Monoamine Oxidase Inhibitors - or MAOI's increase levels of neurotransmitters in the brain, known as monoamines (serotonin, norepinephrine, and dopamine), by blocking a protein known as monoamine oxidase, which is normally responsible for burning them up. Increasing the level of monoamines in the brain has been shown to reduce symptoms of depression. MAOI's are powerful antidepressants.
MAOI's have been shown to have serious side effects, most notably a sudden, sometimes fatal, increase in blood pressure. Consequently, MAOI's are sometimes considered the last-resort antidepressant for serious cases.