Phentermine functions as an appetite suppressant and leads to successful weight loss for many people. As half of the weight-loss drug "fen-phen," phentermine fell under suspicion as a cause of heart valve problems found in weight-loss patients during the late 1990s. The other drug in the combination, fenfluramine, was discovered as the responsible substance and was removed from the market. Phentermine continues to be available as an effective weight-loss agent, reports the medical journal, "Gastroenterology Clinics," in its March 2010 issue. Side effects occur with some frequency, though, so if you use phentermine, follow your doctors instructions and report any problems immediately.
Fast Heart Beat
Phentermine works by keeping norepinephrine -- a component of adrenaline -- available in the bloodstream instead of allowing its usual degradation, explains "Rakel: Integrative Medicine." The stimulant effect of increased norepinephrine suppresses appetite, leading to decreased calorie consumption. As a side effect, increased norepinephrine from phentermine increases heart rate and, less commonly, blood pressure. Palpitations, the sensation of a pounding or rapid heart, causes discomfort for some patients taking stimulant-type appetite suppressants. Your doctor will monitor your vital signs frequently when you first start phentermine to check for dangerous elevations in heart rate and blood pressure. If you have high blood pressure or any heart condition, your doctor may recommend that you do not take phentermine for weight loss.
Dry Mouth
As you start therapy with phentermine, you may notice an unusually dry mouth. According to drugs.com, a bad taste in your mouth may accompany this common side effect. Side effects such a dry mouth, a bad taste and mild nausea cause irritation but present no danger. If any of these persist beyond the first month of treatment, let your doctor know.
Headache
Headaches occur commonly with phentermine use and require you to contact your doctor. Most of the time, your headache represents a simple side effect. Sometimes though, headaches signal a dangerous rise in blood pressure that leads to increased cardiovascular risk for events such as heart attacks and strokes. Reports of major complications such as these remain exceedingly rare, states "Gastroenterology Clinics."
Restlessness
Restlessness and related side effects can become intolerable in a small subset of patients, leading to discontinuation of phentermine. Sleep disturbances and mild hyperactivity usually subside after the first month of treatment. If nervous system side effects continue, you may develop a tremor of your hands, combative outbursts or confusion and require emergency treatment. Further progression to fatigue and depression signals a dangerous overdose, according to DailyMed, a website of the National Institutes of Health.
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