Zoloft is an antidepressant that is used to treat depression, anxiety and obsessive compulsive disorder. It is a selective serotonin reuptake inhibitor Synthroid 25 mg (SSRI) which help to increase the amount of serotonin available in the brain. Zoloft produces fewer side effects than older antidepressants called tricyclics, such as Elavil and Tofranil.