Selective serotonin reuptake inhibitors (SSRIs):

drugs used to treat depression and anxiety. They inhibit the reuptake of serotonin, which causes serotonin levels to increase. Although SSRIs were designed to increase serotonin levels, it is now thought that the mechanism by which they are able to improve depressive symptoms is more complex.

Further reading: The serotonin hypothesis and neurogenesis

                             Autism, SSRIs, and epidemiology