definition of Paxil (paroxetine): Paroxetine is an antidepressant that is a type of selective serotonin reuptake inhibitor (SSRI). It works by restoring the balance of serotonin, a neurotransmitter in the brain, which helps to improve certain mood problems. Paxil® is prescribed for depression, obsessive-compulsive disorder, anxiety disorders, post-traumatic stress disorder, and premenstrual dysphoric disorder. It may also be prescribed for people with autism. One possible serious side effect is increased risk of suicidal thinking and behavior in children, adolescents, and young adults. The risk of this side effect needs to be weighed carefully with the clinical need. Families should be aware of the need for close observation of the child or adolescent taking this medication, and maintain communication with the doctor who prescribed it.