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Sulfasalazine (Azulfidine®)

By: Dr. Nicholas Dodman

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  • Sulfasalazine is a sulfa drug with both antimicrobial and anti-inflammatory properties that is generally employed in veterinary medicine to treat inflammatory bowel disease.
  • Its mode of action is not known with certainty and may be attributable to the parent drug and/or its primary metabolites, 5-aminosalicylic acid (5-ASA) and sulfapyridine (SP).
  • Sulfasalazine has high affinity for connective tissue and reached high concentrations in serous fluids, the liver and intestinal wall. The oral bioavailability of sulfasalazine itself is low (~ 15%).
  • It is metabolized by intestinal bacteria to 5-ASA and SP. The former metabolite also has low bioavailability (approximately 10 to 30%) but the latter is respectably well absorbed (bioavailability approximately 60%).
  • Peak plasma levels of sulfasalazine occur 90 minutes after dosing but peak levels of its metabolites are not attained for approximately 10 hours. The IV plasma half-life of sulfasalazine is 7.6 hours in humans. Sulfasalazine is excreted in the bile. Absorbed 5-ASA and SP and their metabolites are excreted in the urine.
  • Sulfasalazine is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Azulfidine® (Pharmacia & Upjohn) and generic sulfasalazine
  • Veterinary formulations: None

    Uses of Sulfasalazine

  • Treatment of colitis in dogs and inflammatory bowel disease in dogs and cats
  • Treat of rheumatoid arthritis in humans

    Precautions and Side Effects

  • While generally safe and effective when prescribed by a veterinarian, sulfasalazine can cause side effects in some animals.
  • Sulfasalazine should not be used in pregnant animals.
  • Sulfasalazine should not be used in animals with known hypersensitivity or allergy to the drug, sulfonamides, or salicylates.
  • It should also be avoided in patients with intestinal or urinary tract obstruction and those with porphyria.
  • Sulfasalazine should be used with caution, if at all, in patients with pre-existing fever, jaundice, or severe allergy; patients with renal or hepatic disease; low Schirmer tear test results, and patients with blood dyscrasias.
  • Gastrointestinal intolerance may occur to non-enteric-coated forms of sulfasalazine when there is severe inflammation/ulceration of the intestinal wall. Anorexia, vomiting, diarrhea, and gastric distress may occur.
  • Other possible side-effects include: skin rash, pruritus, urticaria, Heinz body anemia, hemolytic anemia, various blood dyscrasias, CNS and PNS disturbances, hepatitis, renal toxicity, decreased sperm count, keratoconjunctivitis sicca (dogs), and hypersensitivity.
  • Monitoring should include: 3-monthly CBCs and liver function tests, and periodic urinalysis.

    Drug Interactions

    Effects of sulfasalazine on the following drugs include:

  • Digoxin – reduced absorption
  • Folic acid - reduced absorption
  • Sulfonylureas – impaired hepatic metabolism

    How Sulfasalazine is Supplied

  • Sulfasalazine is available as 500 mg tablets
  • Sulfasalazine is available as delayed-release, enteric-coated tablets: 500 mg

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • In dogs, the usual dose is 10 to 25 mg per pound (20 to 50 mg/kg) every 8 hours. It may be give for 3 to 6 weeks or longer depending on the response and indication. A maximum of 1 g per dose has been suggested.
  • In cats, the usual dose is 5 to 10 mg per pound (10 to 20 mg/kg) every 12 to 24 hours. It must be used use cautiously as cats are sensitive to salicylates.
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

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