Chocolate contains theobromine which can cause seizures and even death. Domesticated animals metabolize theobromine much more slowly than humans, which leads to complications of theobromine poisoning. Theobromine is also suspected to induce right atrial cardiomyopathy after long term exposure at levels equivalent to around 15 g of dark chocolate per kg of weight and per day. A typical 20 kg (44 lb) dog will normally experience intestinal distress after eating less than 240 g (8.5 oz) of dark chocolate, but won't necessarily experience bradycardia or tachyarrhythmia unless it eats at least 0.5 kg (1.1 lb) of milk chocolate.
Avocados contain persin, which in large amounts, can make your dog extremely sick. Many pet parents have reported that a little avocado has been fine for their dogs. Some pet parents have found that their pets are slightly allergic to avocado, and may experience diarrhea. The best thing is to ask your vet! There is much conflicting evidence surrounding avocados and dogs - in small quantities it may be alright, but definitely not in larger quantities.
3. Onions & Garlic
Onions and garlic can destroy the blood cells in your dog, leading to hemolytic anemia (bursting of red blood cells). The main culprit in onions and garlic is the active compound called thiosulphate. Symptoms of hemolytic anemia include vomiting, diarrhea, liver damage, breathing problems, and discoloured urine. Garlic however is less toxic, and large amounts would need to be consumed to cause illness in dogs.
4. Grapes & Raisins
Grapes & raisins can cause liver damage or kidney failure in your dog. However, in the 10 cases reported to the ASPCA Animal Poison Control Center, each dog ingested between 9 ounces and 2 pounds of grapes or raisins - a couple grapes here and there would not bring about such damaging effects in the organs of your pet (depending on the size of your dog! Little dogs shouldn't be fed a lot of grapes).
5. Milk & Dairy Products
Dairy products cause diarrhea and digestive upset. Some adult dogs do not have sufficient amounts of the enzyme lactase which breaks down lactose in milk (same goes for cats). Lactose-free milk products are available for pets (such as cat milk - however, these are often highly processed, and likely shouldn't be going in the tummies of your pets).
6. Macadamia Nuts
As little as 6 macadamia nuts can be enough to initiate symptoms such as vomiting, tremor, weakness and depression. This is due to the high phosphorous content, and should be avoided at all costs.