Onions can cause hemolytic anemia in dogs. Onions and related vegetables such as shallots and garlic contain sulfoxides and disulfides, compounds that damage oxygen-bearing red blood cells. This impairs the ability of the body to circulate oxygen and causes illness. Dogs and cats are particularly susceptible to the toxic effects.
While onions are less toxic to dogs than garlic, they can still be very dangerous, especially if they are consumed in concentrated form, such as in powders or dehydrated spice mixes. Cooked and raw onions are also dangerous, as the toxic compounds are not affected by heat. Symptoms of hemolytic anemia include fatigue, muscle weakness, and urinating darkly colored or reddish-tinged urine.