As with human beings, no one is sure exactly what causes OCD in animals. However, it is believed that this condition is the result of a chemical imbalance in the brain. People who have OCD are thought to have lower levels of serotonin, which may be why doctors prescribe antidepressant drugs, such as Prozac and Paxil, for this type of obsessive-compulsive disorder. Some experts believe that obsessive-compulsive disorder affects more than ten percent of pets.