Will Phentermine Help You Lose Weight?

Will Phentermine help you lose weight

Phentermine is the number one weight loss medication in the USA. It is a prescription drug similar to amphetamines (known as “speed” on the street), which is used for weight loss in obese patients. Phentermine is labeled under the following brand names:

  • Adipex-P®
  • Anoxine-AM®
  • Fastin®
  • Ionamin®
  • Obephen®
  • Obermine®
  • Obestin-30®
  • Phentrol®

Phentermine claims to help you lose weight by acting as an appetite suppressant.

How It Works

Phentermine works by stimulating the hypothalamus and affecting certain neurotransmitters that decrease your appetite such as serotonin.
At the same time, it is a stimulant drug and puts your body in fight or flight mode, (as if you were running from a wild tiger.) It stimulates your nervous system and increases your heart rate, blood pressure and can cause insomnia. 

Fen-Phen Fiasco

Phentermine received FDA approval in 1959. In 1992, Dr. Michael Weintraub of the University of Rochester published a study showing Fen-Phen as far more effective than dieting or exercise in reducing the weight of the chronically obese.

Fen-Phen is the combination of Fenfluramine ("Fen") and Phentermine ("Phen"). Fenfluramine and Phentermine are prescription medications used as appetite suppressants for the short-term (a few weeks) management of obesity. Together, the two medications produced a powerful diet drug. The FDA had never approved the Fen-Phen combination, but once the agency has approved a drug, doctors may prescribe it at will.

Soon, Fen-Phen was on the market. In just a short time, Fen-Phen became a national sensation, with 6.6 million prescriptions in 1996. Dexfen-Phen - The combination of Redux or Dexfenfluramine - a more refined compound that, like Fen-Phen, affects seratonin levels, but with fewer side effects than Fenfluramine - and Phentermine also became a sensation.

Unfortunately, neither combination was tested for safety. By the summer of 1997, the Food and Drug Administration (FDA) found over 90 cases of heart-valve problems in women who had taken the Fen-Phen combination.
In September 1997, The FDA requested drug manufacturers to voluntarily withdraw Fenfluramine and Dexfenfluramine. The FDA also recommended that patients using either Fenfluramine or Dexfenfluramine stop doing so.  The FDA did NOT, however, request the withdrawal of the third drug (Phentermine) involved in the cocktails.

Scientific Support

There have been very few studies on the effects of phentermine on weight loss and no long-term studies. A 2005 meta-analysis of the pharmacologic treatment of obesity showed that those who took phentermine had an average weight loss at 6 months of 3.6 kg (8 pounds). This is quite low compared to diet and exercise programs which can produce a weight loss of 48 pounds in 6 months.


Common phentermine side effects include:

  • Digestion: Bad taste in mouth, constipation, diarrhea, dry mouth, upset stomach, vomiting
  • Changes in sex drive, impotence
  • Dizziness, headache
  • Exaggerated sense of well being, mood swings, agitation, irritability, nervousness, over-stimulation, restlessness, sleeplessness, difficulty sleeping

Severe side effects (occurs in estimated 2% of the population):

  • Severe allergic reactions (rash, hives, difficulty breathing, tightness in the chest, swelling of the mouth, face, lips, or tongue)
  • Chest pain, fainting, fast heartbeat, pounding in the chest, shortness of breath, swelling of the legs and feet, tremor
  • Bizarre behavior
  • Stroke


Phentermine comes in tablets and extended-release capsules. Phentermine dosage is usually 15 mg - 37.5 mg once per day, before breakfast, or approximately 1-2 hours after breakfast. Also, instead of taking phentermine once a day, some physicians recommend that you take 15 mg - 37.5 mg in divided doses, 1/2 hour before meals.
According to the FDA, it is not recommended to take this drug more than 12 weeks as it can be addictive and its effects can decrease after weeks of continuous use. The FDA also recommends that it be used with exercise, diet, and behavioral modification. Most patients take it for 3-6 weeks.


Average cost is $40/month.


Appetite suppressants do not treat the cause of your weight problem. You should not suppress your appetite because hunger is a signal that your brain needs food. When you restrict your caloric intake with appetite suppressants, you slow your metabolism to a snail’s pace.

The ideal way to eat is to graze--where you eat small meals throughout the day. Grazing keeps your metabolism high and your insulin levels normal. You cannot graze if you are suppressing your appetite. The true cause of weight gain is living an unhealthy lifestyle.

If you are overweight the best thing you can do is join a weight loss program like the one on Enlita.com which helps you make the lifestyle changes you need to make to be thin and also helps you overcome the other factors which also play a role in obesity such as food allergies, toxicity issues, and neurotransmitter deficiencies to name a few.

In addition, stimulant drugs like caffeine and phentermine can increase cortisol levels which can increase abdominal obesity.

About the Author:
Dr. Kendra Pearsall, N.M.D. is a Naturopathic Medical Doctor specializing in natural weight loss and food addiction. She created Enlita.com to help millions of people achieve optimal health, natural weight loss and life success with her free weekly e-newsletter (sign up at the top of this page.)