the colonic mucosa. Its mechanism involves inhibition of cyclooxygenase and lipoxygenase pathways, scavenging of reactive oxygen species, and inhibition of pro-inflammatory cytokines (e.g., TNF-α, IL-1). Sulfapyridine, the absorbed moiety, contributes to immunosuppressive effects and may be responsible for many systemic adverse effects.

Pharmacodynamics: Sulfasalazine reduces inflammation in the gastrointestinal tract, particularly the colon. It also has immunomodulatory effects, including inhibition of neutrophil chemotaxis, decreased production of inflammatory mediators, and modulation of lymphocyte function. In rheumatoid arthritis, it may slow disease progression. The onset of therapeutic effect in inflammatory bowel disease (IBD) is typically 3-6 weeks.

⚡ Pharmacokinetics Summary

Absorption: Sulfasalazine is poorly absorbed from the small intestine (approximately 10-20% absorbed as the parent drug). The majority reaches the colon where it is metabolized by bacteria. The absorbed sulfapyridine is well absorbed, while 5-ASA is poorly absorbed and acts locally.

Distribution: Sulfasalazine and its metabolites distribute widely. Sulfapyridine crosses the blood-brain barrier and placenta, and is secreted into milk. 5-ASA is largely confined to the gastrointestinal tract.

Metabolism: Sulfasalazine is cleaved in the colon by bacterial azoreductases to sulfapyridine and 5-ASA. Sulfapyridine is further metabolized in the liver via acetylation and hydroxylation. 5-ASA is metabolized to N-acetyl-5-ASA in the liver and by intestinal bacteria.

Excretion: Sulfapyridine and its metabolites are excreted primarily in urine. 5-ASA and its metabolites are excreted in feces (unabsorbed) and urine (absorbed portion).

Half-Life: Dog: 5-10 hours (sulfapyridine); Cat: 10-20 hours (sulfapyridine); Horse: 2-4 hours (sulfapyridine)

Bioavailability: Oral bioavailability of parent drug is low (10-20%) due to poor absorption and first-pass metabolism; the active moiety 5-ASA is released locally in the colon.

Protein Binding: Sulfasalazine: ~99% bound to plasma proteins; Sulfapyridine: ~50-70% bound; 5-ASA: ~40% bound

Available Formulations & Strengths

Oral Tablet 500 mg (PO)

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

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to sulfasalazine, sulfonamides, or salicylates
Severe hepatic or renal impairment
Blood dyscrasias (e.g., agranulocytosis, aplastic anemia)
Porphyria
Intestinal or urinary obstruction
Use in animals with known glucose-6-phosphate dehydrogenase deficiency (rare in animals)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal disease, blood dyscrasias, or allergies.
Cats are more prone to adverse effects, especially keratoconjunctivitis sicca (KCS) and hepatotoxicity.
May cause discoloration of urine, skin, or tears (orange-yellow).
Monitor complete blood count (CBC), liver enzymes, and tear production periodically during long-term therapy.
Use with caution in pregnant or lactating animals; safety not well established.
In food animals, observe withdrawal times; extra-label use is prohibited in the US for non-approved species.

Adverse Effects & Reactions

Common:

Anorexia
Vomiting
Diarrhea
Lethargy
Orange-yellow discoloration of urine/tears

Serious / Severe:

Keratoconjunctivitis sicca (KCS)
Hepatotoxicity
Blood dyscrasias (leukopenia, thrombocytopenia, anemia)
Exfoliative dermatitis
Stevens-Johnson syndrome
Neurotoxicity (ataxia, seizures)

Rare:

Pulmonary fibrosis
Pancreatitis
Nephrotoxicity
Hypersensitivity reactions (anaphylaxis)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Digoxin	Sulfasalazine may reduce digoxin absorption, decreasing its efficacy.	Moderate
Folic acid	Sulfasalazine inhibits folate absorption and metabolism, potentially leading to folate deficiency.	Moderate
Warfarin	Sulfasalazine may enhance the anticoagulant effect of warfarin, increasing bleeding risk.	High
Methotrexate	Additive hepatotoxicity and bone marrow suppression.	High
Corticosteroids	May increase risk of GI ulceration when used together.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Nausea
Vomiting
Abdominal pain
Drowsiness
Seizures
Methemoglobinemia
Renal failure

Emergency Treatment Protocol: Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal. Provide supportive care: IV fluids, oxygen, anticonvulsants if seizures occur. Monitor CBC, liver enzymes, and renal function. Methylene blue may be used for methemoglobinemia if severe.

Food Animal Withdrawal Times

Not approved for food animals in the US. Extra-label use in food animals is prohibited. If used in non-approved species, consult regulatory authorities for withdrawal times.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (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.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (Azulfidine).

Extra-Label (Off-Label) Use: In the US, sulfasalazine is not FDA-approved for veterinary use; use in animals is extra-label. Under AMDUCA, extra-label use is permitted in non-food animals by or on the order of a veterinarian. Use in food animals is prohibited.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Sulfasalazine is primarily used in dogs and cats for inflammatory bowel disease, especially colitis. It is a prodrug that delivers 5-ASA to the colon. Response may take several weeks; if no improvement in 4-6 weeks, reassess diagnosis. Cats are particularly susceptible to adverse effects, especially KCS; baseline and periodic Schirmer tear tests are recommended. Monitor CBC and liver enzymes every 3-6 months during long-term therapy. Administer with food to minimize GI upset. Discontinue gradually to avoid relapse. Consider alternative therapies (e.g., mesalamine, corticosteroids, immunosuppressants) if intolerance or lack of efficacy occurs.

References & Literature

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