The MAOIs were actually the first drugs used to treat depression. They work by blocking the breakdown of a number of neurotransmitters involved in depression via an enzyme, monoamine oxidase, located inside specific neurons. The result is more of the needed chemicals become available for the cells to transmit to other cells and normalize chemical function in the brain. Unfortunately, the MAOIs also block the breakdown of an important amino acid called tyramine. If people do not severely restrict their tyramine intake while on an MAOI, tyramine rapidly accumulates in the body, leading to a sharp increase in blood pressure which can be fatal. Used too soon after certain other antidepressant medications are stopped, the MAOIs can cause a serious toxic condition, serotonin syndrome, leading to confusion, agitation and other symptoms. MAOIs are not considered first choice for the majority of depressed people but they are useful in people who have both depression and anxiety, for those who eat and sleep a lot because they are depressed and for those who have not responded well to other antidepressants.
Currently available MAOIs available in Canada to treat depression include phenelzine (Nardil) and tranylcypromine (Parnate).Isocarboxazid (Marplan) is available in the US as a third option.