Can Dogs Eat Onions?

Why Are Onions Toxic to Dogs?

Allium vegetables contain organosulphur compounds that, when a dog ingests them, break down into reactive oxidants in the bloodstream. These oxidants attach to haemoglobin — the protein in red blood cells that carries oxygen — and cause the cells to rupture and be destroyed. When enough red blood cells are destroyed faster than the body can replace them, the dog develops haemolytic anaemia.

Importantly, the toxic compounds are present in all forms of onion: raw, cooked, dehydrated, and powdered. Onion powder — commonly used as a flavouring in processed foods, stocks, soups, and gravies — is actually more concentrated and therefore more dangerous per gram than fresh onion.

How Much Onion Is Toxic to Dogs?

Toxicity depends on the total amount ingested relative to body weight. As a rough guideline, ingesting around 0.5% of body weight in onion (any form) can cause toxicity — that is approximately 5 grams per kilogram of body weight. For a 10 kg dog, this equates to roughly 50 grams of fresh onion (about half a medium onion).

However, cumulative exposure over several days is also dangerous. A dog that eats small amounts of onion-containing food repeatedly — for example, from shared table scraps or food prepared with onion powder — can develop toxicity even if no single meal contained enough onion to cause obvious immediate harm.

Symptoms of Allium (Onion) Poisoning

Symptoms may be delayed by several days after ingestion because red blood cell destruction takes time to accumulate to noticeable levels. Initial signs may include vomiting, diarrhoea, drooling, and abdominal discomfort shortly after eating. Over the following days, as anaemia develops, you may notice weakness and lethargy, pale or yellowish gums, reduced appetite, rapid or laboured breathing, an elevated heart rate, and reddish or brown-tinged urine (caused by haemoglobin from broken blood cells being filtered through the kidneys).

In severe cases, collapse can occur. If your dog's gums appear pale or white rather than healthy pink, this is a veterinary emergency.

Common Foods That Contain Onion or Garlic

Many everyday human foods contain onions, garlic, or related compounds that can harm dogs. Stock cubes and gravy granules are particularly high in onion powder or garlic flavouring. Baby food often uses onion powder as a flavouring — if you use baby food for sick dogs, always check the ingredient list. Soups, stews, casseroles, and curries typically contain onions. Pizza, pasta sauces, and ready meals frequently include onion and garlic. Sausages and processed meats may contain onion or garlic as flavourings.

Always read ingredient labels carefully before sharing any human food with your dog. When in doubt, use TreatCheck to check the specific food or dish before offering it.

What to Do If Your Dog Eats Onion

If you know your dog has eaten a significant amount of onion or garlic — or has been eating small amounts regularly — contact your vet. If the ingestion was recent (within the past hour or two), your vet may recommend inducing vomiting to prevent further absorption.

If symptoms of anaemia are already present, your dog will need urgent veterinary assessment. Treatment may include supportive care, IV fluids, and in severe cases, a blood transfusion. Recovery is possible with prompt treatment, but haemolytic anaemia can be life-threatening if left untreated.

Remember: garlic, which is often promoted as a 'natural' remedy or parasite deterrent, is in fact more concentrated in thiosulphate than onion and should never be given to dogs or cats in any form.