My causins dpitbull was diagnosed with cancer?

he is 2 years old. me and my aunt think its because he gives him some sort of steroids since the dogs was a puppy. and he give the dog some supplement sold in the tractor supply store, that supplement is mainly for horses. i know that he has given the dog different types of steroid and crap. im disgusted by this. he did this because he wanted the dog to have muscle.