How do SSRIs work?
Serotonin is a chemical in the brain known as a neurotransmitter, according to Wikipedia. The imbalance of serotonin related to depression and anxiety is caused by excess quantities of serotonin leaving the synapses, which are the spaces between neurons within the brain, and re-entering the neurons. By inhibiting the reuptake of serotonin into neurons, SSRIs effectively correct the levels of serotonin within the synapses and regulate its transmission into the neurons. Scientific studies show that this process relieves depression and anxiety in some test subjects. Other antidepressants, such as tricyclic antidepressants, momoamine oxidase inhibitors and serotonin norepinephrine reuptake inhibitors, also work by regulating serotonin and other neurotransmitter levels.
Sources: mayoclinic.org, history.nih.gov, en.wikipedia.org