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Phentermine

Phentermine is a phenethylamine primary used as an appetite suppressant. It is typically prescribed for individuals who are at increased medical risk because of their weight, as opposed to cosmetic weight loss. more...

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Phentermine is sold either as an immediate-release formulation (Adipex®) or as a slow-release resin (Ionamin®, Australia:Duromine®).

History

Phentermine is one of two drugs in the Fen-phen anti-obesity medication, the other being fenfluramine or dexfenfluramine. Fenfluramine was withdrawn from the U.S. market in 1997 after reports of valvular heart disease and pulmonary hypertension.

Phentermine is still available by itself in most countries, including the U.S. However, because it is similar to the amphetamines, individuals may develop an addiction to it. Hence, it is classified as a controlled substance in many countries. Internationally, phentermine is a schedule IV drug under the Convention on Psychotropic Substances (PDF file). In the United States, it is classified as a Schedule IV controlled substance under the Controlled Substances Act.

Mechanism of action

Phentermine, as many other prescription drugs, works with neurotransmitters in the brain. It is a centrally-acting stimulant chemically related to the amphetamines. It stimulates neuron bundles to release a particular group of neurotransmitters known as catecholamines; these include dopamine, epinephrine (also known as adrenalin), and norepinephrine (noradrenaline). This is the same mechanism of action as other stimulant appetite suppresants such as sibutramine, diethylpropion, and dextroamphetamine.

The neurotransmitters signal a fight-or-flight response in the body which, in turn, puts a halt to the hunger signal. As a result, it causes a loss in appetite because the brain does not receive the hunger message.

Clinical use

Generally, it is recommended by the FDA that phentermine should be used short-term (usually interpreted as 'up to 12 weeks'), while following nonpharmacological approaches to weight loss such as healthy dieting and exercise. However, recommendations limiting its use for short-term treatment may be controversial. One reason given behind limiting its use to 12 weeks is drug tolerance, whereby phentermine loses its appetite-suppressing effects after the body adjusts to the drug. On the contrary, it has been shown that phentermine did not lose effectiveness in a 36-week trial (PMID 11054601). However, the risk of drug addiction may be a significant reason in limiting phentermine for short-term use.

Due to the risk of insomnia, it is generally recommended that the drug be taken either before breakfast or 1-2 hours after breakfast.

Side effects

Because phentermine acts through sympathomimetic pathways, the drug may increase blood pressure and heart rate. It may also cause palpitations, restlessness, and insomnia. Additionally, individuals taking this drug on a long-term basis may develop euphoria and a psychological addiction to it. Heart valve damage and pulmonary hypertension, severe enough to cause permanent disability or death have been seen with phentermine alone.

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Mixing diet drugs calls for caution - combination of fenfluramine and phentermine may cause heart valve disease in women - Brief Article
From FDA Consumer, 9/1/97

Reports of heart valve disease in women taking the obesity drugs fenfluramine and phentermine together have prompted FDA to advise doctors to closely monitor patients on this therapy.

In a letter to doctors, FDA noted that as of July 8 it had received reports of 33 cases of unusual abnormalities in heart valves in women ages 30 to 72 taking both drugs together for 1 to 28 months. The course of the disease when the drugs are stopped is unknown.

There is no conclusive evidence of a cause-effect relationship between use of the drugs and development of this disease. However, because of the seriousness of the heart problems and their rarity in otherwise healthy obese women, FDA believes patients and health professionals need this information. The agency will continue monitoring adverse events reports.

FDA approved each drug for use alone and short term, not in combination or for long term. In accordance with approved labeling, only obese patients should take these drugs, and they should also follow a weight-loss regimen that includes a reduced-calorie diet and an exercise program. (See "Losing Weight Safely" in the January-February 1996 FDA Consumer, "New Weight-Loss Drug" in the Updates section of the July-August 1996 issue, and "Redux Relabeling Notes Higher Risk" in the Updates of the November 1996 issue.)

Health professionals should give patients on this therapy thorough heart evaluations and, if heart disease develops, pursue further such evaluation. They also should report any heart valve disease or other serious problems associated with the use of phentermine. fenfluramine, and also dexfenfluramine to FDA's MedWatch program.

Editor's Note: Herbal "fen-phen" is not similar to the prescription drug combination of fenfluramine and phentermine. Herbal fen-phen is marketed as a combination of herbals and, like all products sold as dietary supplements, can be marketed without FDA review of safety or effectiveness, in accordance with the Dietary Supplement Health and Education Act of 1994. For further information on a specific dietary supplement product, contact the manufacturer. Check with your health-care professional about use of the product.

COPYRIGHT 1997 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group

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