Zoloft (also known as Lustral in the UK) is the brand name for the drug sertraline, a selective serotonin reuptake inhibitor (SSRI) antidepressant. SSRIs prevent receptors in the human brain from re-absorbing serotonin that has already been released. Serotonin helps maintain feelings of well-being in people, so preventing its re-absorption alleviates negative feelings.