Sulfasalazine
Salicylazosulfapyridine
Also known as: Azulfidine EN-tabs · salazosulfapyridine · salicylazosulfapyridine · sulfasalazinum · sulphasalazine · Aculfin · Azulfin · Colo-Pleon · Pleon RA · Pyralin · SAS · Salazopirina · Salazoprin · Salazopyrina · Salazopyrine · Salisulf Gastroprotetto · Salopyrine · Saridine · Sazo · Sulazine · Ulco
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Inflammatory large bowel disease | 20-40 mg/kg PO q8h for 3 weeks, followed by 20-40 mg/kg q12h for 3 weeks, and 10-20 mg/kg q12h for 3 weeks. | PO | q8h then q12h | 9 weeks | 🌎 NA |
| Inflammatory large bowel disease | 20-48.4 mg/kg (maximum total dose of one gram in refractory patients) PO q8h. | PO | q8h | Minimum 4 weeks | 🌎 NA |
| Chronic colitis | 20-50 mg/kg (up to a maximum of 1 gram) three times daily. | PO | q8h | Variable | 🌎 NA |
| Inflammatory large bowel disease | 10-25 mg/kg PO three times a day for 4-6 weeks. | PO | q8h | 4-6 weeks | 🌎 NA |
| Adjunctive treatment of vasculitis | 20-40 mg/kg PO q8h | PO | q8h | Unspecified | 🌎 NA |
| Adjunctive treatment of vasculitis | 25 mg/kg PO three times a day. | PO | q8h | Unspecified | 🌎 NA |
| All uses (Management of colitis) | 15-30 mg/kg | PO | q8-12h | — | 🌍 EU |
- Inflammatory large bowel disease: May consider an initial dose of 12.5 mg/kg, q8h. After signs resolve, reduce dosage by 25% at 2 week intervals.
- Chronic colitis: Initial dosage usually 20-30 mg/kg TID. Reduce dose at 2-4 week intervals if stool remains normal.
- Inflammatory large bowel disease: With resolution of clinical signs, reduce dose by 25 percent at 2 week intervals.
- All uses (Management of colitis): maximum 6 g/day
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Inflammatory large bowel disease | 10-20 mg/kg PO once daily. | PO | q24h | Unspecified | 🌎 NA |
| Inflammatory large bowel disease | 10-20 mg/kg PO q24 hours (once daily) tapered to the lowest effective dose | PO | q24h | Unspecified | 🌎 NA |
| Inflammatory large bowel disease | 10-20 mg/kg PO q8-12h (maximum of 10 days) | PO | q8-12h | Maximum 10 days | 🌎 NA |
| Inflammatory large bowel disease | 10-20 mg/kg PO q8-24h; up to a maximum of 10 days treatment | PO | q8-24h | Maximum 10 days | 🌎 NA |
| All uses (Management of colitis) | 10-20 mg/kg | PO | q8-12h | — | 🌍 EU |
- Inflammatory large bowel disease: Use cautiously in cats because of their sensitivity to salicylates.
Ferrets
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| General | 10-20 mg/kg PO 2-3 times a day | PO | q8-12h | Unspecified | 🌎 NA |
| General | 25 mg (total dose) PO twice daily | PO | q12h | Unspecified | 🌎 NA |
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Sulfasalazine is a unique, locally-acting prodrug that combines a sulfonamide antibiotic (sulfapyridine) and a salicylate anti-inflammatory (mesalamine or 5-ASA) via an azo bond.
- Primary Use: It is a cornerstone therapy for idiopathic inflammatory bowel disease (IBD), specifically plasmacytic-lymphocytic colitis (large bowel diarrhea) in dogs and cats. It has also been used adjunctively for vasculitis in dogs.
- Limitations: It is not effective for small intestinal disease because it requires colonic bacteria for activation.
- Clinical Pearl: While highly effective for colitis, its use in veterinary medicine requires careful monitoring due to the risk of keratoconjunctivitis sicca (KCS) or "dry eye" in dogs, which can be irreversible if not caught early. Cats require cautious dosing due to their inherent sensitivity to salicylates.
Mechanism of action
Sulfasalazine acts as a targeted delivery vehicle to transport anti-inflammatory medication directly to the colon.
- Delivery: The intact drug passes mostly unabsorbed (only 10-33% absorbed) through the stomach and small intestine.
- Activation: In the colon, bacterial azoreductase enzymes cleave the diazo bond.
- Mechanism:
- 5-Aminosalicylic Acid (5-ASA / Mesalamine) → Acts locally on the colonic mucosa to inhibit cyclooxygenase (COX) and lipoxygenase (LOX) pathways → Decreases production of pro-inflammatory prostaglandins and leukotrienes. It also acts as a local free radical scavenger.
