Chocolate: While a small amount will likely only give your dog an upset stomach, a large amount can be deadly. Large quantities of chocolate in your pets' system can cause muscle tremors, seizures, irregular heartbeat, internal bleeding or a heart attack. Note, the onset of theobromine poisoning is usually marked by severe hyperactivity.
Xylitol: Foods that contain this sugar alternative include gum, candy, toothpaste, mouthwashes and baked goods. When ingested by dogs it can cause vomiting, loss of coordination, seizures and in severe cases, liver failure. Symptoms typically develop within 15 to 30 minutes of ingestion.
Garlic & Onions: While people frequently use these to spice up their food, it can cause health problems in dogs and cats whether they are fresh, cooked or in a dried/powered form. The toxicity lies in Allium species of plants (which also include leeks, scallions, chives and shallots). When a pet ingests these products red blood cells can be damaged and can cause anemia.
Grapes & Raisins: Although the exact substance that causes a toxic reaction is not yet known, we do know that even small amounts of grapes or raisins can be fatal. Dogs of any age, breed and gender can be affected. The most serious complications is acute kidney failure with lack of urine production.