Like most antidepressants, MAOIs work by changing the levels of one or more of these naturally occurring brain chemicals. 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.This is thought to boost mood by improving brain cell communication. 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.ide effects of MAOIs. Because of side effects and safety concerns, MAOIs are most often tried when other antidepressants don't work. The most common side effects of MAOIs include: 1  Dry mouth. 2  Nausea, diarrhea or constipation. 3  Headache.