Onions contain compounds called disulfides and thiosulphates which can be toxic to cats and dogs if ingested. The severity can range from minor gastrointestinal effects to a potentially serious, life-threatening haemolytic anaemia. Poisoning has occurred from the ingestion of whole raw onions, but also from chopped, cooked and dehydrated onions as well as products containing onion powder. Garlic, chives, leeks and other Allium species contain the same compounds.
Although cats are more susceptible, dogs are also at risk if a large enough amount is consumed. Initial signs of ingestion may include vomiting and diarrhea, however more concerning signs may take days to appear as red blood cell changes occur. These include pale mucous membranes, loss of appetite, lethargy, rapid breathing and a rapid heart rate.