Serotonin is a type of chemical known as a neurotransmitter. It helps carry electrical impulses from one neuron to the next within the brain. Normally, the serotonin is quickly reabsorbed by the first neuron. SSRIs, however, slow down the reabsorption process. This allows the serotonin to remain in the synaptic gap, which is the space between the neurons, and stimulate further impulses in the second neuron.
Serotonin has been identified in research as one of the neurotransmitters that specifically affect mood and anxiety. Creating the right balance of serotonin is the ultimate goal of treatment with SSRIs.
The use of SSRIs gradually creates important changes in the brain. Both the sending neuron and the receiving neuron adapt to the increased level of serotonin by altering the production and reception processes. These processes may take two weeks to two months to fully adapt.
Therefore, you should not expect a dramatic immediate effect from your medication. You may experience some changes in mood, and even increased anxiety, during the first few weeks. Let your doctor know about these changes, but remain on your medication unless your doctor tells you otherwise. Over time, your brain should adapt, and you should notice a decrease in side effects. Keep in mind, however, that SSRIs are not without risks. See "SSRIs and Suicide" for more information on possible concerns.
Source:“How SSRIs (probably) Work.” Norfolk and Waveney Mental Health. National Health Service Foundation Trust. April 13, 2008. http://www.nmhct.nhs.uk/pharmacy/moa-ssri.htm
