8 Common Questions About Phentermine for Weight Loss

Dr. Sharon Orrange
Dr. Orrange is an Associate Professor of Clinical Medicine in the Division of Geriatric, Hospitalist and General Internal Medicine at the Keck School of Medicine of USC.
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Phentermine (Adipex-P), an amphetamine derivative, is the most prescribed weight loss medication in the United States. Available as a low cost generic, phentermine has been approved for weight loss since 1973.

Here are the great questions I hear from my patients over and over:

  1. How does phentermine work? Phentermine increases release of neurotransmitters which simulate metabolism and suppress appetite. Phentermine is a “sympathomimetic” because it simulates the release and inhibition of certain neurotransmitters in your brain. Essentially it tricks your mind into feeling full!
  2. Release of which neurotransmitters? A large body of evidence has observed that phentermine works by causing the release of two neurotransmitters, dopamine and norepinephrine, in your brain to suppress hunger.
  3. Wait, wasn’t this the scary fen-phen drug? Kind of. Here is the story: fen-phen was a mixture of fenfluramine (the “fen”) and phentermine (the “phen”). The fen-phen combination was very popular in 1996, with 6.6 million prescriptions dispensed in the US. Soon after, reports of heart valve disease and primary pulmonary hypertension were reported, caused by fenfluramine (the “fen” part of the drug). In September 1997, the FDA recommended that fenfluramine be withdrawn and that patients should immediately stop taking it. So don’t worry, phentermine was not part of these warnings and has not been shown to contribute to heart valve disease or any lung problems.
  4. What is the difference between phentermine, cocaine, and amphetamines? A lot. While it is true that they are all considered sympathomimetics, the addictive effects of cocaine and methamphetamines do not exist with phentermine. This is because cocaine and methamphetamines are much more potent releasers of norepinephrine and dopamine, which cause users to experience the addictive effects. 
  5. Who should not take phentermine? Phentermine is not a good idea in patients with coronary heart disease, uncontrolled high blood pressure, hyperthyroidism, or in patients with a history of drug abuse.
  6. Why is phentermine a “scheduled” drug like Xanax, Ativan or Ambien? Phentermine is a schedule IV drug, which suggests the potential for abuse, although the actual potential is low. If you are concerned, talk to your doctor about the potential for addictive effects.
  7. How much weight will I loose with phentermine? Phentermine 37.5mg daily for 12 weeks leads to an average weight loss of 15.8 pounds (7.2 kgs).
  8. Will phentermine give me side effects? Dry mouth is the one i hear most often from my patients, and it doesn’t seem to bother them. Phentermine can also cause increased head rate, blood pressure, insomnia, constipation and nervousness. So keep an eye on your blood pressure if you do decide to start on it.

What has your experience been?

Dr. O

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