Monday Pharmaceutical Mystery: November 25

Why is a woman being prescribed phentermine for ADHD?

After filling a new prescription for a 32-year-old female patient, GS, for phentermine, you meet her at the consultation window to go over the medication. When you mention that this drug is used for weight loss, she replies, “Oh, I’m not taking it for that. I’m taking it for ADHD (Attention-Deficit/Hyperactivity Disorder).” You are stumped, and think she must be confused.

Mystery: Why is GS being prescribed phentermine for ADHD?

Solution: Phentermine is a sympathomimetic amine (an amphetamine-like analogue) that has CNS stimulant and appetite suppressant effects.1 The drug increases norepinephrine levels, and increases serotonin effects. Phentermine seems to cause less tachycardia, elevated blood pressure, insomnia, and restlessness than amphetamines.2

Considering its similarity to amphetamines, phentermine has anecdotally been used off label to treat ADHD. It is possible that increasing norepinephrine concentrations in the brain could help improve function and symptoms, such as improved attention, reduced impulsivity, in patients with ADHD.3

Phentermine may also increase dopamine, leading to improved attentiveness, memory, and motivation.3


  • The US National Library of Medicine: phentermine Accessed November 22, 2019.
  • Isidro, M. Cordido, F. Approved and Off-Label Uses of Obesity Medications, and Potential New Pharmacologic Treatment Options Accessed November 22, 2019.
  • Mental Health Daily. Phentermine For ADHD: An Uninvestigated Treatment Accessed November 22, 2019.