Home/Medicines/Trimethoprim-Sulfamethoxazole
All medicines

Trimethoprim-Sulfamethoxazole

PrescriptionCDSCO approved
Potentiated sulfonamide antibiotic
Last reviewed 19 Apr 2026 · PetCare.AI Editorial Team
Species
Dog, Cat
Brands
3 available
Interactions
1 documented
Formulations
4

Mechanism of action

Sequential blockade of folate synthesis — sulfamethoxazole inhibits dihydropteroate synthase, trimethoprim inhibits dihydrofolate reductase

At a glance

Class
Potentiated sulfonamide antibiotic
Schedule
Prescription
Storage
Store below 25°C, protect from light
CDSCO (India)
Vet-approved — 1975-01

Dosing

🐕

Dog

Urinary tract infections
Dose
15–30 mg/kg
Route
PO
Frequency
BID
Max dose
960 mg/dose; 1920 mg/day
Duration: 10-14 days
Skin infections (Staphylococcal)
Dose
15–30 mg/kg
Route
PO
Frequency
BID
Max dose
960 mg/dose; 1920 mg/day
Duration: 21-30 days
Protozoal infections (Coccidiosis)
Dose
15–30 mg/kg
Route
PO
Frequency
BID
Max dose
960 mg/dose; 1920 mg/day
Duration: 5-7 days
🐈

Cat

Urinary and respiratory infections
Dose
15–30 mg/kg
Route
PO
Frequency
BID
Max dose
480 mg/dose; 960 mg/day
Duration: 7-14 days

Formulations

💊

Tablet — 2

Strength
480mg (80+400mg)
Available in India
Strength
960mg (160+800mg)
Available in India
🧴

Suspension — 1

Strength
240mg/5mL
Available in India
💉

Injectable — 1

Strength
480mg/5mL
Available in India

Storage

Store below 25°C, protect from light

Safety

Absolute contraindications — do not use

  • Known sulfonamide hypersensitivity
    Dobermans have increased risk of idiosyncratic polyarthritis/hepatopathy
    Doberman Pinscher
  • Severe hepatic disease
    Hepatically metabolised — risk of hepatotoxicity

Use with caution

  • Keratoconjunctivitis sicca (KCS) history
    Sulfonamides can cause or worsen KCS
  • Dehydration
    Risk of crystalluria — maintain hydration

Adverse effects

Common
Vomiting
Diarrhoea
Anorexia
Serious
Keratoconjunctivitis sicca (KCS)
Hepatotoxicity
Blood dyscrasias (thrombocytopenia, anaemia)
Drug eruption
Polyarthritis (Dobermans)

Monitoring parameters

Schirmer tear test (baseline and during treatment)CBCLiver enzymesUrinalysisHydration status

Interactions

Contraindicated — 1

Methotrexate
contraindicated
Both are folate antagonists; combined use causes severe bone marrow suppression
Management: Do not combine. If antibiotic needed during methotrexate therapy, use non-folate antagonist.

Brands

International

Bactrim
Roche

India

Septran
GSK
TMP-SMX Vet
Virbac

FAQs

Frequently asked questions

›What is Trimethoprim-Sulfamethoxazole?
Trimethoprim-Sulfamethoxazole is a potentiated sulfonamide antibiotic used in pets. Sequential blockade of folate synthesis — sulfamethoxazole inhibits dihydropteroate synthase, trimethoprim inhibits dihydrofolate reductase
›What is Trimethoprim-Sulfamethoxazole used for in pets?
Trimethoprim-Sulfamethoxazole is used in veterinary medicine for: Urinary tract infections; Skin infections (Staphylococcal); Protozoal infections (Coccidiosis); Urinary and respiratory infections.
›What is the Trimethoprim-Sulfamethoxazole dose for dogs?
For dogs, Trimethoprim-Sulfamethoxazole is typically dosed as follows — Urinary tract infections: 15–30 mg/kg PO BID; Skin infections (Staphylococcal): 15–30 mg/kg PO BID; Protozoal infections (Coccidiosis): 15–30 mg/kg PO BID. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What is the Trimethoprim-Sulfamethoxazole dose for cats?
For cats, Trimethoprim-Sulfamethoxazole is typically dosed as follows — Urinary and respiratory infections: 15–30 mg/kg PO BID. Always consult your veterinarian for a dose tailored to your pet's weight, age, and condition.
›What are the side effects of Trimethoprim-Sulfamethoxazole?
Common: Vomiting, Diarrhoea, Anorexia. Serious (call your vet immediately): Keratoconjunctivitis sicca (KCS), Hepatotoxicity, Blood dyscrasias (thrombocytopenia, anaemia), Drug eruption, Polyarthritis (Dobermans).
›Does Trimethoprim-Sulfamethoxazole need a prescription?
Yes. Trimethoprim-Sulfamethoxazole is a prescription medication and should only be administered under veterinary supervision.
›When should Trimethoprim-Sulfamethoxazole not be used?
Do not use Trimethoprim-Sulfamethoxazole if: Known sulfonamide hypersensitivity; Severe hepatic disease.

