Your dog has a different digestive system than you do. If you have a dog, do not leave these food in a place where he or she can get to them.
Avocados

You may love guac, but your dog will not. Avocados contain a toxin called persin. Dogs can develop an upset stomach, breathing difficulties, and fluid buildup in the chest after eating an avocado. Also, the pit is slippery and can easily be swallowed by a dog.
Onion

Onions contain a compound that damages red blood cells in dogs. After eating onions, a dog will move around less and become weaker. If the dog has eaten too many onions, a blood transfusion may be necessary.
Chocolate

You've probably already heard that chocolate is bad for your dog, and the common belief is true. The caffeine and theobromine in chocolate is toxic to dogs, and can cause vomiting, dehydration, seizures, and death among other negative side effects. While all chocolate is dangerous for dogs, darker chocolate is the most toxic with white chocolate being the safest.
Bacon

Unfortunately, dogs shouldn't be given bacon, as its high fat content can lead to pancreatitis in dogs. Dogs that eat bacon will experience problems with digestion and nutrient absorption.
Macadamia Nuts

Scientists have only recently discovered that macadamia nuts are toxic to dogs. Although the exact reason for the toxic reaction is still unknown, dogs that have eaten macadamia nuts will experience a weakness or inability to walk, especially in their hind legs.
Milk

Like humans, dogs will drink their mother's milk after being born, but lose the enzymes to break it down as they grow older. Drinking milk can cause severe gastrointestinal problems in dogs, and even bacterial exposure that could eventually lead to death.
Cheese

Like milk, dogs don't have the enzymes to break down cheese, so it can cause diarrhea and vomiting if a dog eats it in excess.
Sugarless Gum

If you love chewing on sugar-free gum, just be sure you keep it away from your dog. Most sugarless gum contains the artificial sweetener Xylitol, which can cause liver damage in dogs. In addition, Xylitol can cause a life-threatening drop in blood sugar in dogs.
Grapes

The severity of symptoms vary between dogs, but grapes have been known to cause kidney failure. Kidney failure can cause death in three to four days.
Raisins

Since raisins are just dried grapes, they have the same toxic effects on dogs. Raisins can cause dehydration, vomiting, and kidney failure in dogs who eat them.
Yeast Dough

The yeast dough used to make bread can ferment in a dog's stomach and become toxic. It can also expand in the dog's stomach, potentially leading to a ruptured stomach or intestinal tract.
Garlic

Garlic is part of the onion family, so it's another no-no for dogs. Similar to the effects of onions, dogs will become more lethargic after eating garlic and may need a blood transfusion.
Apple Cores

Be extra careful not to leave apple cores lying around if you have a dog. Apple seeds actually contain cyanide, which can induce seizures, hyperventilation, and even comas in dogs.
Alcohol

Most people aren't sharing beers or cocktails with their dogs, and that's a good thing. If your dog ingests alcohol, it could have seizures.