Trimethoprim-Sulfadiazine (± Sulfamethoxazole)

What is this medication for?

• UTI, pyoderma, soft tissue infection, systemic infection, protozoal disease (toxoplasmosis, coccidiosis, Neospora, Hepatozoon — combined with other agents)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Hypothyroidism (With extended therapy, reversible)
• Anorexia, crystalluria, hematuria, low white blood cell counts/low red blood cell counts (Reported)
• Transient itching (post-into a vein) (Reported)

Worth a phone call to your veterinarian

• Acute hepatitis/icterus, hemolytic low red blood cell counts, polyarthritis, facial swelling (Type I/III hypersensitivity)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Keratoconjunctivitis sicca (may be irreversible) (Documented, can be permanent)

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Sulfonamide/thiazide/sulfonylurea hypersensitivity; severe renal/hepatic impairment; Doberman Pinschers — increased susceptibility to sulfonamide hypersensitivity, use with caution
• has or has had: Urolithiasis risk (avoid in dogs prone to/with uroliths, dehydration, acidic urine)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Cyclosporine
  • Digoxin
  • Methotrexate
  • Phenytoin
  • Warfarin
  • Thiazide diuretics

Storage and handling

Room temperature, protect from light.

Good to know

• Watch for dry eye with prolonged use — this can become permanent if not caught early; consider baseline and periodic Schirmer tear tests on long courses.
• Can cause a reversible functional hypothyroidism in dogs after ~2 weeks of treatment — don't misinterpret thyroid panels drawn during therapy.
• Ineffective for abscesses/anaerobic infections — necrotic tissue material antagonizes the drug's mechanism.