Theobromine can trigger epileptic seizures, fatal cardiac irregularity, can irritate the gastrointestinal tract and cause internal bleeding which can kill the dog within a day or so. Theobromine is also present in differing amounts in different kinds of chocolate. Theobromine will stay in the bloodstream between 14-20 hours. It goes back into the bloodstream through the stomach lining and takes a long time for the liver to filter out. If your dog has eaten chocolate, contact your vet immediately.
Many dogs react adversely to the chemicals in tea, coffee and cola.
The best defense is prevention. Keep all such products where your dog can't get at them. Onions, especially raw onions, should be avoided as they can trigger fatal hemolytic anemia in dogs. Potato poisonings have occurred both in dogs and people from the solanum alkaloids found in green sprouts and green potato skins which occurs when the tubers are exposed to sunlight during growth or after harvest. Cooked, mashed potatoes are fine to eat, but keep the raw ones away from your dog.