Leeks, chives, onions, garlic, shallots and scallion are collectively known as the allium species and this whole group of common vegetables may be toxic if consumed by dogs.

When broken down in the digestive system and metabolised by the body they release by-products that can cause oxidative damage to red blood cells leading to serious anaemia and damaged haemoglobin. This results in a reduced ability to carry oxygen around the body.

The most common signs of toxicity

- Vomiting

- Unwillingness to eat

- Weakness

- Diarrhoea.

Less commonly

- Excess salivation

- Incoordination

- Blood in the urine

- Rapid respiratory and heart rate

- Abdominal pain

- Tiredness

- Seizures

The degree to which consumption of these vegetables is potentially dangerous depends on the dose i.e. it will vary with the amount consumed and the size of the dog. 

More than 5 grams of onion per kg body weight of dog is considered potentially toxic

It is a similar situation for garlic though as little as 15g per kg of garlic could be highly toxic. 

(As a general guide a small onion may weigh 60-130g and a medium onion 130-250g. A medium sized onion is also equivalent to roughly 1 cup of chopped onion which is also equivalent to 1tbsp onion powder(5.91g). A clove of garlic may weigh 3-7g and a whole bulb 60-75g). 

So you can see that a single medium sized onion could be toxic for a 20kg or 45lb dog. Or a whole garlic bulb could be very dangerous for a 5kg or 12lb dog.

Treatment

Your vet may choose to make your dog vomit to empty the stomach if this can be done before the ingested onions pass through to the intestines. For this to be effective it must be done within approximately 2 hours of the consumption of the onions etc.

It is not recommended to make dogs vomit if they are already showing signs of toxicity in case some of the toxic signs should make them more likely to inhale any material brought up.

Activated charcoal may be given orally to reduce absorption of toxic material

Further supportive treatment may include

Intravenous fluids to counteract dehydration

Anti oxidants 

Antiemetics (to control vomiting) 

There is no specific antidote for onion toxicity.

The prognosis (likely outcome)

This will vary with the quantity of material consumed and the treatment provided especially if there are symptoms of poisoning. The prognosis is generally good with decontamination and appropriate supportive care.

The outlook is less good in severely affected cases without aggressive and timely treatment