The MAO Inhibitors can be further categorized based on their reversibility and their selectivity.
Selectivity
There are two MAO enzymes that are involved when people discuss MAO inhibitors. Like LDL and HDL cholesterols, they have been somewhat inappropriately labeled, "good" and "bad" MAO enzymes. The first enzyme, MAO-A, is known as the "good" one. The inhibition of this enzyme is responsible for some of the desirable anti-depressant activity as it allows a greater amount of neurotransmitters, specifically norepinephrine, serotonin and dopamine, to accumulate unmolested at the receptor sites in the synaptic cleft. The second enzyme, MAO-B, is known as the "bad" one. This enzyme degrades only dopamine from the neurotransmitter group. Selectivity refers to the ability of an MAO inhibitor to target a specific MAO enzyme while not disturbing the other. The majority of the current MAO inhibitors non-selectively target both "good" and "bad" MAO enzymes. Both MAO enzymes disable tyramine, so selectively inhibiting one of the two (usually MAO-A) leaves one to manage tyramine and greatly reduces the danger associated with traditional non-selective MAO inhibitors. From this, it should be clear that "good" and "bad" are misnomers and only serve as arbitrary labels.
Reversibility
The reversibility of the MAO inhibitors is based upon the mechanism by which the MAO inhibitors act on the MAO enzymes. Non-reversible MAO inhibitors form bonds with the MAO enzymes that are very difficult to sever. For all intents and purposes, once the MAO inhibitor has bonded to the MAO enzyme, it is out of the game for the duration. Further synthesis of MAO enzymes is required to rebuild the MAO enzyme population. The reversible MAO inhibitors can be displaced from the MAO enzymes by tyramine so that the danger of the so-called "cheese effect" (the heightened risk from tyramine which is often found in aged cheeses, among other foods) is significantly reduced.
It should be noted that not all cheeses contain tyramine in levels that should cause concern. Cream cheese, ricotta and generally any processed cheese or unaged cheese is "safe" when using MAO inhibitors of any type. Also, despite common misconception, bananas are relatively safe, whereas banana peels are not as the peels contain significant amounts of tyramine.
A selection of currently available MAO inhibitors:
WARNING - Regarding the dietary restrictions associated with taking MAO inhibitors, consult your physician or psychiatrist if you have one. If you are in it for recreational purposes then you've already decided to fly solo and should be as thoroughly educated on the topic as possible.
1. The reversible MAO inhibitors, aka. RMAIs, are currently available in Europe and Canada but not in the United States.