Olsalazine Sodium
Olsalazine sodium is a locally acting gastrointestinal anti-inflammatory agent primarily used in veterinary medicine to treat **chronic colitis** in dogs. It serves as an alternative for patients that are unresponsive to or cannot tolerate **sulfasalazine** (often due to the sulfa moiety causing side effects like keratoconjunctivitis sicca). Structurally, it is a prodrug consisting of two molecules of **5-aminosalicylic acid (5-ASA, mesalamine)** linked by an azo bond. Because it lacks the sulfapyridine moiety found in sulfasalazine, it generally has a lower risk of inducing immune-mediated side effects, though tear production issues can still occasionally occur. *Clinical Pearl:* While olsalazine is more expensive than sulfasalazine, its targeted delivery and lack of a sulfa component make it a valuable second-line therapy for inflammatory bowel disease (IBD) localized to the large intestine.
Mechanism: Olsalazine is a prodrug that passes through the stomach and small intestine largely intact. Upon reaching the colon, **bacterial azoreductase enzymes** (produced by normal colonic flora) cleave the azo bond, releasing two molecules of **5-ASA (mesalamine)**. * **5-ASA** acts locally on the colonic mucosa. * It inhibits the **cyclooxygenase (COX)** and **lipoxygenase (LOX)** pathways → decreasing the production of pro-inflammatory prostaglandins and leukotrienes. * It also acts as a **free radical scavenger**, reducing oxidative tissue damage in the inflamed colon.
Dosing by species
- For dogs who cannot tolerate sulfasalazine · 10-20 mg/kg · PO · three times daily
- Chronic colitis · 10-20 mg/kg · PO · q8h · Has had limited use in dogs without causing apparent side effects.
- Chronic colitis · 10-15 mg/kg · PO · q8-12h
- Chronic colitis · 5-10 mg/kg · PO · three times daily · Initially, then reduce gradually.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypersensitivity to olsalazine or salicylates
Adverse effects
- Keratoconjunctivitis sicca (KCS / dry eye) - less frequent than with sulfasalazine
- Diarrhea (paradoxical worsening reported in humans)
- Vomiting (especially with overdose)
- Decreased motor activity (with overdose)
- Elevated ALT or AST
Drug interactions
- Warfarin · Olsalazine may increase prothrombin times in patients receiving warfarin.
Monitoring
- Clinical efficacy (resolution of colitis signs)
- Adverse effects (especially signs of KCS/dry eye)
- Schirmer Tear Test (STT) periodically
- Serum ALT and AST
Overdose
Overdosage in dogs may cause **vomiting, diarrhea, and decreased motor activity**. * **Toxicity:** Dosages up to 2 grams/kg were not lethal in dogs. * **Treatment:** Treat symptomatically and supportively. GI decontamination may be considered if a massive overdose was recently ingested.
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.