Zoloft (Sertraline HCl) is one of a class of antidepressant drugs known as selective serotonin reuptake inhibitors (SSRIs). These drugs work by increasing the amount of serotonin that is available in the brain. Serotonin Is doxycycline a penicillin involved in regulating mood and Is doxycycline a penicillin general feelings of well-being.