The use of monoamine oxidase inhibitors (MAOIs) by psychiatrists has declined over the past several decades with the increase of other drug options. This trend has resulted from concerns about food and drug interactions and side effects, as well as decreased doctor experience with these medications. 
MAOIs are used to treat atypical depression with symptoms such as a histrionic need for attention, interpersonal sensitivity, vulnerability to rejection, early onset of chronic dysphoria, association with panic attacks, association with anxiety, increased appetite, weight gain, hypersomnia or initial insomnia, and increased libido respond better to MAOIs than other drugs.
In bipolar disorder, there is evidence that MAOIs reduce the severity of manic episodes while still treating depressive symptoms. MAOIs may be more effective than tricyclical antidepressants in the treatment of bipolar depression, which manifests as fatigue, psychomotor retardation, and at least one symptom which is the opposite of those expected, such as weight gain.
MAOIs also affect other neurotransmitters in the brain and digestive system, causing side effects. MAOIs are sometimes used to treat conditions other than depression, such as Parkinson's disease.
The Mayo Clinic explains MAIOs can ease depression by impacting chemical messengers -neurotransmitters - used to communicate between brain cells. Similar to other types of antidepressants, MAOIs work by causing changes in brain chemistry and how brain cells communicate in ways which regulate mood, to help relieve depression. An enzyme called monoamine oxidase is involved in removing the neurotransmitters norepinephrine, serotonin and dopamine from the brain. MAOIs prevent this from happening, which makes more of these brain chemicals available to effect changes in both cells and circuits impacted by depression.
Examples include (brand name italicized):
- Isocarboxazid (Marplan)
- Phenelzine (Nardil)
- Selegiline (Emsam)
- Tranylcypromine (Parnate)
- Dry mouth
- Nausea, diarrhea or constipation
- Dizziness or lightheadedness
- Skin reaction at the patch site
- Involuntary muscle jerks
- Low blood pressure
- Reduced sexual desire or difficulty reaching orgasm
- Weight gain
- Difficulty starting a urine flow
- Muscle cramps
- Prickling or tingling sensation in the skin (paresthesia)
 Fiedorowicz, J. G., & Swartz, K. L. (2004). The Role of Monoamine Oxidase Inhibitors in Current Psychiatric Practice. Journal of Psychiatric Practice, 10(4), 239–248.
 Mayo Clinic Staff. “Monoamine Oxidase Inhibitors (MAOIs).” Mayo Clinic, Mayo Foundation for Medical Education and Research, 8 June 2016, www.mayoclinic.org/diseases-conditions/depression/in-depth/maois/art-20043992.