Monoamine oxidase inhibitors (MAOIs) are a class of medications used primarily to treat depression. These drugs work by inhibiting the activity of monoamine oxidase, an enzyme responsible for breaking down neurotransmitters such as serotonin, norepinephrine, and dopamine in the brain. By preventing this breakdown, MAOIs help increase the levels of these neurotransmitters, which can improve mood and alleviate depressive symptoms.