Paxil Paxil Paxil is an agent in a newer class of antidepressant medication known as selective serotonin reuptake inhibitors (SSRIs). Depression and anxiety disorders might be caused by a chemical imbalance in the brain. One of these chemicals is serotonin, which helps send electrical signals from one nerve cell to another. In the process, serotonin is released from one nerve cell (the sender) and travels to the next (the receiver), where it is either absorbed or returns back to the original sender cell.