Antidepressants act on the theory that depression is caused by a lack of the key neurotransmitters responsible for mood: serotonin, norepinephrine, and dopamine. The idea is to keep these neurotransmitters "on" by preventing them from early reuptake after they are released into the synapses, or from being broken down by monoamine oxidase.

There are four main classes of antidepressants. In order of earliest to latest, they include the monoamine oxidase inhibitors (MAOIs), tricyclics, selective serotonin reuptake inhibitors (SSRIs), and norepinephrine-serotonin reuptake inhibitors (NSRIs). All antidepressant medications can have side effects, including weight gain and sexual dysfunction.

Scientists are not sure about the actual relationship between neurotransmitters and depression. For example, many people who are depressed have low levels of norepinephrine. Some antidepressants can increase the level of norepinephrine in the brain and subsequently relieve depressive symptoms. Conversely, other people who are depressed have high levels of norepinephrine, and the same scenario may hold true for other neurotransmitters.

So, while we know there is a strong relationship between neurotransmitter levels in the brain and clinical depression, and that antidepressant medications work for a great many people, we are not absolutely certain of the actual relationship between neurotransmitters and depression.