Antidepressants correct imbalances in the levels of chemical messengers - neurotransmitters - in the brain, particularly serotonin and noradrenaline. The types of antidepressants that are available include:

	serotonin and noradrenaline re-uptake inhibitors (SNRIs)
	selective serotonin re-uptake inhibitors (SSRIs)
	serotonin 5-HT2 blockers
	tricyclic antidepressants (TCAs)
	monoamine oxidase inhibitors (MAOIs)
	tetracyclic antidepressants
	noradrenaline and specific serotonin antidepressants (NaSSAs

SNRI antidepressants improves the levels of serotonin and noradrenaline in the brain. In contrast, the SSRIs and 5-HT2 blockers improve the levels of serotonin only. Tricyclic and tetracyclic antidepressants affect the levels of serotonin and noradrenaline either alone or together, depending on the drug used. MAO inhibitors work by blocking the metabolism of serotonin and noradrenaline and increase levels in the brain. NaSSAs also work by increasing the levels of serotonin and noradrenaline.
Although antidepressant medications begin to alter brain chemistry with the very first dose, it usually takes 2-4 weeks before there is a marked improvement in symptoms. It may be necessary to take antidepressants regularly for up to 8 weeks before the full effect occurs. Even after symptoms have improved, the medication is usually taken for at least 6-12 months to sustain recovery. It is important not to stop taking antidepressants until advised by a doctor. Questions about any antidepressant prescribed, or problems that may be related to the medication, should be discussed with the doctor who prescribed it.