Onion
Allium cepa
Yes — onions are toxic to cats. The ASPCA lists Allium cepa as toxic to cats, dogs, and horses. The toxic principle is N-propyl disulfide, which destroys red blood cells and causes hemolytic (Heinz body) anemia. All forms are dangerous — raw, cooked, dried, and powdered.
Plate IAllium cepa — onion. The ASPCA lists it as toxic to cats: the N-propyl disulfide in all parts destroys red blood cells, causing hemolytic anemia.
Three plants that look the part, without the risk.
If you want kitchen herbs that are safe to grow around a cat, these ASPCA non-toxic plants are good alternatives to the entire Allium family.
Basil
A non-toxic culinary herb that cats usually ignore. Safe on a sunny windowsill and the most popular kitchen herb — the Allium-free substitute for flavour.
Rosemary
A woody, aromatic herb that is non-toxic to cats. The strong scent deters most cats from sampling, and it grows into a sturdy kitchen shrub.
Catnip
A non-toxic herb that cats actively enjoy. If your cat is drawn to plants, catnip gives them a safe option to chew and roll in instead of your cooking onions.
What onions do to a cat's blood.
Yes — onions are toxic to cats. The ASPCA lists Allium cepa as Toxic to Dogs, Toxic to Cats, Toxic to Horses. The toxic principle is N-propyl disulfide, which destroys red blood cells and causes hemolytic (Heinz body) anemia. Every form is dangerous — raw, cooked, dried, and powdered.
Onions (Allium cepa, family Liliaceae) are one of the most clinically important food-related poisonings in cats. Cats are more sensitive than dogs because of their smaller body size and the structure of their haemoglobin. The ASPCA lists the following clinical signs: vomiting, breakdown of red blood cells (hemolytic anemia, Heinz body anemia), blood in urine, weakness, high heart rate, and panting.
ASPCA Data
According to the ASPCA, onions are listed under plants toxic to cats:
Toxic to Dogs, Toxic to Cats, Toxic to Horses.
Clinical signs: Vomiting, breakdown of red blood cells (hemolytic anemia, Heinz body anemia), blood in urine, weakness, high heart rate, panting.
The toxic principle is N-propyl disulfide — a compound that does not break down with cooking, drying, or processing.
Why onions are so dangerous for cats
The N-propyl disulfide in onions oxidises the haemoglobin inside red blood cells, causing Heinz bodies — clumps of damaged haemoglobin — to form. The damaged cells lose their flexibility and rupture as they pass through the spleen. This produces hemolytic anemia, meaning the blood can no longer carry enough oxygen.
Two things make onions particularly dangerous for cats:
- Cats are the most sensitive species. Their haemoglobin is more susceptible to oxidative damage, and their small body size means a smaller dose causes clinical signs. A slice of onion that would barely affect a large dog can be dangerous for a 4 kg cat.
- All forms are toxic. Cooking, drying, and powdering do not neutralise N-propyl disulfide. Onion powder in baby food, gravy, broth, and soup is a classic poisoning route — it is concentrated, and owners may not realise it is present.
The Allium family
Onions belong to the genus Allium, which also includes garlic and chives — both also toxic to cats by the same N-propyl disulfide mechanism. Garlic is roughly five times more potent than onion per gram of body weight. If you are avoiding onions for your cat, avoid the entire Allium family.
What to do if your cat eats onion
Onion toxicity is a medical emergency. Red blood cell destruction can continue for days after ingestion:
- Call your vet or ASPCA Animal Poison Control at (888) 426-4435 immediately. Do not wait for symptoms — the damage to red blood cells can be silent at first.
- Tell the vet what form of onion was eaten (raw, cooked, powdered), how much, and when.
- Do not induce vomiting unless instructed by a veterinarian.
Safe kitchen herbs instead
If you want to grow herbs in a kitchen a cat can reach, skip the entire Allium family. Basil, rosemary, and catnip are all ASPCA-listed non-toxic to cats — and catnip gives a plant-curious cat a safe option to chew on instead of your cooking ingredients.
The bottom line
Onions are toxic to cats — the N-propyl disulfide in every form (raw, cooked, dried, powdered) destroys red blood cells and causes hemolytic anemia. Cats are the most sensitive species, and onion powder in human food is a classic poisoning route. Keep all Alliums — onions, garlic, and chives — away from cats, and call your vet immediately if ingestion occurs.
What we have actually seen.
Hemolytic (Heinz body) anemia
The N-propyl disulfide in onions damages the surface of red blood cells, causing them to rupture. This produces hemolytic anemia — specifically Heinz body anemia, where damaged haemoglobin clumps inside the cell. Cats are more sensitive than dogs because of their smaller body size and haemoglobin structure.
Vomiting and gastrointestinal signs
The ASPCA lists vomiting as a clinical sign. Onions irritate the gastrointestinal lining on contact, so drooling, vomiting, and diarrhoea can appear within hours of ingestion.
Weakness, high heart rate, and panting
As red blood cells are destroyed, the blood carries less oxygen. The ASPCA lists weakness, high heart rate, and panting as clinical signs — the body's attempt to compensate for the oxygen deficit caused by anemia.
Blood in urine
The ruptured red blood cells release haemoglobin, which is filtered through the kidneys and appears in the urine. The ASPCA lists blood in urine as a clinical sign of onion toxicity.
- ASPCA Animal Poison Control Center. Toxic and Non-Toxic Plants: Onion.Accessed June 2026 · aspca.org