VA Class:GA751
Phentermine is an amphetamine congener that is used as an anorexigenic agent.
Phentermine is used as an adjunct to exercise, behavioral modification, and caloric restriction in the short-term management (a few weeks) of exogenous obesity. Phentermine therapy is indicated for patients with no underlying risk factor but a pretreatment body mass index (BMI) of 30 kg/m2 or greater and those with an underlying risk factor (e.g., hypertension, diabetes mellitus, hyperlipidemia) and a pretreatment BMI of 27 kg/m2 or greater.154 Phentermine is indicated only for monotherapy in the management of exogenous obesity; the drug should not be used in combination with selective serotonin-reuptake inhibitor antidepressants (e.g., fluoxetine, fluvoxamine, paroxetine, sertraline) or monoamine oxidase (MAO) inhibitors.154 To help bring about and maintain loss of weight, the patient must be taught to curtail overeating and to consume a suitable diet. Phentermine also has been used for longer periods† combined with fenfluramine† (no longer commercially available in the US) in selected patients for the management of this condition.100,101,103,104,105,106,107,108,109,110,112,113 Such combined† long-term therapy had been used widely in the 1990s in the management of exogenous obesity.100,103,111,148 However, because of accumulated data on adverse effects associated with the drugs, fenfluramine hydrochloride (Pondimin®) and its dextrorotatory isomer dexfenfluramine hydrochloride (Redux®) were withdrawn from the US market in 1997.114,115,116,135 (See Cautions and also see Cautions, in the Amphetamines General Statement 28:20.04.)
Phentermine is administered orally in the form of the hydrochloride salt or the resin complex.
The usual adult dosage of phentermine hydrochloride is 8 mg 3 times daily, given 30 minutes before meals. Alternatively, 15 or 30 mg of phentermine as the resin complex, or 15-37.5 mg of phentermine hydrochloride, may be given as a single daily dose in the morning.
In 1997, during postmarketing surveillance, abnormal heart valve findings, including echocardiographic features, dyspnea, chest pain, syncope, lower extremity edema, and/or heart murmurs, were reported in some patients who were receiving phentermine in combination with fenfluramine or dexfenfluramine for the management of obesity.114,115,116,118,122,125,127,128,129,130,131,132,133,134,158,166,167,168,169 Preliminary analysis by the US Food and Drug Administration (FDA) of pooled data from several medical centers revealed abnormal echocardiographic findings in about 32% of 291 evaluated asymptomatic patients receiving fenfluramine or dexfenfluramine for up to 24 months, usually in combination with phentermine.113,114,115,116,148,152,160,162 Preliminary data also suggest that the incidence of heart valve abnormalities may be higher in patients exposed to the anorexigenic agents for 6 months or longer when compared with those receiving the drugs for less than 6 months.148 Since a temporal association between use of fenfluramine and dexfenfluramine and these abnormal heart valve findings (e.g., development of unusual mitral, aortic, tricuspid, and/or pulmonary valvular [usually multivalvular]) and echocardiographic abnormalities (that sometimes occurred concomitantly with pulmonary hypertension, occasionally required open heart surgery, and rarely were fatal)113,114,115,118,122,125,127,128,129,130,131,132,133,134,136,148,154,155,156,157,166,167,168,169 were established, the manufacturer of fenfluramine (Pondimin®) and dexfenfluramine (Redux®) voluntarily withdrew these anorexigenic agents from the US market in 1997.114,115,116,135,160 (See Cautions, in the Amphetamines General Statement 28:20.04.)
