General Information: Phentermine is an oral sympathomimetic amine used as an adjunct for short-term (e.g., 8—12 weeks) treatment of exogenous obesity. The pharmacologic effects of phentermine are similar to amphetamines. Phentermine resin complex was approved by the FDA in 1959, but is no longer marketed in the US. Phentermine hydrochloride was FDA approved in 1973. In the mid-90s, there was renewed interest in phentermine in combination with another anorectic, fenfluramine, for the treatment of obesity and substance abuse, however, little scientific data support this practice. On July 8, 1997, the FDA issued a ‘Dear Health Care Professional’ letter warning physicians about the development of valvular heart disease and pulmonary hypertension in women receiving the combination of fenfluramine and phentermine; fenfluramine was subsequently withdrawn from the US market in fall of 1997. Use of phentermine with other anorectic agents for obesity has not been evaluated and is not recommended. In May 2011, the FDA approved a phentermine hydrochloride orally disintegrating tablet (Suprenza) for the treatment of exogenous obesity.
Mechanism of Action: Limited data are available in reference texts regarding the mechanism of action of this drug. Phentermine is an analog of methamphetamine. Similar to the amphetamines, phentermine increases the release of norepinephrine and dopamine from nerve terminals and inhibits their reuptake. Thus, phentermine is classified as an indirect sympathomimetic. Other effects include a weak ability to dose-dependently raise serotonin levels, although the effect on serotonin occurs is less potent than that of methamphetamine itself. Clinical effects include CNS stimulation and elevation of blood pressure. Appetite suppression is believed to occur through direct stimulation of the satiety center in the hypothalamic and limbic region.
Tolerance to the anorexiant effects of phentermine usually develops within a few weeks of starting therapy. The mechanism of tolerance appears to be pharmacodynamic in nature; higher doses of phentermine are required to produce the same response. When tolerance develops to the anorexiant effects, it is generally recommended that phentermine be discontinued rather than the dose increased.