The combination of low nutrition and a high chemical load from your dog’s diet and medication, has the effect of causing liver (in particular, but other organs are not unaffected) toxicity and a low immunity. The effect of a compromised liver can lead to anger and aggression, as well as digestive problems and cancer.
The effect of a compromised immune system means your dog is far more likely to contract other health related problems and is easy prey to passing epidemics.
Simply changing the diet can start to unload the toxins in your dog and improve his/her immune system. The best food for your dog is to make it yourself from quality, human grade ingredients. You can find more information on this subject here. http://naturallyhealthydogs.com/indexnn.html