References

CDSCO approvals (India) — 3

Trimethoprim +Sulphadoxine
M/s Hoechst · Approved 1973- September
Trimethoprim+sulphadoxime
M/s Burroghs wellcare Co., Bombay · Approved 1974-July
Cosumix Plus Alpha Chloropyridazine Sodium 10gm + trimethoprim 2gm per 100 gm po
M/s Hindusan Ciba geigy · Approved 01.05.1990
Each 100 gm cosumix plus powder contains; alpha chloropyridazine sodium-10g, methoprin-2gm
Source: CDSCO Veterinary Drug Approval Registry (1969–2026)

References

Textbooks & handbooks

  • Plumb, D.C. Plumb's Veterinary Drug Handbook. 10th ed., Wiley-Blackwell, 2023.
  • Vail, D.M., Thamm, D.H., & Liptak, J.M. (eds.). Withrow & MacEwen's Small Animal Clinical Oncology. 6th ed., Saunders/Elsevier, 2020.
  • Riviere, J.E., & Papich, M.G. (eds.). Veterinary Pharmacology and Therapeutics. 10th ed., Wiley-Blackwell, 2018.
  • National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press, Washington DC, 2006.
  • The Merck Veterinary Manual. Merck & Co., Online edition. https://www.merckvetmanual.com/

Clinical guidelines & consensus

  • Fletcher, D.J., Boller, M., Brainard, B.M., et al. "RECOVER Evidence and Knowledge Gap Analysis on Veterinary CPR." Journal of Veterinary Emergency and Critical Care, 2012;22(S1):S102–S131.
  • American Animal Hospital Association. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. AAHA Press.

Journals & peer-reviewed studies

  • Hogan, D.F., Fox, P.R., Jacob, K., et al. "Secondary prevention of cardiogenic arterial thromboembolism in the cat: The FAT CAT study." Journal of Veterinary Cardiology, 2015;17(Suppl 1):S306–S317.
  • Boswood, A., Häggström, J., Gordon, S.G., et al. "Effect of Pimobendan in Dogs with Preclinical Myxomatous Mitral Valve Disease and Cardiomegaly: The EPIC Study — A Randomized Clinical Trial." Journal of Veterinary Internal Medicine, 2016;30(6):1765–1779.
  • ASPCA Animal Poison Control Center. Toxicology and Poison Management Guidelines. American Society for the Prevention of Cruelty to Animals. https://www.aspca.org/pet-care/animal-poison-control

Regulatory & approvals

  • Central Drugs Standard Control Organisation (CDSCO), Government of India. Veterinary Drug Approval Registry, 1969–2026. Directorate General of Health Services. https://cdsco.gov.in/

Databases

  • Washington State University, College of Veterinary Medicine. Veterinary Clinical Pharmacology Laboratory (VCPL) — MDR1 Multidrug Sensitivity Database. https://vcpl.vetmed.wsu.edu/
Educational reference only
This information is provided for educational purposes and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult a qualified veterinarian before administering any medication to your pet. Find a vet near you →

Related medicines

Other medicines in the same class (Potentiated sulfonamide antibiotic).

Sulfa-Trimethoprim (TMP-Sulfa)
Rx
Sequential inhibition of folate synthesis: sulfonamide inhibits dihydropteroate synthase (DHPS) and trimethoprim inhibits dihydrofolate reductase (DHFR). Combined, they are synergistically bactericidal despite each being bacteriostatic alone. 1:5 TMP:sulfa ratio.
dogcat
Sulfadimethoxine/Ormetoprim
Rx
Synergistic combination: sulfadimethoxine inhibits DHPS and ormetoprim inhibits DHFR (same sequential folate blockade as TMP-sulfa). Long half-life allows once-daily dosing. FDA-approved for dogs.
dogcat