Onion & Garlic Toxicity

Every part of the allium family — onions, garlic, leeks, chives and shallots — is toxic to cats, and garlic is actually more potent by weight than onion. The danger lies in a group of sulphur-containing compounds that, once digested, damage the membranes of red blood cells. The injured cells form clumps of oxidised haemoglobin called Heinz bodies and are then destroyed, producing a haemolytic anaemia.

Cats are unusually vulnerable to this kind of oxidative damage because feline haemoglobin has more of the sulphur-bearing sites the toxins attack. A quantity of onion that a dog might tolerate can make a cat seriously ill. Crucially, cooking or processing does not neutralise the poison — dehydrated onion and garlic powder are, gram for gram, even more concentrated. This is why the most common real-world poisonings come not from a cat gnawing a raw onion but from small amounts hidden in human food: baby food, pasta sauces, stocks and gravies, and seasoned meats.

The insidious feature of allium toxicity is delay. A cat may eat the food and seem perfectly well, because it takes a few days for enough red cells to be destroyed to produce visible illness. By the time an owner notices pale gums, weakness, fast breathing, or urine turning a reddish-brown colour, significant anaemia has already developed.

There is no specific antidote, so treatment is a combination of decontamination if the cat is seen soon after eating, and supportive care to help it through the anaemia — fluids, oxygen, and transfusion in severe cases. The far better strategy is strict avoidance: no onion, no garlic, no allium-containing scraps, and particular caution with any "natural" garlic-based flea or tonic products marketed for pets, which have no place in feline care.