Brewer's Yeast

Brewer's yeast is Saccharomyces cerevisiae that has been used in brewing, then harvested, dried and deactivated. The deactivation matters enormously and we will come back to it, because the live version of this organism is dangerous to dogs. Dried brewer's yeast is a concentrated source of B vitamins and protein, has a savoury flavour animals generally like, and has spent decades being sold for something it does not do.

What It Contains

• B vitamins — thiamine, riboflavin, niacin, pantothenic acid, B6, folate and biotin. Note that it contains essentially no B12 unless the product has been fortified, since B12 comes from bacterial rather than fungal synthesis.

• Protein, around 45 to 50 percent by weight, with a reasonable amino acid profile.

• Trace minerals including chromium and selenium.

• Cell wall components — beta-glucans and mannan-oligosaccharides, which act as prebiotics feeding beneficial gut bacteria.

It May Already Be in Your Dog’s Food

Brewers dried yeast is a standard pet food ingredient. As the Pet Food Institute explains, the AAFCO definition requires it to come from brewing beer or ale for human consumption and to be sterilised and rendered inactive. Manufacturers use it as a protein source and because dogs and cats find it palatable, even though people find it bitter.

So check the ingredient list before adding more. A dog on a complete diet that already contains it has its B vitamins covered, and a topper is mostly about taste. It is not calorie-free either — PetMD puts it at about 80 calories per ounce, which matters for a dog on a weight plan.

The Flea Claim, Which Was Actually Tested

Brewer's yeast has been sold as a natural flea repellent for as long as anyone can remember, usually in tablets combined with garlic. It is worth knowing that this was put to a controlled test in dogs and brewer's yeast did not reduce flea burden. The study compared supplemented and unsupplemented dogs and found no repellent effect.

That is unusual and valuable. Most supplement claims in this category simply have no data behind them; this one has data against it. If fleas are your reason for buying brewer's yeast, buy a veterinary preventative instead and treat the environment as well.

There is a second, sharper warning attached to this claim. Many of the brewer's yeast flea products on the market contain garlic, and garlic is in the allium family and toxic to dogs and cats, causing oxidative damage to red blood cells. A product combining an ineffective ingredient with a harmful one is the worst of both. Read the panel.

What It Is Genuinely Good For

• Coat and skin condition. The B vitamin and biotin content supports keratin production, and this is the most defensible use. Expect four to six weeks before you see a difference in coat.

• Palatability. It tastes savoury and is one of the easiest ways to make food more interesting to a fussy eater or an older animal who has gone off its meals.

• Digestive support via the cell wall oligosaccharides, with reasonable supporting work in livestock species.

• Nutritional top-up for animals on home-prepared diets, where B vitamin coverage is often the weak point.

Never Use Active Yeast

This is the safety point that matters most. Brewer's yeast is deactivated. Baking yeast and raw bread dough are alive. Live yeast swallowed by a dog continues to ferment in the warm stomach, producing carbon dioxide and ethanol. The gas expands the stomach, which can progress to a life-threatening bloat or gastric dilatation, and the ethanol causes genuine alcohol poisoning. Raw bread dough is a recognised veterinary emergency. Keep rising dough where a dog cannot reach it, and never substitute baking yeast for brewer's yeast.

Nutritional yeast, by contrast, is also a deactivated S. cerevisiae product, grown on molasses rather than recovered from brewing. It is nutritionally similar and an acceptable alternative, often less bitter.

How Much to Give

Animal · Daily amount

• Cat — 1/4 to 1/2 teaspoon

• Dog under 20 lbs — 1/2 teaspoon

• Dog 20 to 40 lbs — 1 teaspoon

• Dog 40 to 70 lbs — 1.5 to 2 teaspoons

• Dog over 70 lbs — 1 tablespoon

• Horse — 1 to 2 ounces

Roughly a teaspoon per 20 to 30 lbs of body weight is the working rule. Start at half and build over a week, since a sudden introduction of any fermentable material tends to produce gas.

How to Give It

Sprinkle the powder over food and stir it in; it is savoury enough that most animals take it readily. Flakes are milder than powder. Tablets exist but are usually the garlic-containing flea products, so check before buying. Store it sealed and dry — it absorbs moisture and clumps.

Dogs, Cats and Horses

Cats take it well and it is a useful appetite prompt, though the amounts are small.

Horses are where yeast has its strongest evidence, but you need to know that a different product is involved. Live Saccharomyces cerevisiae yeast cultures — alive, and safe in horses because a hindgut fermenter is built for microbial activity — have reasonable research support for improving fibre digestibility and stabilising hindgut pH, particularly in horses on grain-heavy diets. Dried dead brewer's yeast is a nutrient source; live yeast culture is a digestive aid. Both are sold for horses, they do different jobs, and the labels rarely make that clear.

What to Look For When Buying

• No garlic. The most important line on the panel.

• "Debittered" brewer's yeast, or nutritional yeast, if palatability is the goal.

• Single ingredient, no added sweeteners or flavourings, and no xylitol in any chewable version.

• Species and form stated — dried deactivated yeast for nutrients, named live yeast culture with a guaranteed cell count for equine digestive use.

• A date and a sealed container.