- Sulfapyridine → Provides local antibacterial effects, though it is rapidly absorbed systemically and is responsible for the majority of the drug's adverse effects (e.g., KCS, hepatotoxicity).
Safety & warnings
Contraindications
- Hypersensitivity to sulfasalazine, sulfonamides, or salicylates
- Intestinal or urinary obstructions
- Doberman pinschers (highly susceptible to sulfonamide-induced polysystemic immune complex disease)
- Dobermanns (appear to be highly sensitive to adverse effects associated with sulfapyridine)
- Patients with known hypersensitivity to sulfonamides or salicylates
Adverse effects
- Keratoconjunctivitis sicca (KCS) in dogs
- Anorexia
- Vomiting
- Cholestatic jaundice
- Hemolytic anemia
- Leukopenia
- Decreased sperm counts
- Allergic dermatitis
Precautions
Use with caution in animals with preexisting liver, renal, or hematologic diseases.
Feline Warning: Because cats have a deficient glucuronidation pathway, they are highly sensitive to salicylates. Use extreme caution and strict dosing when using this drug in cats.
Use with caution in nursing patients, as sulfonamides are excreted in milk and may cause kernicterus in newborns.
Drug interactions
Hypoglycemic effects could be potentiated
Sulfasalazine may reduce absorption of digoxin
May decrease the blood levels of sulfasalazine if administered concurrently
Oral absorption of folic acid may be inhibited
Potentially sulfasalazine could potentiate warfarin
Measurement of serum folate concentration may be affected
May cause a reduction in serum thyroxine concentrations
Monitoring
- Clinical efficacy (resolution of diarrhea/colitis signs)
- Schirmer tear tests (STT) prior to therapy and on rechecks (especially in middle-aged to older dogs) to monitor for KCS
- Complete Blood Count (CBC) occasionally with chronic therapy
- Liver function tests occasionally with chronic therapy
- Schirmer tear test (STT) periodically
- Clinical signs of KCS (ocular discharge, blepharospasm)
- Liver enzymes (due to risk of cholestatic jaundice)
- Thyroid panel (if on long-term therapy)
Pharmacokinetics
Absorption
Only about 10-33% of an orally administered dose is absorbed. The unabsorbed drug is cleaved in the colon. The sulfapyridine component is rapidly absorbed from the colon, but only a small percentage of the 5-ASA is absorbed.
Distribution
Sulfonamides are excreted in milk and can cross the placenta.
Metabolism
Unabsorbed and biliary excreted drug is cleaved into 5-ASA and sulfapyridine in the colon by bacterial flora. Absorbed sulfapyridine and 5-ASA are hepatically metabolized.
Elimination
Renally excreted. Some of the initially absorbed intact drug is excreted unchanged in the bile.
Overdose
Massive overdoses could cause significant salicylate and/or sulfonamide toxicity.
- Treatment: Standard protocols for decontamination (empty stomach, cathartics, etc.) should be considered.
- Supportive Care: Urine alkalinization and forced diuresis may be beneficial in selected cases to enhance elimination.
Available products
Formulations
- Delayed-Release Tablets (enteric coated)
- 500 mg tablet
- 250 mg/5 ml oral suspension
Human-labeled
- Sulfasalazine Tablets: 500 mg (Azulfidine, generic)
- Sulfasalazine Delayed-Release Tablets: 500 mg (enteric coated) (Azulfidine EN-tabs, generic)
- Salazopyrin 500 mg tablet
- Salazopyrin 250 mg/5 ml oral suspension
Regulatory status
Used under the cascade. POM (Prescription Only Medicine).
Used under the prescribing cascade. POM-V.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets (plain or enteric-coated) should be stored at temperatures less than 40°C, preferably at room temperature (15-30°C) in well-closed containers. Oral suspension should be stored at room temperature; avoid freezing.
Client information
CRITICAL WARNING: This medication can cause "dry eye" (Keratoconjunctivitis sicca) in dogs, which can lead to permanent blindness if ignored.
- Monitor your pet's eyes closely. Contact your veterinarian immediately if you notice squinting, redness, thick yellow/green eye discharge, or if the eyes appear dull and dry.
- Administration: Give with food to reduce stomach upset. If using enteric-coated tablets, do not crush or break them.
- Cats: Cats are highly sensitive to this medication. Follow your veterinarian's exact dosing instructions and do not exceed the prescribed duration.
- Side Effects: Watch for loss of appetite, vomiting, or yellowing of the gums/eyes (jaundice).
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.