Phentermine and phentermine hydrochloride are the technically correct names for a very popular type of prescription diet pill, one that has been around since the late 1950s. Medical research studies of this drug show that it is both safe and effective when used according to a doctor's instructions. For most people, the drug significantly increases weight loss as compared to dieting alone.
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How Phentermine Enhances Weight Loss
According to the National Library of Medicine, phentermine's primary mode of action is that of a sympathomimetic amine, meaning that it increases the body's release of catecholamines, specifically, epinephrine and norepinephrine, and possibly dopamine, as well. These catecholamines cause a decrease in appetite and an increase in energy expenditure by their influence on a variety of neurotransmitters and neuropeptides. Quite simply, when you are taking phentermine, you tend to feel less hungry but more energetic and alert. Increased energy expenditure will burn more calories. Weight loss results from a combination of these influences.
Monthly Weight Loss
How much weight you will lose varies from person to person and depends on a number of factors: the diet that you follow, the drug dosage taken, your starting weight, your gender and any number of other genetic and environmental variables. That said, existing research studies can give you a realistic estimate of what to expect from this medication.
British Medical Journal Study
The British Medical Journal published the results of a double-blind trial of phentermine that was conducted in 1968. This study included three groups of 36 women each, two of which were given phentermine to augment their weight loss efforts. One group was given 15 mg per day continuously, and the other was given the same dose but only every other month. Both were put on a 1,000-calorie daily diet. The continuous group lost a total of 27 lbs. over the period, and the intermittent group lost slightly more at 28.7 lbs. Although weight loss is not necessarily consistent from month to month, the average monthly weight loss in this study was 3.2 lbs. for the first group, and 3.4 lbs. for the second.
Korean Phentermine Study
A more recent trial of phentermine's weight loss effects was conducted in 2005 in Korea. This time, the standard dosage was 37.5 mg of phentermine HCL per day, and the drug was tested on a total of 34 people, both men and women. These subjects were counseled to follow a 1,500 calories per day diet. After 14 weeks of study, the drug treatment group lost 16.5 lbs., which works out to about 5 lbs. per month.
UCLA Very Low Calorie Diet Study
In a 2003 study from UCLA, data on 188 male and female phentermine users was collected. These dieters consumed a very low calorie diet of only 500 to 800 calories per day, and were given phentermine at doses ranging from 8 mg to 30 mg per day. They were also told to exercise about 45 minutes a day, three times a week. After 12 weeks, the women had lost an average of 17.6 lbs., and the men, 21.3 lbs., for an average monthly rate of 6.3 and 7.6 lbs., respectively.
In all three studies, the rate of weight loss was highest in the early months, then tapered off, which is a pattern common to almost all weight loss methods. This explains why the longest study, the British Medical Journal one, shows the lowest rate of loss. It wasn't that people actually lost more slowly in this study, but rather, that their weight loss significantly slowed after the first three or four months. However, unlike the placebo users in the study, the phentermine users maintained their weight loss during the last three to four months.
Based on these three studies, you can expect to lose roughly 3 to 6 lbs. per month, if you are a woman, and 5 to 8 lbs. per month, if you are a man, using a combination of phentermine and a low-calorie diet.