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Although all antidepressants are about equally effective against depression, the different types of antidepressants vary greatly. In general, they are grouped into classes according to which neurotransmitters they affect and how they affect these neurotransmitters.  

Monoamine oxidase inhibitors (MAOIs) were the first antidepressants to be developed. They work by blocking monoamine oxidase enzymes (the chemicals that break down neurotransmitters). Currently, MAOIs are rarely prescribed to a patient initially because when they are taken with certain foods they can cause life threatening problems, and  newer antidepressants have fewer side effects. However, MAOIs remain quite effective for certain disorders, especially when other antidepressants do not work.

MAOIs include:

Tricyclic antidepressants (TCAs) were the only alternatives to MAOIs available for many decades. TCAs block the reuptake of the neurotransmitters norepinephrine, serotonin and, to lesser degrees, dopamine in the brain. In general, physicians are prescribing TCAs less often as the first course of treatment because the newer antidepressants have fewer side effects.

Tricyclic antidepressants include:

In addition, the drug clomipramine (Anafranil) may be prescribed in an "off-label" manner.  This means that it is prescribed for a purpose other than which it was originally intended, and has not been approved for use as an antidepressant by the U.S. Food and Drug Administration.  

Selective serotonin reuptake inhibitors (SSRIs) are newer antidepressants that have fewer side effects than MAOIs or TCAs. They work by specifically blocking the reuptake of serotonin without affecting levels of norepinephrine or dopamine.

SSRIs include:

In some cases, the drug fluvoxamine (Luvox) may also be prescribed off-label to treat depression.

Norepinephrine and dopamine reuptake inhibitors (NDRIs) work by blocking the reabsorption of dopamine and norepinephrine without affecting serotonin levels. This allows the medications to avoid many serotonin-related side effects (e.g., sexual side effects). The only NDRI available in the United States is bupropion (Wellbutrin).

Serotonin and norepinephrine reuptake inhibitors (SNRIs) block the reuptake of serotonin and norepinephrine without affecting dopamine levels. These include duloxetine (Cymbalta) and venlafaxine (Effexor). TCAs are also primarily SNRIs.

Norepinephrine-serotonin modulators block certain receptors that accept serotonin and norepinephrine. The only medication in this class available in the United States is mirtazapine (Remeron).

Some antidepressants specifically block both the reuptake and receptors of serotonin. These serotonin modulators include nefazodone (Serzone) and trazodone (Desyrel).

In addition, newer antidepressant medications are under development.