Acidophilus is the most commonly used probiotic, or “friendly” bacteria. Many healthy bacteria live in the dogs intestines, where they protect against “bad” bacteria that can cause disease. They do this is a couple of ways: for example, when acidophilus breaks down food in the intestine, several substances are formed (such as lactic acid and hydrogen peroxide) that create an unfriendly environment for “bad” bacteria. Probiotics are often suggested as a supplement when you give your dog antibiotics. Antibiotics kill bacteria, but don’t discriminate between “friendly” and “unfriendly” organisms, so the balance between good and bad bacteria in the intestines can be upset. It is thought that taking probiotics helps restore the healthy balance of bacteria.