There is only a short amount of time that a neurotransmitter has to relay it's message before it is destroyed by enzymes or taken back up by the cell. This process is referred to as reuptake. Once reuptake occurs, the neurotransmitter has no affect on the brain.
In people suffering from depressive illness fewer of these neurotransmitters are being produced. Therefore antidepressant medications known as selective serotonin reuptake inhibitors (SSRIs) delay the reuptake of one of these neurotransmitters, serotonin, thus raising levels in the brain. With more natural levels of serotonin in the brain, mood is elevated from the depressed state to a more normal state of mind. SSRIs are very popular because they seem to have the least side-effects among antidepressants. Additionally, side-effects tend to be mild to moderate and are transitory, usually disappearing after 1-3 weeks. Some commonly known SSRI medicines are Zoloft, Prozac, Paxil and Lexapro.
The video below is an animation of how an SSRI prevents reuptake.
