Trimethoprim-sulfamethoxazole (SMZ)
Bactrim, Septra (human) · generic sulfamethoxazole-trimethoprim · Uniprim, Tucoprim (veterinary trimethoprim-sulfadiazine powders)
Trimethoprim-sulfa is the most prescribed oral antibiotic in horses, for wounds, abscesses, respiratory and urinary infections and after surgery. The human double-strength tablets are the usual form, filled at any pharmacy on a vet’s prescription; a horse takes many tablets a day, so the tablet price matters.
What it is used for
The combination of trimethoprim with a sulfonamide blocks two steps in bacterial folate synthesis and covers many of the bacteria behind equine infections. It is given twice a day by mouth, usually as human double-strength tablets crushed into feed or dissolved and syringed, and courses run from a week to several weeks. Veterinary powders combining trimethoprim with sulfadiazine are an alternative that mixes into feed.
Horses tolerate it well. Diarrhoea is the main risk, as with any antibiotic in horses, and a horse that develops diarrhoea on it needs the vet promptly. In dogs the same combination is used for urinary, skin and some protozoal infections, with more caution because dogs are prone to sulfa reactions.
Safety points that differ from human use
- Diarrhoea in a horse on antibiotics can become serious quickly; stop and call the vet the same day.
- Keep the horse drinking; sulfonamides can crystallise in the urine of a dehydrated animal.
- Dogs: dry eye, joint pain, skin reactions and blood changes are known sulfa effects; the vet monitors on long courses.
- Withdrawal rules apply before competition.
- Double-strength tablets (human, 800/160 mg)
- Oral suspension (human)
- Powders for feed (veterinary)
- Paste (veterinary, some markets)
- 800/160 mg and 400/80 mg human tablets
A full-size horse takes several double-strength tablets per dose, so a course is a large count of tablets.
Why the cash price varies
The human double-strength tablet is a commodity generic on most flat-price lists, but a horse course uses dozens to hundreds of tablets, so a pharmacy without a flat price can quote many times what another does for the same prescription. The veterinary powders are sold through veterinary channels and priced by the tub.
How to pay less
- Ask the vet to write the prescription for human double-strength tablets at the full course quantity.
- Ask whether sulfamethoxazole-trimethoprim is on the pharmacy’s flat-price generic list, and compare a second pharmacy.
- Upload the prescription to Script Unlock so licensed pharmacies can quote. You pay the pharmacy directly.
Questions for your vet
- How many tablets per dose, and for how many days?
- What should I do if my horse develops diarrhoea?
- Tablets or the veterinary powder for my horse?
- Will the infection need a culture or a recheck?
- What withdrawal time applies before competition?
Rules on refills, transfers and controlled substances differ by state; see pharmacy rules by state.
How Script Unlock gets you a cash price for your pet’s trimethoprim-sulfamethoxazole (smz) prescription
Prices on this site come from pharmacies responding to a real prescription. We do not publish estimated, average or "typical" prices, because none of those would be your price.
- 1You upload your prescription
A photo of your your pet’s trimethoprim-sulfamethoxazole (smz) prescription prescription is enough. Nothing is sold on the site; you are asking pharmacies to quote.
- 2Licensed pharmacies respond
Pharmacies that Script Unlock has verified see the request and each can reply with its own cash price for your exact drug, strength and quantity.
- 3You choose and pay the pharmacy
You compare the responses, pick a pharmacy, and pay that pharmacy directly when you collect. Script Unlock never takes a patient payment and never sets the price.
Related pet medications
Sources
- Merck Veterinary Manual
- FDA Approved Animal Drug Products (Green Book)
- FDA Orange Book (approved generics and therapeutic equivalence)
- ASPCA Animal Poison Control
Written by the Script Unlock editorial team from the sources above. Not yet reviewed by a veterinarian or veterinary pharmacist. Last checked 2026-10-03. Nothing here replaces your vet\u2019s directions; doses for animals are set by weight and species.
Script Unlock is a marketplace that lets licensed pharmacies quote a cash price on your prescription. It does not prescribe, dispense or sell medication, does not process patient payments, and does not give medical advice. Always follow the instructions of your prescriber and pharmacist and read the medication guide supplied with your medicine.