By Dr. Michelle Frye, DVM · Published 2026-05-01
TL;DR. Ronidazole is a compounded nitroimidazole used to treat Tritrichomonas foetus ("Trich") in cats — the chronic foul diarrhea that PCR diagnoses and metronidazole misses. Dosed 30 mg/kg once daily for 14 days. Compounded only — no FDA-approved veterinary product exists. Handle with gloves.
The Diarrhea That Doesn't Quit
The shelter cat has had cow-pat diarrhea for two months. She's bright, eating well, and looks normal otherwise — just leaving disasters in the litter box that smell, frankly, criminal. The vet has dewormed her twice, treated empirically with metronidazole, switched her diet, added probiotics. Nothing. The stool sample finally goes for a PCR panel. Tritrichomonas foetus positive. Suddenly the picture makes sense, and ronidazole is the answer.
What Ronidazole Actually Does
Ronidazole is a nitroimidazole — same drug class as metronidazole, but with much greater activity against the trichomonad protozoa that infect the feline large intestine. It works by interfering with the parasite's DNA synthesis, killing it where metronidazole only suppresses it.
- Dose: 30 mg/kg orally once daily for 14 days.
- Forms: pharmacy-compounded capsules, flavored oral suspension (no FDA-approved veterinary product exists).
- Onset of effect: stool quality often begins improving in the first week.
- Diagnosis is by PCR on a fresh fecal sample — routine fecal floats and centrifugation usually miss it.
Things People Are Wrong About
Myth 1: "Metronidazole treats Trich." Metronidazole controls the diarrhea symptomatically by partially suppressing the organism, but it does not eliminate it. Symptoms come back. Ronidazole actually clears the infection in most cats.
Myth 2: "All chronic cat diarrhea is IBD." True IBD is common, but Trich, giardia, food-responsive enteropathy, and dietary indiscretion all imitate it. Diagnosis matters.
Myth 3: "It's just dewormer, I can dose more if needed." No. Ronidazole has a narrow therapeutic window and rare neurologic side effects. Stick to the prescribed dose.
Myth 4: "My multi-cat household will be fine without testing the others." Trich is highly contagious between cats living together. If one tests positive, the others should be tested.
When NOT to Use Ronidazole
Avoid in pregnant queens, nursing queens, and very young kittens unless specifically directed. Stop and call your veterinarian immediately if your cat develops lethargy, inappetence, ataxia (wobbliness), tremors, or other neurologic signs — rare but reported. Wear gloves to handle the medication, especially if you are pregnant. Keep away from children and other pets.
What I Tell Owners After 30 Years
Tritrichomonas was the diagnosis we used to miss for years — the shelter and breeder cats with chronic foul diarrhea who got rounds of antibiotic and dietary trials and never quite improved. PCR testing made the diagnosis easy; ronidazole made the treatment effective. It is one of the genuinely satisfying interventions in feline GI medicine. The catch: it has to be compounded, the dose matters, and it needs to be sourced from a licensed compounding pharmacy — not the cheapest international site Google surfaces.
Frequently Asked Questions
What is ronidazole used for in cats?
Ronidazole is used to treat Tritrichomonas foetus infection of the large intestine in cats — a protozoal cause of chronic, foul-smelling, cow-pat diarrhea diagnosed by PCR fecal testing.
How long does ronidazole take to work?
Stool quality often begins improving within the first week of a 14-day course. Full clearance of the organism is the goal of the complete treatment course.
How much ronidazole do I give my cat?
The standard dose is 30 mg/kg orally once daily for 14 days, prescribed by your veterinarian and dispensed by a licensed compounding pharmacy.
Why isn't there an FDA-approved ronidazole for cats?
Ronidazole has never been submitted for FDA veterinary approval in the United States. It is dispensed exclusively as a pharmacy-compounded medication from a state-licensed compounding pharmacy.
What are the side effects of ronidazole?
Most common are mild GI upset and lethargy. Rare but serious neurologic side effects (tremors, ataxia, seizures) require immediate discontinuation and veterinary evaluation.
Can ronidazole treat Giardia?
No. Giardia is a different protozoal infection treated with fenbendazole or metronidazole. Ronidazole is specifically used for Tritrichomonas foetus.
Can dogs take ronidazole?
Ronidazole is occasionally used in dogs for specific protozoal infections, but its primary clinical use is feline trichomoniasis.
Where can I buy ronidazole for my cat?
Ronidazole is prescription-only and compounded. Smarty Vets dispenses pharmacy-compounded ronidazole capsules and flavored oral suspension from a licensed compounding pharmacy.
This article is for educational purposes and is not a substitute for veterinary advice. Always consult your veterinarian before starting, stopping, or changing any medication for your pet.