Onions contain a substance that harms dogs' red blood cells. Cooked or raw, keep onions away from dogs.

Grapes and raisins can cause kidney failure in dogs. Experts aren't sure exactly why, but it's best to avoid them.

This sugar substitute found in many gums and baked goods is highly toxic to dogs and can lead to liver failure.

Macadamia nuts cause vomiting, weakness, and hyperthermia in dogs. Keep them out of reach.

Like onions, garlic and chives contain compounds that damage dog's red blood cells even in small amounts.  

The pit and skin of avocados contain persin, which is dangerous to dogs if ingested. The flesh is also risky.