Phentermine is a sympathetic amine, which means it is a stimulant very similar to an amphetamine. Phentermine has one purpose -- as a short-term pharmacologic, or drug, therapy for weight loss in obese patients. Although approved for only one indication, the effects of phentermine are felt across multiple body systems. You should not use phentermine with other diet medications. Tell your doctor if you are taking any over-the-counter or prescription medications, vitamins, supplements or herbs before taking phentermine.
Phentermine causes vasoconstriction, or tightening and narrowing of blood vessels. This causes an elevation in blood pressure. If you have a history of hypertension, including mild hypertension, you should use phentermine with care, checking your blood pressure daily to monitor for elevation, advises the National Institutes of Health website.
The effects of phentermine include cardiac stimulation, which is when the heart muscle becomes increasingly excitatory, meaning more easily stimulated to beat. This can lead to arrhythmias, or irregular heart rates and rhythms, as noted in "Pearson Nurse's Drug Guide 2010." Prolonged rapid heart rate, or tachycardia, can cause chest pain, shortness of breath and syncope, or fainting. Phentermine can also cause serious arrhythmias in which only the ventricles, or lower chambers, of the heart beat very rapidly, making it difficult to sustain your blood pressure. Before taking phentermine, you should have a baseline cardiac assessment completed and have your doctor evaluate your heart rate throughout therapy.
The stimulation effects of phentermine also affect the central nervous system. Patients taking phentermine may experience extreme anxiety because of overstimulation. Nervousness, restlessness and insomnia are all symptoms of anxiety that are often reported by patients taking phentermine.
The appetite suppression effects of phentermine are greatest during the initial two weeks of therapy, according to the NIH website. 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 thus, weight loss.