SSRIS are considered first line treatment for depression, obsessive compulsive disorder, anxiety and a variety of other conditions. They are some of the most commonly prescribed medications in North America. So how do these medications work? The short answer is they increase the level of serotonin in your brain. Normally excess serotonin in a synapse gets recycled back to the presynaptic neuron. Selective serotonin reuptake inhibitors antagonize or block the serotonin reuptake transporter so that more serotonin is readily available to bind to the post synaptic cleft leading to a positive effect on mood.