Tricyclic antidepressants appear to work by blocking the reuptake of serotonin and norepinephrine. As a result, the levels of these two neurotransmitters in the brain are increased. Tricyclic antidepressants are usually used to treat depression and obsessive-compulsive disorder (OCD). Because of their side effects, they have been largely replaced in clinical use in by newer antidepressants.