Phentermine

Phentermine is a phenethylamine, more importantly a derivative of amphetamine, primarily 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. 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 Fenfluramine was withdrawn from the U.S. market in 1997 after reports of valvular heart disease and pulmonary hypertension.

Mechanism of action

Phentermine, like many other prescription drugs, works with neurotransmitters in the brain. It is a centrally-acting stimulant. 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). The anorectic activity seen with these compounds would thus seem likely due to this effect on the central nervous system, which is consistent with current knowledge about central nervous system systems and feeding behaviour. This is the same mechanism of action as other stimulant appetite suppressants such as diethylpropion, and phendimetrazine.

The neurotransmitters signal 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 Food and Drug Administration (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). Due to the risk of insomnia, it is generally recommended that the drug be taken either before breakfast or 1-2 hours after breakfast.

Contraindications

Phentermine should generally be avoided in patients with:

Allergy/Hypersensitivity to other sympathomimetic amines
Atherosclerosis
Cardiovascular disease
Glaucoma
High blood pressure
Hyperthyroidism
History of drug abuse
Additionally, this drug should not be used at the same time or within 14 days following the use of monoamine oxidase inhibitors.

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