Because of the severity of the mentioned cardiac effects, the US Department of Health and Human Services (DHHS) issued in 1997 interim recommendations that were developed by FDA in conjunction with the US Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH) (the National Heart, Lung, and Blood Institute and the National Institute of Diabetes and Digestive and Kidney Diseases) and in consultation with the American Heart Association (AHA), the American College of Cardiology (ACC), and the American Dental Association (ADA) for individuals who received fenfluramine or dexfenfluramine as monotherapy or in combination with other drugs (e.g., phentermine).146,147,148,161,163 These interim recommendations include information concerning detection and immediate management of heart valve disease associated with these anorexigenic agents.146,147,148,159,163 (See Cautions, in the Amphetamines General Statement 28:20.04.) However, because of uncertainties about the described heart valve abnormalities (e.g., unknown incidence of substantial heart valve abnormalities; uncertainty about which patients might be at high or low risk for developing such abnormalities and whether such abnormalities would be reversible upon discontinuance of the anorexigenic drugs; uncertainty about the optimal timing of follow-up echocardiograms to determine progression, regression, or stabilization of cardiac valve lesions), the DHHS states that clinicians should exercise their best judgment based on the individual patient's history and clinical and cardiac status to determine the need for additional echocardiographic follow-up.147,148,163 The DHHS anticipates that within 1 year sufficient data will become available to make further recommendations about such acquired cardiac valvular disease.148 In addition, one manufacturer of phentermine (Ionamin®) states that patients who have received fenfluramine or dexfenfluramine either as monotherapy or in combination with phentermine should undergo cardiac evaluation before initiating any new treatment for exogenous obesity.156 This manufacturer also states that safety and efficacy of phentermine in patients with existing heart valve abnormalities and/or heart murmur, in whom increased sympathomimetic activity is not desirable, have not been established; therefore, phentermine should not be used in such patients.156
As of mid-September 1997, recommendations concerning phentermine monotherapy for obesity were not affected by the recall of fenfluramine and dexfenfluramine and patients were not being advised to necessarily discontinue such therapy if indicated.121,156 However, one manufacturer of phentermine (Fastin®) states that heart valve abnormalities have been reported rarely in patients receiving monotherapy with phentermine.168 The etiology of these valvulopathies has not been elucidated and the course of these heart valve abnormalities in patients who have discontinued the anorexigenic agents also is not known.168,169 (See Valvulopathy and Pulmonary Hypertension: Mechanism of Cardiac Abnormalities, under Cautions in the Amphetamines General Statement 28:20.04.) However, the manufacturers state that the drug only should be used for short-term management (a few weeks) of exogenous obesity and should not be used in combination with selective serotonin-reuptake inhibitor antidepressants (e.g., fluoxetine, fluvoxamine, paroxetine, sertraline) or monoamine oxidase (MAO) inhibitors.153,154,156,158,168,169
In addition, abnormal heart valve findings and/or primary pulmonary hypertension have been reported in some patients receiving phendimetrazine tartrate who had a history of receiving at least one other anorexigenic agent.165 One manufacturer of phendimetrazine tartrate (Plegine®) states that since the withdrawal of fenfluramine and dexfenfluramine from the US market, there have been reports that clinicians started prescribing phendimetrazine in combination with phentermine for the management of exogenous obesity in a limited number of patients.165 One manufacturer of phendimetrazine tartrate (Plegine®) also states that phendimetrazine should be used only for short-term management (a few weeks) of exogenous obesity and should not be used in combination with other anorexigenic agents (e.g., phentermine).165,167
Palpitation, tachycardia, and increased blood pressure may occur in patients receiving phentermine.
Adverse nervous system effects of phentermine may include overstimulation, restlessness, insomnia, tremor, dizziness, headache, euphoria, and dysphoria. Rarely, psychotic episodes may occur in patients receiving recommended dosages.
GI effects of phentermine may include dryness of the mouth, unpleasant taste, diarrhea, and vomiting.
Urticaria, impotence, and changes in libido may occur in patients receiving phentermine.
Precautions and Contraindications 
Patients should be warned that phentermine may impair their ability to perform activities requiring mental alertness or physical coordination (e.g., operating machinery, driving a motor vehicle). Phentermine should be used with caution in patients with mild hypertension, and blood pressure should be closely monitored.
