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Ketoconazole is an antifungal tablet. In dogs it treats yeast infections of the skin, ears and paws, and the usual dose is 5 to 10 mg per kg by mouth once daily, given with food, for about three weeks.1,2
It's a drug I reach for often, and it's also one of which I watch the liver on most closely.
Key Takeaways
The dose depends on what's being treated, and the range is wide. Yeast on the skin needs far less than a fungus in the lungs.
| What's being treated | Typical dose | Duration |
|---|---|---|
| Yeast (Malassezia) skin, ear or paw infection | 5 to 10 mg/kg by mouth once daily | About 3 weeks2 |
| Ringworm (dermatophytosis) | 10 mg/kg by mouth once daily | 4 to 8 weeks2 |
| Systemic fungal infection | 5 to 20 mg/kg by mouth, split every 12 hours | Months1,9 |
| Long-term maintenance | 2.5 to 5 mg/kg | Set case by case2 |
Doses above 10 mg/kg per day are rarely used outside systemic disease, because toxicity climbs.2
Give it with a full meal. Ketoconazole needs an acidic stomach to dissolve, and it's poorly absorbed in small animals to begin with.1 Food fixes both problems at once, and it's the single easiest thing to get right at home.
Follow the label on your own bottle rather than the table above. The dose your vet picked accounts for your dog's liver, their other medications and what grew on the cytology.
Give it when you remember, then carry on with the normal schedule. If the next dose is nearly due, skip the missed one. Never double up to catch up.
Ketoconazole blocks a fungus from building its cell membrane. The cell wall and the cell membrane are different structures, and only the membrane is the target here.
Fungi need a sterol called ergosterol to hold their cell membrane together, the same job cholesterol does in our own cells. Ketoconazole inhibits lanosterol 14α-demethylase, a fungal cytochrome P450 enzyme, which is the step that turns lanosterol into ergosterol.1 Without ergosterol the membrane goes leaky and the cell can't maintain itself.
That same enzyme explains the drug's problems. Mammals run closely related cytochrome P450 enzymes to build cortisol and testosterone and to clear other medications from the body.1 Ketoconazole doesn't distinguish perfectly between the fungal version and ours. Every interaction, the adrenal effects and the liver risk all trace back to that one lack of selectivity.
Yeast accounts for most of the prescriptions I write. Ringworm and the systemic fungal diseases make up the rest.
Malassezia is a yeast that lives on normal skin and overgrows when the skin barrier breaks down, which is why it so often rides along with allergies. It turns skin red, greasy and itchy, and gives ears and paws a distinct smell. Ketoconazole at 5 to 10 mg/kg daily is very effective against it.2
This is where I use it most. Yeast is so often the secondary layer on an allergy case that clearing it settles dogs who weren't responding to allergy control alone. If the yeast is on the paws, our article on pododermatitis in dogs covers what else drives it. Bacteria frequently join in too, which is a different problem with a different treatment, covered in pyoderma in dogs.
It's uncommon in dogs, and it's contagious to people. Despite the name it's a fungus rather than a worm, and it leaves itchy, scaly, hairless patches. Treatment runs 4 to 8 weeks.2
In Ontario that mostly means blastomycosis, and it's a local disease rather than a textbook one. The fungus lives in warm, moist soil near water. The highest-risk areas in this province are around Georgian Bay, northwestern Ontario and the Ottawa region.9 Dogs inhale the spores from disturbed soil, usually while digging or nosing along a shoreline, and they catch it far more often than people do.
It starts out looking like a chest infection: fever, cough, fast breathing, appetite loss. Left alone it can reach the eyes, skin, bones and brain.9 If your dog has been coughing since a trip up north, say so at the appointment.
Ketoconazole is not the first choice for blastomycosis. Itraconazole is, because ketoconazole has lower potency, more toxicity and a narrower spectrum.2 Ketoconazole still gets used where cost makes itraconazole impossible, which is a real consideration on a treatment course this long.
This one is historical. Ketoconazole suppresses cortisol production, so it was once used to control hyperadrenocorticism. Trilostane has replaced it and does the job better.
Three weeks for a yeast infection, 4 to 8 weeks for ringworm, and 4 to 6 months for blastomycosis.2,9 That last figure surprises people, and it's the one worth knowing before you start.
