phentermine

phentermine, drug used in the treatment of obesity. Phentermine is the most commonly prescribed drug for weight loss, and it is the oldest such drug still in use, having been first made available by prescription in 1959. Phentermine is available in capsule or tablet form and is used only as a short-term treatment, in conjunction with a healthy eating plan and exercise, to help patients lose weight. It is known by several trade names, among them Adipex-P, Duromine, Lomaira, and Suprenza. An anti-obesity treatment marketed as Qsymia consists of a combination of phentermine and topiramate, a drug used in the treatment of epilepsy and migraine.

Mechanism of action

Phentermine is classified as a sympathomimetic amine. It activates the sympathetic nervous system in a manner similar to naturally occurring catecholamine substances in the body, and, similar to those substances, its chemical structure contains an amine (a nitrogen-containing compound), the presence of which influences drug activity. Catecholamines include the neurotransmitters dopamine, epinephrine, and norepinephrine. Although the precise mechanism of action of phentermine remains unclear, the drug appears to stimulate the sympathetic nervous system by prompting the release of catecholamines to varying degrees. The drug also appears to prevent the reuptake of catecholamines by neurons, thereby extending their activity, and may affect the release and reuptake of the neurotransmitter serotonin.

The most notable actions associated with phentermine involve norepinephrine. Norepinephrine activates the fight-or-flight response, which is marked by changes in activity in the nervous system and the endocrine system that prepare an individual to react or retreat in response to a threat. Among the effects of these changes are suppression of appetite and increased breakdown of stored fat for energy. Via these processes, phentermine helps reduce sensations of hunger and promotes fat use, leading to weight loss. The amount of weight that is lost varies by individual, and most weight loss occurs within the first weeks of treatment, with many patients losing about 3 percent of body weight by the end of three months.

Side effects and drug interactions

Side effects of phentermine are largely associated with its stimulatory actions on the nervous system and may include dizziness, headache, high blood pressure, insomnia, irregular heart rate, restlessness, and tremor. Gastrointestinal disturbances are common; examples include constipation, diarrhea, and vomiting. Some patients also experience changes in libido or impotence.

Phentermine is usually taken as a monotherapy (single-drug therapy), owing to its high likelihood for drug-drug interactions, wherein phentermine reduces the effectiveness of certain drugs, other drugs reduce the effectiveness of phentermine, or the drugs interact in ways that significantly increase the risk for severe and potentially fatal reactions. Notable interactions include those that can occur between phentermine and monoamine oxidase inhibitors (MAOIs) to induce a hypertensive crisis, in which blood pressure escalates to dangerously high levels. The combination of fenfluramine-phentermine (fen-phen), an anti-obesity treatment that was popular in the 1990s, causes potentially fatal cardiovascular disease; owing to this risk, the combination was withdrawn from the market in 1997.