Toxic Food & Plants For Dogs - Onion

Onions are toxic to dogs. The toxicity is dose dependent, so the bigger the dog, the more onion need be consumed to cause toxicity.

Onion toxicity causes a Heinz body anemia. Heinz bodies are small bubble-like projections, which protrude from a red blood cell and can be seen when the cells are stained. This "bubble" is a weak spot in the red blood cell and, therefore, the cell has a decreased life span and ruptures prematurely.
If numerous red cells are affected and rupture, anemia can result. It is a form of hemolytic anemia. Onions are only one of the substances, which can cause Heinz body anemia. Other substances such as Acetaminophen (Tylenol) and Benzocaine (used as a topical pain reliever, or in cough drops) can also cause Heinz body anemia in the dog.
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.

The hemolytic episode usually occurs several days after onion ingestion (lowest hematocrit around day 5 post ingestion). Daily feeding of onions could have a cumulative effect due to ongoing formation of Heinz bodies versus a single exposure with a wide gap until the next exposure, allowing the bone marrow time to regenerate the prematurely destroyed red cells.
Symptoms include labored breathing, liver damage, vomiting, diarrhea, and discolored urine.

While garlic also contains the toxic ingredient thiosulphate,
It is safe for your dog used in moderation and can help with a myriad of things such as gas, flea prevention and it has natural antibiotic properties.