Don't stop early because the skin looks better. Yeast clears visually before it clears on cytology, and a course cut short is the most common reason an infection comes straight back. Your vet should be rechecking a sample under the microscope rather than going by appearance.
For blastomycosis, recovery happens in roughly 50 to 75% of dogs, and relapse is possible even after a full course, so monitoring continues past the last tablet.9
Most are mild and involve the stomach. In a retrospective study of 632 dogs on ketoconazole, 14.6% had an adverse effect: vomiting in 7.1%, loss of appetite in 4.9%, lethargy in 1.9% and diarrhea in 1.1%.3 Giving the tablet with food, or splitting the daily dose in two, usually settles it. Reversible lightening of the coat also happens and is harmless.2
The liver is the reason this drug needs supervision.
Stop the tablet and call your vet today if your dog goes off food, starts vomiting, becomes lethargic, or the whites of their eyes or gums look yellow. These are the signs of drug-induced liver injury, and they can appear within the first week and a half of treatment. Don't wait for the next scheduled recheck.
In a series of 15 dogs with probable ketoconazole-induced liver injury, every one was being treated for an ordinary skin yeast infection. The median dose was 7.8 mg/kg, well inside the normal range. Six had been on the drug for 10 days or less, and Cocker Spaniels made up 5 of the 15.4 The reaction is idiosyncratic, so neither the dose nor the length of the course predicts it. Bloodwork is the only way to catch it early.
I run bloodwork before the first tablet, again at two to four weeks, then every three to six months on anything long-term. If enzymes climb, our article on elevated liver enzymes in dogs explains what the numbers mean, and a bile acid test is usually the next step to see whether the liver is still doing its job.
Adrenal suppression catches people off guard. Because ketoconazole blocks the enzymes that build cortisol, a dog on it can end up unable to mount a stress response. A boxer on 4.5 mg/kg twice daily for Malassezia pododermatitis developed hypoadrenocorticism after 9 days, and recovered fully once the drug was stopped.5 It's uncommon and it's reversible. It also looks like the same vague lethargy and appetite loss as everything else on this list, which is why you test rather than wait it out at home.
Ketoconazole has the broadest inhibitory effect on drug metabolism of any azole.1 It blocks CYP3A12, the liver enzyme dogs use to clear a long list of medications. Those medications then stay in the body longer and climb higher than intended.
| Medication | What happens | What it means |
|---|---|---|
| Cyclosporine | Blood levels rise up to five-fold7 | Used deliberately to cut the dose and the cost |
| Ivermectin and other macrocyclic lactones | Exposure roughly doubles8 | Raises the risk of neurological signs, even in dogs without the MDR1 mutation |
| Phenobarbital | Each drug alters the other | Seizure control and antifungal effect both need watching |
| Omeprazole, antacids | Ketoconazole absorption drops1 | The antifungal may simply not work |
| Prednisone and other steroids | Steroid levels rise, natural cortisol falls | Compounded adrenal suppression |
| Digoxin, midazolam, warfarin | Levels rise1 | Heart rhythm, sedation depth and bleeding risk all increase |
The ivermectin combination gets overlooked more than any other. Ketoconazole also blocks P-glycoprotein, the pump that keeps ivermectin out of the brain. Ten days of ketoconazole at 10 mg/kg roughly doubled ivermectin exposure in dogs and slowed its clearance by a factor of 2.2.8 In the 632-dog study, ataxia showed up in 16 dogs, and 3 of the 4 worst cases were on ivermectin at the same time.3 Herding breeds with the MDR1 mutation start from a worse position, but this can happen in a dog with entirely normal genetics.
Antacids cause a quieter kind of failure. Ketoconazole needs stomach acid to dissolve, so omeprazole or any similar acid reducer can leave you giving a tablet that never gets absorbed. If your dog is on one, say so before the antifungal is dispensed rather than after three weeks of no improvement.
Both directions apply to phenobarbital and to prednisone. Give your vet the complete list of what your dog takes, supplements included.
This is the interaction I use on purpose. Cyclosporine is an excellent drug for allergic skin disease and immune-mediated conditions. For a large dog it's also expensive enough that owners stop using it.
