Which vasoconstrictor is contraindicated with concomitant MAO inhibitor therapy?

The vasoconstrictor that is contraindicated with concomitant MAO inhibitor therapy is phenylephrine. This is because MAO inhibitors can prevent the breakdown of certain amines, including phenylephrine, leading to significantly increased levels of the drug in the body. This heightened level can cause severe hypertensive crises due to excessive vasoconstriction.

When understanding the pharmacological interactions, it’s essential to recognize that MAO inhibitors affect the metabolism of catecholamines and other biogenic amines. Other agents like norepinephrine, epinephrine, and levonordefrin can also be affected by MAO inhibitors; however, the specific contraindication with phenylephrine is particularly notable due to its potency and the risk of acute hypertensive reactions if used together with MAO inhibitors.

In summary, the contraindication for phenylephrine in the context of MAO inhibitor therapy stems from the risk of markedly increased blood pressure due to impaired breakdown, which is crucial knowledge for safe patient management.