Phentermine is a stimulant that mainly works on the central nervous system. It simulates a brain region called the hypothalamus to release norepinephrine, which in turn reduces appetite, according to Merck Manuals Online Medical Library. Norepinephrine is a hormone released while the body is under stress or excitement. It triggers a series of physiological "fight-or-flight" reactions which includes appetite suppression.
Heart Disease
When phentermine stimulates the central nervous system, many functions and organs can be affected, including the heart, blood vessels, stomach and lungs. Similar to all stimulants, phentermine increases the heart rate and raises the blood pressure. The use of phentermine has been associated with the development of certain heart diseases. Patients with cardiac medical history and high blood pressure should ask their doctors before using phentermine.
Primary Pulmonary Hypertension
Primary pulmonary hypertension, a rare disease of the lungs that is often fatal, has been reported to affect patients receiving a combination of phentermine and fenfluramine or dexfenfluramine. Although these cases involved drugs besides phentermine, it is possible that phentermine alone has the potential to cause primary pulmonary hypertension, according to Merck.
Addiction
Phentermine, like other central nervous stimulants, may be addictive to some people, particularly those with history of drug abuse. In addition, prolonged use may also increase the possibility of dependency. Therefore, patients who are prescribed phentermine should closely monitor their medicine bottles to avoid misuse by people who the prescription is not intended to. It is advised that prescriptions should be written for the smallest quantity consistent with good patient care to minimize possibility of overdose.
Weight Regain
Because the clinical trials of phentermine were short in duration, phentermine was only approved by the U.S. Food and Drug Administration for short-term obesity treatment. Therefore, phentermine should not be used more than 12 weeks and is not recommended as a long-term solution to weight management, according to MayoClinic.com. In addition, once the treatment of phentermine is stopped, it has been reported that the weight previously lost is quickly regained. To achieve the long-term effect of weight loss, the phentermine treatment should be combined with healthy diet and proper physical activities exercise.
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