I've mostly heard of them in the context of treatment for depression (in a lower-division biopsych course, not personal experience). My understanding was that they'd be taken on their own, and the Wiki article doesn't obviously disagree.

http://en.wikipedia.org/wiki/Monoamine_oxidase_inhibitor

"Monoamine oxidase inhibitors (MAOIs) are chemicals which inhibit the activity of the monoamine oxidase enzyme family. They have a long history of use as medications prescribed for the treatment of depression. They are particularly effective in treating atypical depression."

Though there is admittedly a "citation needed".