Side Effects and Cautions

• Gas and loose stool, particularly at the start or if you begin at a full serving.

• Purines. Yeast is high in purines, which are metabolised to uric acid. Avoid it in Dalmatians and any dog with a history of urate bladder stones — Dalmatians have a breed-specific defect in uric acid handling that makes them uniquely prone to urate urolithiasis, and a purine-rich supplement works directly against management.

• Yeast allergy. Some atopic dogs react to yeast as an allergen. Itching, ear inflammation or digestive upset after starting it means stop.

• A myth worth correcting: feeding brewer's yeast does not cause yeast infections. The skin organism involved in canine yeast dermatitis is Malassezia, a completely different genus from Saccharomyces, and eating one does not seed the other. Skin yeast overgrowth is usually secondary to allergic disease, as our article on allergies in dogs explains.

• Kidney disease — the phosphorus and protein content is a consideration in animals on restricted diets.

• Never live yeast or bread dough, covered above.

Medication Interactions

This is the section most brewer’s yeast pages leave out. In people, WebMD’s brewer’s yeast monograph lists interactions with MAOI drugs, diabetes medication, azole antifungals and immunosuppressants such as cyclosporine, while noting the interactions are not fully understood. There is no canine interaction data, but several of those drugs are common in dogs:

• Selegiline, an MAOI prescribed for canine cognitive dysfunction. PetMD specifically advises avoiding brewer’s yeast in dogs taking it, and in dogs on anxiety medications.

• Insulin and other diabetes treatment — brewer’s yeast may affect blood sugar, so clear it with your vet for a diabetic dog.

• Ketoconazole, itraconazole and fluconazole — azole antifungals, which dogs are often prescribed for skin yeast and other fungal infections.

• Cyclosporine, widely used in allergic dogs. WebMD also warns yeast products may raise infection risk in people with weakened immune systems, and PetMD lists compromised immunity as a reason to avoid it in dogs.

None of this makes brewer’s yeast dangerous for a healthy dog on no medication. It does mean it belongs on the list you give your vet.

When to Talk to Your Veterinarian

Before using it in a Dalmatian, any dog with a urate stone history, or an animal with kidney disease. Immediately if a dog has eaten raw bread dough or active yeast — that is an emergency, not a wait-and-see. And instead of using it for fleas, which is what the trial data tells us.

How It Fits With Hemp

The overlap is skin and coat, and they work from different directions. Brewer's yeast supplies the B vitamins and biotin that keratin production depends on; the fatty acids in hemp seed oil contribute to the skin barrier itself. Owners running both for a dull coat are not duplicating effort.

Brewer's yeast also has a purely practical use with hemp: its savoury flavour covers the taste of oil, so a dog that objects to a hemp serving on plain kibble will often take it mixed with yeast. Our serving guide has amounts by weight, our hemp treats are the pre-measured alternative if you would rather not dose oil at all, and every batch has a certificate of analysis.

Frequently Asked Questions

Does brewer's yeast repel fleas on dogs? No. A controlled trial in dogs found no effect on flea burden. Use a veterinary preventative and treat the environment.

How much brewer's yeast should I give my dog? About a teaspoon per 20 to 30 lbs of body weight daily. Start at half that for a week to avoid gas.

Is brewer's yeast the same as baking yeast? No, and the difference is serious. Brewer's yeast is deactivated. Baking yeast is alive and will ferment in the stomach, producing gas and ethanol. Raw bread dough is a veterinary emergency.

Can brewer's yeast cause yeast infections in dogs? No. Canine skin yeast is Malassezia, an unrelated organism. Skin yeast overgrowth is usually secondary to allergic skin disease.

Which dogs should avoid brewer's yeast? Dalmatians and any dog with urate bladder stones, because yeast is high in purines. Also dogs with a known yeast allergy, and animals on phosphorus-restricted kidney diets.

Is brewer's yeast good for horses? Dried brewer's yeast is a B vitamin and protein source. The better-supported equine product is a live yeast culture for hindgut fibre digestion, which is a different thing sold under similar names.

The flea work is Baker and Farver's controlled trial on brewer's yeast as a flea repellent in dogs in the Journal of the American Veterinary Medical Association. For why raw dough is an emergency, the ASPCA Animal Poison Control Center covers people foods to avoid feeding pets.

Can I give my dog human brewer’s yeast?

The organism is the same, but PetMD recommends a product made for dogs, ideally carrying the NASC quality seal, because human tablets can include added ingredients. If you use a human powder, it should be single-ingredient, debittered brewer’s yeast with no garlic, sweeteners or xylitol.

Does brewer’s yeast help dogs with anxiety or stress?

Some product pages claim so, but we know of no controlled study in dogs showing it. Its B vitamins support normal metabolism; that is not the same as a calming effect. If your dog is on anxiety medication, check with your vet before adding it.

What are the signs my dog is allergic to brewer’s yeast?

Itching, ear inflammation, hives, facial swelling, vomiting or diarrhoea after starting it. Stop it, and see your vet promptly for any facial swelling or breathing difficulty.

Why is brewer’s yeast in my dog’s food?

It is a protein source and a palatability enhancer. Under the AAFCO definition it must be inactive yeast from brewing, so it cannot ferment like baking yeast.