Phentermine hydrochloride is a sympathomimetic amine with pharmacologic activity like that of the prototype drugs of this category used in obesity -- the amphetamines. Behaviors include central nervous system stimulation and increase of blood pressure. Tachyphylaxis and tolerance have been demonstrated with all drugs of this class in which these phenomena have been sought out.

Drugs of this class used to treat obesity are commonly known as "anorectics" or "anorexigenics." It has not been established that the action of such drugs in treating obesity is mainly one of appetite suppression. Other central nervous system actions, or metabolic effects may be involved, for example.

Adult overweight subjects instructed in dietary management and treated with "anorectic" drugs, lose more weight on average than subjects who are treated with placebo and diet, as evidenced in relatively short-term clinical trials.

The degree 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 highest 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 because of the various drug effects are not established. The amount of weight loss because of 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 allow 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 complete impact of drug-induced weight loss over that of diet alone must be considered clinically limited.