Sulfasalazine
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: 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)** โ Acts locally on the colonic mucosa to inhibit **cyclooxygenase (COX)** and **lipoxygenase (LOX)** pathways โ Decreases production of pro-inflammatory **prostaglandins** and **leukotrienes**. It also acts as a local free radical scavenger. - **Sulfapyridine** โ 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).
Dosing by species
- Inflammatory large bowel disease ยท 10-20 mg/kg PO once daily. ยท PO ยท q24h ยท Unspecified ยท Use cautiously in cats because of their sensitivity to salicylates.
- Inflammatory large bowel disease ยท 10-20 mg/kg PO q24 hours (once daily) tapered to the lowest effective dose ยท PO ยท q24h ยท Unspecified
- Inflammatory large bowel disease ยท 10-20 mg/kg PO q8-12h (maximum of 10 days) ยท PO ยท q8-12h ยท Maximum 10 days
- Inflammatory large bowel disease ยท 10-20 mg/kg PO q8-24h; up to a maximum of 10 days treatment ยท PO ยท q8-24h ยท Maximum 10 days
- All uses (Management of colitis) ยท 10-20 mg/kg ยท PO ยท q8-12h
- General ยท 10-20 mg/kg PO 2-3 times a day ยท PO ยท q8-12h ยท Unspecified
- General ยท 25 mg (total dose) PO twice daily ยท PO ยท q12h ยท Unspecified
- Inflammatory large bowel disease ยท 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. ยท PO ยท q8h then q12h ยท 9 weeks
- Inflammatory large bowel disease ยท 20-48.4 mg/kg (maximum total dose of one gram in refractory patients) PO q8h. ยท PO ยท q8h ยท Minimum 4 weeks ยท May consider an initial dose of 12.5 mg/kg, q8h. After signs resolve, reduce dosage by 25% at 2 week intervals.
Routes of administration
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
- Keratoconjunctivitis sicca (KCS)
Drug interactions
- Chlorpropamide ยท Hypoglycemic effects could be potentiated
- Digoxin ยท Sulfasalazine may reduce absorption of digoxin ยท moderate
- Ferrous sulfate (or other iron salts) ยท May decrease the blood levels of sulfasalazine if administered concurrently
- Folic acid ยท Oral absorption of folic acid may be inhibited
- Warfarin ยท Potentially sulfasalazine could potentiate warfarin
- Folate ยท Measurement of serum folate concentration may be affected ยท minor
- Thyroxine ยท May cause a reduction in serum thyroxine concentrations ยท minor
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)
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.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.