Any drug used to treat depression. The three main types inhibit the metabolism of serotonin and norepinephrine in the brain. The aim is to keep these monoamine neurotransmitters from dropping to levels associated with depression. The drugs may take a few weeks to show any effect. Tricyclic antidepressants, which inhibit inactivation of norepinephrine and serotonin, help more than 70% of patients. Monoamine oxidase (MAO) inhibitors apparently block the action of MAO, an enzyme that helps break down norepinephrine, serotonin, and dopamine in neurons. They have unpredictable side effects and are usually given only when tricyclic drugs do not help. Selective serotonin reuptake inhibitors (SSRIs) apparently block reabsorption only of serotonin, allowing its levels to build up in the brain. SSRIs, which include fluoxetine (trade name Prozac), often help with depression unrelieved by tricyclics or MAO inhibitors and have milder side effects.