QUICKLINKS AND VIEW OPITONS
How Does Phentermine Work?
Monday, August 13, 2012 2:37 pm Email this article
The main effect of phentermine is to reduce hunger, but there is a second way in which the med helps dieters. It causes them to feel more awake at night after dinner, so they are less likely to cheat on their diet. If you read The (Honest) Truth About Dishonesty by Dan Ariely, you know that stress and fatigue are the enemies of conscience and good sense.
Phentermine causes the neurons that make norepinephrine to release more NE. The NE-secreting reticular activating system (RAS) in the brainstem sends fibers to the cerebral cortex. Each morning when you wake up, the RAS showers the cerebral cortex with NE, enabling you to think in a focused way and inhibit unhealthy impulses. The problem for patients with ADD is that their RAS underperforms, leaving them unable to concentrate and more prone to impulsive behavior. This is why fat patients with ADD are such a challenge for bariatricians. Fortunately, phentermine works as well as Ritalin or Adderall. Since phentermine is cheap, long-lasting, less regulated, and (in a dose of 37.5 mg or less) not addictive (Adderall is addictive), phentermine should be the first-line treatment for ADD in general, and should be continued after extra weight is lost. It’s an off-label use, I know, but so what? It works.
At night, everyone effectively has ADD, since over the day the activity of the RAS declines. As evening approaches, serotonin is released making us more emotional. Finally, late at night, the RAS shuts down and acetylcholine rises, making us fall asleep. You see why you should avoid arguments and important decisions at night.
Evolution has ensured that the RAS is reignited by the sudden appearance of a threat. A caveman confronted by a bear had to concentrate on fight-or-flight. In such a fix, NE from the RAS reduced distractions. It inhibited the drive to look for food (accounting for the appetite-reducing effect of phentermine). In addition, NE inhibited the urge to have sex or evacuate the bladder or colon (explaining some side-effects of phentermine--anorgasmia, constipation and trouble urinating). NE increased muscle tone and blood flow (sweating, tremors). Dieters hope the greatest effect of phentermine will be the appetite-reducing effect and not the other effects. It usually works.
NE and phentermine behave one way when experienced acutely, but another way over the long haul. Phentermine has its greatest effect on NE at the beginning. Over time, the response is attenuated as neuronal stores of NE decline a little. Thus phentermine raises pulse and blood pressure in the first week; both return to normal in the second week. I tell patients to start phentermine at the half-dose level for the first week before progressing to the whole dose. Other doctors don’t know to do that, so many of their patients don’t tolerate phentermine. All of mine do.
The side-effects of phentermine diminish over time, but so does the appetite-reducing effect. In the 1998 Eli Lilly observational study in which I participated, phentermine monotherapy was not effective for weight loss after six weeks. Fortunately, I discovered back in 1995, and published, that adding Prozac to phentermine restored and preserved the anorectic effect of phentermine. My next article will deal with how I made that discovery, and I will offer up my best guess–I can only guess–why the combination works.
Articles on the same subject can be found here:
Please feel free to share your comments about this article.
© Copyright 2003-2017 - Michael Anchors, MD, PhD - All Rights Reserved.