SSRIs - How They Work

Selective serotonin reuptake inhibitors (SSRIs) work by increasing the levels of a substance called serotonin in the brain.

Normal function and reuptake of serotonin

Serotonin is a chemical messenger or a neurotransmitter. These messengers carry signals from one part of the brain to another. Serotonin regulates several aspects of brain function such as mood, sleep and emotion.

Serotonin is sent from one nerve cell called a presynaptic neuron across the synaptic cleft to a receiving nerve cell called a postsynaptic neuron. Once the presynaptic neuron has finished transmitting the information to the postsynaptic neuron, it reabsorbs the serotonin in a process called reuptake.

Serotonin in depression

People with depression have lowered levels of serotonin. SSRIs block the reuptake of serotonin, meaning more serotonin than usual remains available in the synaptic space between the two nerves. This eases the symptoms of depression resulting from the low levels of serotonin.

SSRIs may take around two to four weeks to raise the levels of serotonin in the brain and the treatment benefits may not be seen for a month or so. Once serotonin levels are raised however, mood is usually improved in individuals with depression.

SSRIs in depression

SSRIs can only control the symptoms of depression by increasing the serotonin levels in the brain. They fail to address and cure the underlying cause of depression or other conditions such as obsessive compulsive disorder and phobias. The underlying cause of these conditions may be far more complex than a low serotonin level in the brain. Furthermore, an individual's response to SSRIs may be governed somewhat by their genetics.

While some people benefit from these drugs, others may not respond. SSRIs are mostly used in conjunction with other medications or other treatment approaches such as psychotherapy, in order to address the more complex underlying causes of depression.

Further Reading

Last Updated: Feb 27, 2019

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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  1. Vickie Clayton Vickie Clayton Australia says:

    This is an interesting article but I'm wondering how levels of serotonin are actually measured in the brain? What tests are available for this? Thank you.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.