Weight Loss Drugs Prescription and OTC »
Taking a weight loss drug may not result in weight loss by itself. But using diet pills can help an overweight person stay on a diet because nearly all of these medications work on suppressing appetite. "Feeling full" is believed to be related to a number of biochemical processes in the body. Signals to indicate fullness come from fat cells and the gastrointestinal tract; these converge with signals in the central nervous system. Appetite suppressants target a couple of key neurotransmitters in this process: serotonin and norepinephrine. Increased levels of serotonin result in a feeling of fullness. Increasing norepinephrine levels stimulate the central nervous system, decreasing appetite. Only one drug among the weight loss medications works in a different way. Orlistat (Xenical, Alli) works in the gastrointestinal tract to prevent absorption of about a third of ingested fat.
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Please Note: This Brand Name drug is no longer available in the US.Phentermine hydrochloride is a sympathomimetic amine with pharmacologic activity similar to the prototype drugs of this class used in obesity, the amphetamines. Actions include central nervous system stimulation and elevation of blood pressure. Tachyphylaxis and tolerance have been demonstrated with all drugs of this class in which these phenomena have been looked for.
Drugs of this class used in obesity are commonly known as "anorectics" or "anorexigenics." It has not been established that the action of such drugs in treating obesity is primarily one of appetite suppression. Other central nervous system actions, or metabolic effects may be involved, for example.
Adult obese subjects instructed in dietary management and treated with "anorectic" drugs, lose more weight on the average than those treated with placebo and diet, as determined in relatively short-term clinical trials.
The magnitude of increased weight loss of drug-treated patients over placebo-treated patients is only a fraction of a pound a week. The rate of weight loss is greatest in the first weeks of therapy for both drug and placebo subjects and tends to decrease in succeeding weeks. The possible origins of the increased weight loss due to the various drug effects are not established. The amount of weight loss associated with the use of an "anorectic" drug varies from trial to trial, and the increased weight loss appears to be related in proof to variables other than the drugs prescribed, such as the physician-investigator, the population treated, and the diet prescribed. Studies do not permit conclusions as to the relative importance of the drug and non-drug factors on weight loss.
The natural history of obesity is measured in years, whereas the studies cited are restricted to a few weeks duration; thus, the total impact of drug-induced weight loss over that of diet alone must be considered clinically limited.
Last reviewed on RxList: 12/8/2004
This monograph has been modified to include the generic and brand name in many instances.
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