Selective serotonin reuptake inhibitors (SSRIs) are a class of antidepressant medication. SSRIs work by inhibiting the reuptake of the neurotransmitter serotonin. When serotonin binds to its receptors in the brain it has a positive effect on mood. Therefore by inhibiting the reuptake or “capture” of serotonin by cells there will be an increase in the amount of serotonin able to bind.