Garlic, onion and Allium: toxic to dogs and cats

Why are Allium species dangerous?

Garlic, onion, chives, leek and all other species from the genus Allium contain organosulphur compounds that cause oxidative damage to red blood cells in dogs and cats [1][2]. These damaged cells are broken down by the animal's own body — haemolytic anaemia. A characteristic intermediate step is the formation of Heinz bodies: abnormal protein clumps in the red blood cells that severely disrupt their oxygen transport [3]. The process is dose-dependent but there is no established safe lower limit [2].

All forms are dangerous

Many owners know that raw onion is harmful, but think that garlic powder in dog biscuits or a leftover piece of fried onion at the bottom of the pan is harmless. This is incorrect. Dried and ground variants — powder, flakes, granules — are more concentrated than the fresh plant and therefore more dangerous per gram [1][2]. Allium processed in sauces, soups or stock also counts.

Cats are extra sensitive

Cats are more sensitive than dogs due to differences in haemoglobin structure and a more limited ability to neutralise certain sulphur compounds [1][2]. In cats, measurable damage can occur even with small amounts. In dogs the effect appears more strongly dose-dependent, but here too: no safe lower limit has been scientifically established [2]. Japanese breeds such as the Akita and Shiba Inu appear to be extra sensitive [1][2].

'Medicinal' garlic: also dangerous

Garlic is promoted online as a natural remedy against fleas or to support the immune system. There is no scientific evidence that this has an effect in dogs or cats, while the toxicological risk is real [2]. A recent in-vitro study confirmed that garlic — both raw and heated — clearly causes Heinz body formation in canine erythrocytes, even at concentrations common in supplement recommendations on the internet [3]. The unfavourable safety balance therefore also applies when garlic is administered intentionally.

Symptoms and timing

Poisoning often proceeds with a delay: symptoms can appear one to five days after ingestion, once the anaemia has developed [1][2]. Early complaints such as vomiting and diarrhoea may be present sooner.

Typical symptoms:

• Pallor or jaundice of gums, eyes or skin
• Dark red or brown urine (haemoglobinuria)
• Extreme fatigue, no interest in moving
• Rapid or laboured breathing
• Increased heart rate
• Vomiting and diarrhoea
• Collapse in severe cases

What to do if poisoning is suspected

Contact a veterinary surgeon or the animal poison control centre immediately. Note how much and which form of Allium was ingested, and when. In cases of early exposure, vomiting can be induced — but only by a veterinary surgeon, never at home without consultation. In severe cases, blood transfusion is necessary [1].

Do not wait. Because symptoms appear with a delay, an animal may initially feel well after ingestion. This is not proof that nothing is wrong.