Mechanism of Action of Phentermine
Phentermine works by increasing norepinephrine levels in the central nervous system, which reduces appetite through stimulation of the hypothalamus. 1, 2
Primary Mechanism
Phentermine is a sympathomimetic amine that functions as a noradrenergic drug with two key actions:
- Enhancement of norepinephrine release in the CNS
- Blockade of norepinephrine reuptake at nerve terminals
This dual action increases norepinephrine concentrations in brain regions controlling appetite, particularly the hypothalamus, leading to appetite suppression 3, 1.
Pharmacologic Classification
Phentermine has pharmacologic activity similar to amphetamines (d- and dl-amphetamine), though it is not optically active like amphetamine 4. The FDA label explicitly states that while phentermine is classified as an "anorectic" or "anorexigenic" drug, it has not been definitively established that appetite suppression is the sole mechanism, as other CNS actions or metabolic effects may contribute to its weight loss effects 2.
Neurotransmitter Specificity
Research demonstrates phentermine's selectivity for norepinephrine over other monoamines:
- Preferentially increases extracellular dopamine in the nucleus accumbens (to a lesser degree than norepinephrine) 5, 6
- Has negligible activity at the serotonin transporter 4
- Does not significantly affect serotonin (5-HT) release, unlike fenfluramine 5, 6
This noradrenergic selectivity distinguishes phentermine from fenfluramine (which increases serotonin) and explains why the cardiovascular toxicity seen with the fen-phen combination was attributed to fenfluramine's serotonergic effects on cardiac tissue, not phentermine 1.
Additional Pharmacodynamic Effects
Beyond appetite suppression, phentermine produces typical amphetamine-like effects including:
Tachyphylaxis and tolerance have been demonstrated with phentermine, as with all drugs in this class 2.
Clinical Implication
The mechanism explains why phentermine should be taken early in the day (to minimize insomnia from CNS stimulation) and why it's contraindicated with MAO inhibitors (risk of hypertensive crisis from excessive norepinephrine) 1.