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Effective antidepressants work by increasing the amount of norepinephrine and serotonin available to the receptor, although other neurotransmitters may also play an important role in mood regulation. This increase in neurotransmitters may be achieved by causing more to be released or by delaying their breakdown, or metabolism.

The side effects of antidepressants are related to neurotransmitters and receptors as well. Although the relationship between neurotransmitter and receptor can be very specific, frequently the antidepressants are not specific in their action. For example, the serotonin “key” can work at many different types of serotonin receptors or “locks.” These receptors can be found in sites they are meant to influence (for example, nerve cells in the brain) as well as in areas they were not intended to influence (for example, serotonin receptors in the digestive system). This is why these medications can decrease appetite or cause loose stools, until the body adjusts to them.

Norepinephrine can affect other receptors that can lead to dry mouth and constipation, for example. Both kinds of antidepressants can cause sexual side effects, like difficulty achieving orgasm. Side effects frequently cease after the body gets accustomed to the medication; or, they can be minimized with help from your doctor. Researchers continue to look for antidepressants that act only on targeted receptors.