MAOIs are antidepressants which are effictatious in the treatment of atypical depression (vegitative symptoms, anxiety, initial insomnia, etc), panic disorder, anxiety and (recently discovered) borderline personality disorder.

MAO metabolizes serotonin, norepinephrine and dopamine. By inhibiting this, MAOIs increase levels of those neurotransmitters. Phenelzine (Nardil) and tranycypromine (Parnate) are the two popular MAOIs. moclobemide (Aurorex -- called something else in canada) is a recently introduced MAOI which is slightly different and apparently not as effictatious, although it eliminates the hazards associated with the earlier MAOIs.

Tyramine is an amino acid which is normally metabolized by MAO in the gut. After taking tranylcypromine or phenelzine (but not, apparently, moclobemide) this MAO is inhibited and any dietary Tyramine will not be metabolized and will cause an increase in tyramine levels in blood. This is bad due to tyramine's effect on norepinephrine and leads to a hypertensive crisis. Blood pressure goes up to 180+ over something, heartbeat goes to well over 100 bpm, and people who suffer one of these typically report getting the most painful headache of their lives. Sometimes a Ca-blocker like nifedipine is prescribed for the user of the MAOI to take to combat this effect should it occur.