MAO inhibitors

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Letter to the editor

Editor, – Dietary restrictions are needed for combination therapy with selegiline and moclobemide.

Professor D. Story's article (Aust Prescr 1993;16:54-7) on selective monoamine oxidase (MAO) inhibitors states that the reversible (selective) MAO-A inhibitors are less likely than older MAO inhibitors to cause dangerous adverse effects arising from their interaction with sympathomimetic amines. MAO-B inhibitors, despite MAO-B predominance in the liver, do not appear to be associated with the interaction with indirectly-acting sympathomimetics.

While there is no need for a low tyramine diet with these drugs when used separately in recommended doses, dietary tyramine can cause a hypertensive reaction when MAO-A and B inhibitors are prescribed concurrently. The mechanism for this observed interaction is not understood.

Patients being treated with both moclobemide (for depression) and selegiline (for Parkinson's disease) should be on a low tyramine diet. The patient should also be aware of the potential for drug interactions as with irreversible, non-selective MAO inhibitors.

J.W.G. Tiller
Department of Psychiatry
University of Melbourne
Melbourne, Vic.

Editor, – A good review of the clinical usefulness of MAO inhibitors, such as that of David Story (Aust Prescr 1993;16:54-7) in relation to the new reversible drugs, is always welcome; especially since so much of what has been written in articles and textbooks in the past has been misleading or unhelpful.

I would like to add one or two small comments. I think it is preferable for the word 'dangerous' to be avoided when talking about the problems, especially the hypertensive reaction to tyramine, caused by MAO inhibitors. I did a calculation some years ago that suggested NSAIDs produced a much greater risk of morbidity and mortality than MAO inhibitors. If one takes into account the fact that NSAIDs are not given for a life-threatening condition, then one can see everything in amore objective and useful perspective. Although the cheese reaction is much talked about, the actual mortality relative to the high risk of suicide in severely ill, depressed patients is extremely small. If one considers the risk benefit ratio of other drugs, it would be logical to extend the use of the word 'dangerous', with an appropriate qualifying superlative, to many other drugs prescribed on a daily basis. This would be highly undesirable, if only from a medico-legal point of view.

The selectivity of these drugs is relative. Moclobemide (300-600 mg daily) gives a 30% inhibition of MAO-B. The extent to which its various active metabolites are selective is difficult to ascertain from the current literature. Selectivity is further diminished when higher doses are used. Indeed, the whole concept of selectivity may be quixotic.

P.K. Gillman
Mackay, Qld