Because ketoconazole blocks the enzyme that clears cyclosporine, adding it lets you get the same blood level from far less cyclosporine. Ketoconazole at 8 to 10 mg/kg daily allows a 70 to 90% reduction in the cyclosporine dose.6 A study at Ohio State found that cyclosporine and ketoconazole given together at 2.5 mg/kg each may work as well for canine atopic dermatitis as cyclosporine alone at 5 mg/kg.7
That turns an unaffordable treatment into an affordable one for a number of my patients. It needs monitoring. You're running two drugs that both affect the liver, and adverse effects were more frequent in dogs on ketoconazole plus cyclosporine than on ketoconazole alone.3 Our article on cyclosporine for dogs covers that side of the combination.
Prolonged ketoconazole also suppresses testosterone alongside cortisol, and infertility in intact males has been reported and resolved after stopping the drug.1 If you're breeding, raise it before treatment starts.
Ketoconazole also comes as a shampoo, cream or ointment. For localized yeast the topical version can do the whole job, with none of the liver risk.
Shampoos are usually combined with chlorhexidine, which handles the bacteria that show up alongside the yeast. The part people get wrong is contact time. The lather needs to sit on the skin for about ten minutes before rinsing, and a 30-second wash does close to nothing. Twice weekly is a common starting frequency.
If the yeast sits on two paws and a pair of ears, I'd rather bathe and treat the ears than put the whole dog on a systemic antifungal. On a dog that's greasy and red from neck to tail, the tablets do the heavy lifting and the shampoo supports them.
If your dog is itchy, smelly, or already on ketoconazole and not improving, give us a call at (519) 896-0532.
For a yeast infection of the skin, ears or paws, the usual dose is 5 to 10 mg per kg of body weight by mouth once daily, given with food, for about three weeks. Ringworm is treated at 10 mg/kg once daily for four to eight weeks. Systemic fungal infections such as blastomycosis need more, often 5 to 20 mg/kg split into two daily doses and continued for months. Doses above 10 mg/kg per day are rarely used outside systemic disease because toxicity climbs. Always follow the label on your own bottle rather than a general guide, because your vet set that dose around your dog's liver values and other medications.
You will often see a yeasty dog looking less red and smelling better within the first week, but looking better is not the same as being clear. Yeast disappears from the skin's appearance before it disappears from a cytology sample, which is why your vet should be rechecking a slide under the microscope rather than going by how the dog looks. A full course for a skin or ear yeast infection is about three weeks. Systemic fungal infections take months and improvement is slower and less obvious. If there is no change at all after seven to ten days, call your vet rather than continuing on the same dose.
It can, and that risk is the main reason the drug needs monitoring. Liver injury from ketoconazole is idiosyncratic, which means it is not predicted by the dose or by how long your dog has been taking it. In one series of 15 dogs with probable ketoconazole-induced liver injury, all were being treated for ordinary skin yeast infections at normal doses and six had been on the drug for ten days or less. Cocker Spaniels were overrepresented. Get bloodwork before the first tablet, again at two to four weeks, and every three to six months on long-term treatment. Stop the drug and call your vet the same day if your dog goes off food, vomits, becomes lethargic, or the gums or whites of the eyes look yellow.
No. Stopping early is the most common reason a yeast infection comes straight back. The skin clears visually well before the yeast is actually gone, so a course that ends when the redness fades usually ends too soon. Finish the full course your vet prescribed and go to the recheck appointment, because the decision to stop should be based on a cytology sample rather than appearance. This is even more true of systemic fungal infections such as blastomycosis, where treatment runs four to six months and relapse is possible even after a complete course.
No. Ketoconazole is a prescription medication and it interacts with a long list of other drugs, including ivermectin, cyclosporine, phenobarbital, prednisone, omeprazole, digoxin and warfarin. It is also unsafe in pregnant dogs and a poor choice for a dog with existing liver disease. Beyond the safety issue, a yeast infection and a bacterial skin infection look similar to the eye and need different treatment, so guessing without a cytology sample often means treating the wrong organism. Ketoconazole shampoos are more accessible than the tablets, but even those are worth clearing with your vet first so the underlying cause gets addressed.
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