Pulmonary hypertension has been reported in patients receiving phentermine in combination with dexfenfluramine (no longer commercially available in the US), fenfluramine (no longer commercially available in the US), and in those with a history of receiving at least one other anorexigenic agent;133,153,154,155,156,165,166,167,168 however, the possibility of an association between pulmonary hypertension and the use of phentermine as monotherapy cannot be ruled out.154,168,169 Primary pulmonary hypertension is a rare, frequently fatal pulmonary disease.154 The initial symptom of pulmonary hypertension generally is dyspnea.154,164,168,169 Other initial manifestations include angina pectoris, syncope, or edema of the lower extremities.154,164,168,169 Phentermine should be discontinued in any patient who develops new, unexplained symptoms of dyspnea, angina, syncope, or edema of the lower extremities.154,164,168,169 Such patients should be evaluated for the possible presence of pulmonary hypertension.164 In addition, patients receiving phentermine should be advised to report immediately any deterioration in exercise tolerance.154,168,169
Phentermine should not be used in combination with selective serotonin-reuptake inhibitor antidepressants (e.g., fluoxetine, fluvoxamine, paroxetine, sertraline) or MAO inhibitors,154,168,169 since severe adverse reactions may occur.158 In addition, one manufacturer of phendimetrazine tartrate (Plegine®) states that phendimetrazine should not be used in combination with other anorexigenic agents (e.g., phentermine) since valvulopathy and primary pulmonary hypertension have been reported in patients receiving phendimetrazine who had received at least one other anorexigenic agent.165
Habituation or addiction has been reported with similar drugs and the possibility of its occurrence should be considered with phentermine.
Phentermine is contraindicated in patients with advanced arteriosclerosis, hyperthyroidism, moderate to severe hypertension, symptomatic cardiovascular disease, agitated states, a history of drug abuse, glaucoma, or known hypersensitivity or idiosyncrasy to sympathomimetic amines. In addition, the drug is contraindicated during or within 14 days of administration of monoamine oxidase inhibitors.
Use of phentermine in children younger than 16 years of age is not recommended.
Safe use of phentermine during pregnancy has not been established. During pregnancy it is questionable whether potential benefits from anorexigenic agents outweigh potential risks and this condition should probably be considered a contraindication to their use, especially during the first trimester.
Insulin requirements in patients with diabetes mellitus may be decreased in association with the use of phentermine and the concomitant dietary regimen and weight loss; therefore, phentermine should be used with caution in patients with diabetes mellitus.
Like amphetamines, phentermine may decrease the hypotensive effect of guanethidine.
In experimental animals and in clinical studies in humans, phentermine apparently produces an anorexigenic effect and loss of weight. As with other amphetamine derivatives, no primary effect on appetite has been demonstrated with phentermine and it is probable that its anorexigenic action is secondary to CNS stimulation. In the past, it was suggested that combined therapy with phentermine, which inhibits uptake of norepinephrine and dopamine, and fenfluramine (no longer commercially available in the US), which affected serotoninergic mechanisms, may provide complementary anorexigenic affects; therefore, such combined therapy had been used in the management of obesity.100,103,111 In addition, while phentermine produced CNS stimulation, fenfluramine produced CNS depression.103,111 However, because of accumulated data on adverse effects associated with the drugs, fenfluramine hydrochloride (Pondimin®) and its dextrorotatory isomer dexfenfluramine hydrochloride (Redux®) were withdrawn from the US market in 1997.114,115,116,135 (See Cautions and also see Cautions, in the Amphetamines General Statement 28:20.04.)
Phentermine is an amphetamine congener that is used as an anorexigenic agent. Phentermine is available as the hydrochloride salt and as a cation exchange resin complex of sulfonated polystyrene. Phentermine hydrochloride occurs as a bitter, white, odorless, hygroscopic, crystalline powder and is soluble in water and in alcohol. Phentermine resin is a coarse granular substance and is practically insoluble in water.
Phentermine hydrochloride capsules and tablets should be stored in tight and well-closed containers, respectively, at a temperature less than 40°C, preferably at 15-30°C. Capsules containing phentermine as a resin complex should be stored in tight containers at 15-30°C.
Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.
Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.
Phentermine preparations are subject to control under the Federal Controlled Substances Act of 1970 as schedule IV (C-IV) drugs.
Routes | Dosage Forms | Strengths | Brand Names | Manufacturer |
|---|---|---|---|---|
Oral | Capsules | 37.5 mg* | Adipex-P® (C-IV) | |
Tablets | 37.5 mg* | Adipex-P® (C-IVscored) | Gate |
* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name
Only references cited for selected revisions after 1984 are available electronically.
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