Phentermine (a,a-dimethylphenethylamine) is the oldest appetite suppressant,
having been approved by the US Food and Drug Administration in 1959.
Phentermine was the first of a chemical class of appetite suppresing
medications that now includes sibutramine (Meridia), diethylpropion (tenuate)
and others.
Phentermine works on a small part of the brain called the hypothalamus and
exerts its effects there by stimulating the release of a neurotransmitter called
The release of norepinephrine in the hypothalamus caused by phentermine
triggers several effects that include:

Appetite suppression
Increased satiety (earlier 'fullness' when eating)
Stimulation of the 'sympathetic' branch of the nervous system

Effectiveness of Phentermine

Phentermine is not only effective but eficacious, however the magnitude of its
effect is not huge. In most clinical studies with the drug, people taking it lost
ten to twenty pounds more than those taking placebo (fake phentermine).
Thus phentermine is far from a cure. It is rather an adjunct, a sometimes
helpful addition, to dietary modification and exercise. In my experience
prescribing phentermine for patients, it is most helpful in getting a patient
started on a long term weight control plan. It can mean the difference, for
some people, between several months of hard work that is followed by limited
weight loss, and a rather more encouraging start.

Duration of Use

Phentermine is approved by the FDA as a "short-term" adjunct to diet and
exercise and therefore the official prescribing guidelines for phentermine
reccomend it be discontinued after three months. This recomendation is based
largely upon the original research on the drug done in the late 1950s and many
physicians believe that it can be safely and effectively used for longer than
this. They argue that since obesity is a chronic illness it deserves chronic
treatment in much the same way that, say, diabetes does. Prescription of
phentermine for longer than three months is perfectly legal but falls into a
category of medication prescription called "off-label" use.

Side Effects of Phentermine:

Phentermine is a stimulant and therefore it is capable of causing insomnia,
irritabilty and anxiety. These side effects are dosage and duration related: the
lower the dose and or the longer a person has taken the drug, the less likely
become these side effects. Phentermine can also cause dry mouth and
constipation. These side effects are usually mild.

Safety of Phentermine

Because phentermine has been on the market for over 50 years, it has been
prescribed for a lot of people with very few serious problems. If you wish to
read a much more detailed discussion on the safety of appetite suppressants,
please click here.
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