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...

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

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 Fenfluramine was withdrawn from the U.S. market in 1997 after reports of valvular heart disease and pulmonary hypertension.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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