Selective serotonin reuptake inhibitors (SSRIs) form a major class of antidepressant medicines. In 2012 over 27 million prescriptions for SSRIs were dispensed in England in the primary-care setting. SSRIs accounted for more than half of all antidepressant prescriptions in England.
SSRIs inhibit presynaptic reuptake of the neurotransmitter serotonin (5-hydroxytryptamine, 5HT), increasing the availability of serotonin at synapses and enhancing stimulation of postsynaptic neurones. Changes in receptor sensitivity (eg downregulation) arising from increased synaptic serotonin are thought to contribute to the antidepressant activity of SSRIs.
The adverse effects of SSRIs are distinct from those of tricyclic antidepressants; SSRIs have less marked antimuscarinic (anticholinergic) activity and, in overdosage, the characteristic features of tricyclic antidepressant poisoning—respiratory failure, defective cardiac conduction and coma—are largely absent in overdose with SSRIs. Characteristic adverse effects—such as those affecting the gastrointestinal tract and sexual function—can be attributed to the stimulation of 5HT receptors.
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