Phentermine

As a stimulant it raises heart rate and blood pressure and can cause insomnia, dry mouth, jitteriness, and, with misuse, dependence; it is meant for short-term use only. It must not be combined with MAOIs (risk of a hypertensive crisis) and is not for people with significant heart disease, uncontrolled hypertension, or hyperthyroidism. Not medical advice.

Phentermine is a weak inhibitor of human monoamine oxidase A with a Ki of 498 microM, approximately 6-fold weaker than the rat enzyme and substantially weaker than classic MAO inhibitors like moclobemide ^[4]. In rat studies, phentermine showed MAO-A inhibition potency of 85-88 microM, placing it in the range of reversible MAO inhibitors, and when combined with other reversible MAO inhibitors such as pseudoephedrine or ephedrine, the degree of MAO inhibition was additive, though clinical relevance remains unclear ^[5]. Human enzyme kinetics indicate that phentermine's inhibition of both MAO-A and MAO-B is too weak to be of pharmacological importance at therapeutic doses ^[4]. Because phentermine is a sympathomimetic amine structurally related to amphetamine and methamphetamine, it can theoretically potentiate serotonergic agents; however, studies examining this interaction in humans are largely absent from the literature.

Phentermine's interactions with serotonin reuptake inhibitors remain inadequately studied despite historical concerns from the fen-phen era. The interaction with classic MAO inhibitors has never been formally documented, leaving a significant gap in the clinical evidence base.

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