Many ingredients that are perfectly safe for human beings are poisonous to dogs, sometimes in miniscule amounts. The following list includes items that your dog should never ingest:
Chocolate contains a compound known as theobromine. This is present at higher concentrations in dark or baker's chocolate and is highly toxic to dogs.
Grapes and raisins, even in small amounts, are capable of causing complete kidney failure in dogs. The reason for this is not understood, but the connection has been recently documented.
Onions are toxic, raw or cooked, and foods containing onions should not be available to your dog.
Caffeine is dangerous in even small amounts.
Alcohol and recreational drugs are potentially lethal, in any amount.
Animal fats, including oil, are difficult for your dog to digest and can cause pancreatitis, a very painful condition that can lead to death if not treated immediately.
The general rule is to avoid feeding your boxer anything other than his regular diet, whether you use commercial dog food or home-prepared foods. If you need to feed a treat, give you boxer something healthy, like a bit of raw carrot, apple, or plain cooked chicken breast, or a good quality dog biscuit.