Phentermine is a sympathetic amine, which means it is a stimulant very similar to amphetamine. Phentermine has one purpose, and that is as a short term pharmacologic (drug) therapy for weight loss in obese patients. Although approved for only one indication, the effects of phentermine are felt across multiple body systems. Phentermine is not to be used with other medications due to potential dangerous drug interactions.
Hypertension
Phentermine causes vasoconstriction, or tightening and narrowing of blood vessels. This causes an elevation in blood pressure. The National Institutes of Health advises that phentermine should be used with caution in any individuals with a history of hypertension, including mild hypertension. A patient taking phentermine should check his blood pressure daily to monitor for elevation.
Arrhythmias
The effects of phentermine include cardiac stimulation. The heart muscle becomes increasingly excitatory, which means it is more easily stimulated to beat. Billie Ann Wilson, Ph.D., Margaret Shannon, Ph.D., and Kelly Shields, Pharm.D., authors of "Pearson Nurse's Drug Guide 2010," explain how this can lead to arrhythmias or irregular heart rates and rhythms. For example, phentermine may cause a rapid heart rate, called tachycardia. If the tachycardia is prolonged, it can cause chest pain, shortness of breath and syncope, or fainting. Patients may also experience palpitations, or strong forceful beats that can be felt in the chest or throat. Phentermine can also cause more serious arrhythmias like premature ventricular contractions (extra beats from the ventricle) or ventricular tachycardia; only the ventricles (lower chambers of the heart) are beating, and they are beating very rapidly. Blood pressure cannot be sustained with this. A patient taking phentermine should have a baseline cardiac assessment completed before taking the medication, and she should also be evaluated throughout therapy.
Anxiety
The stimulation effects of phentermine also affect the central nervous system. Patients taking phentermine may experience extreme anxiety because of over-stimulation. Nervousness, restlessness and insomnia are all symptoms of anxiety that are often reported by patients taking phentermine.
Appetite Suppression
The NIH reports that the appetite suppression effects of phentermine are greatest during the initial two weeks of therapy. The exact reason for this, or even for how phentermine suppresses appetite, is unclear. Phentermine does cause altered taste sensations, constipation and dry mouth. These symptoms, along with anxiety and an increased metabolic consumption rate, may lead to loss of appetite and weight loss.
References
- National Institute of Health Daily Med Sheet: "Phentermine"
- "Pearson Nurse's Drug Guide 2010"; Wilson, B. A., Shannon, T. M., & Shields, K. M.; 2010
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