Onions cause hemolytic anemia, which means that the red blood cells break down leaving the dog short of oxygen. Onion poisoning can occur with a single ingestion of large quantities or with repeated meals containing small amounts of onion. The condition generally improves once the dog is prevented from eating any further onion. The poisoning may occur a few days after the dog has eaten the onion. At first dogs affected by onion poisoning show gastroenteritis with vomiting and diarrhea, weakness and show little or no interest in food. The red pigment from the burst blood cells appears in an affected dog's urine making it dark colored. The dog will experience shortness of breath because the red blood cells that carry oxygen through the body are reduced in number. Other symptoms are elevated body temperature, confusion, and increased heart rate
The quantity of onions, raw or cooked, required is high enough that dogs can generally tolerate small doses of onions without any problem and moderate amounts of onion without apparent signs of onion poisoning. All forms of onion can be a problem including dehydrated onions, raw onions, cooked onions and table scraps containing cooked onions, left over pizza, chinese dishes and commercial baby food containing onion