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Medication Safety Issues
Sound-alike/look-alike issues:
Phentermine may be confused with phentolamine, phenytoin
Pronunciation
(FEN ter meen)
U.S. Brand Names
Index Terms
Generic Available
Yes
Canadian Brand Names
Pharmacologic Category
Pharmacologic Category Synonyms
Use: Labeled Indications
Short-term (few weeks) adjunct in exogenous obesity
Restrictions
C-IV
Pharmacotherapy for weight loss is recommended only for obese patients with a body mass index ?30 kg/m2, or ?27 kg/m2 in the presence of other risk factors such as hypertension, diabetes, and/or dyslipidemia or a high waist circumference; therapy should be used in conjunction with a comprehensive weight management program. Rule out organic causes of obesity (eg, untreated hypothyroidism) prior to use.
Note: Phentermine is not approved for long-term use. The limited usefulness of medications in this class should be weighed against possible risks associated with their use. Consult weight loss guidelines for current pharmacotherapy recommendations.
Pregnancy Risk Factor
C
Pregnancy Considerations
Reproduction studies have not been conducted.
Contraindications
Hypersensitivity or idiosyncrasy to phentermine or other sympathomimetic amines or any component of the formulation; advanced arteriosclerosis, cardiovascular disease, moderate-to-severe hypertension; pulmonary hypertension; hyperthyroidism, glaucoma, agitated states, patients with a history of drug abuse; use during or within 14 days following MAO inhibitor therapy
Warnings/Precautions
Concerns related to adverse effects:
• CNS effects: Amphetamines may impair the ability to engage in potentially hazardous activities.
• Primary pulmonary hypertension (PPH): A rare, frequently fatal disease of the lungs, PPH has been reported to occur in patients receiving a combination of phentermine and fenfluramine or dexfenfluramine. The possibility of an association between PPH and the use of phentermine alone cannot be ruled out.
• Valvular heart disease: The use of some anorexigens, including phentermine, has been associated with the development of valvular heart disease. Avoid stimulants in patients with known serious structural cardiac abnormalities, cardiomyopathy, serious heart rhythm abnormalities, or other serious cardiac problems that could increase the risk of sudden death that these conditions alone carry.
Disease-related concerns:
• Diabetes: Use with caution in patients with diabetes mellitus; antidiabetic agent requirements may be altered with anorexigens and concomitant dietary restrictions.
• Hypertension: Use with caution in patients with hypertension and other cardiovascular conditions that might be exacerbated by increases in blood pressure or heart rate.
• Seizure disorders: Use with caution in patients with a history of seizure disorders.
• Tourette's syndrome: Use with caution in patients with Tourette's syndrome; stimulants may unmask tics.
Concurrent drug therapy issues:
• Anorexigens: Safety and efficacy have not been established for use with other weight loss medications, including over-the-counter or herbal products. Not recommended for use in patients who have used other anorectic agents within the past year.
Special populations:
• Pediatrics: Safety and efficacy have not been established in children ?16 years of age.
Other warnings/precautions:
• Abuse potential: Phentermine is pharmacologically related to the amphetamines, which have a high abuse potential; prolonged use may lead to dependency. Prescriptions should be written for the smallest quantity consistent with good patient care to minimize possibility of overdose.
• Discontinuation of therapy: Discontinue if satisfactory weight loss has not occurred within the first 4 weeks of treatment, or if tolerance develops.
Adverse Reactions
Frequency not defined.
Cardiovascular: Hypertension, palpitation, primary pulmonary hypertension and/or regurgitant cardiac valvular disease, tachycardia
Central nervous system: Dizziness, dysphoria, euphoria, headache, insomnia, overstimulation, psychosis, restlessness
Dermatologic: Urticaria
Endocrine & metabolic: Changes in libido
Gastrointestinal: Constipation, diarrhea, unpleasant taste, xerostomia
Genitourinary: Impotence
Neuromuscular & skeletal: Tremor
Drug Interactions
Alkalinizing Agents: May decrease the excretion of Amphetamines. Risk D: Consider therapy modification
Ammonium Chloride: May decrease the serum concentration of Amphetamines. This effect is likely due to an enhanced excretion of amphetamines in the urine. Risk C: Monitor therapy
Analgesics (Opioid): Amphetamines may enhance the analgesic effect of Analgesics (Opioid). Risk C: Monitor therapy
Antacids: May decrease the excretion of Amphetamines. Risk C: Monitor therapy
Antihistamines: Amphetamines may diminish the sedative effect of Antihistamines. Risk C: Monitor therapy
Antipsychotics: May diminish the stimulatory effect of Amphetamines. Risk C: Monitor therapy
Atomoxetine: May enhance the hypertensive effect of Sympathomimetics. Atomoxetine may enhance the tachycardic effect of Sympathomimetics. Risk C: Monitor therapy
Cannabinoids: May enhance the tachycardic effect of Sympathomimetics. Risk C: Monitor therapy
Carbonic Anhydrase Inhibitors: May decrease the excretion of Amphetamines. Exceptions: Brinzolamide; Dorzolamide. Risk C: Monitor therapy
Ethosuximide: Amphetamines may diminish the therapeutic effect of Ethosuximide. Amphetamines may decrease the serum concentration of Ethosuximide. Risk C: Monitor therapy
Gastrointestinal Acidifying Agents: May decrease the serum concentration of Amphetamines. Risk C: Monitor therapy
Iobenguane I 123: Sympathomimetics may diminish the therapeutic effect of Iobenguane I 123. Risk X: Avoid combination
Lithium: May diminish the stimulatory effect of Amphetamines. Risk C: Monitor therapy
MAO Inhibitors: May enhance the hypertensive effect of Amphetamines. Risk X: Avoid combination
Methenamine: May decrease the serum concentration of Amphetamines. This effect is likely due to an enhanced excretion of amphetamines in the urine. Risk C: Monitor therapy
PHENobarbital: Amphetamines may decrease the serum concentration of PHENobarbital. Risk C: Monitor therapy
Phenytoin: Amphetamines may decrease the serum concentration of Phenytoin. Risk C: Monitor therapy
Sympathomimetics: May enhance the adverse/toxic effect of other Sympathomimetics. Risk C: Monitor therapy
Tricyclic Antidepressants: May enhance the stimulatory effect of Amphetamines. Tricyclic Antidepressants may also potentiate the cardiovascular effects of Amphetamines. Risk C: Monitor therapy
Mechanism of Action
Phentermine is a sympathomimetic amine with pharmacologic properties similar to the amphetamines. The mechanism of action in reducing appetite appears to be secondary to CNS effects, including stimulation of the hypothalamus to release norepinephrine.
Pharmacodynamics/Kinetics
Duration: Resin produces more prolonged clinical effects
Absorption: Well absorbed; resin absorbed slower
Excretion: Primarily urine
Dosage
Oral: Children >16 years and Adults: Obesity:
Phentermine hydrochloride: 18.75-37.5 mg/ day
Phentermine resin: 15-30 mg/day
Administration: Oral
Phentermine hydrochloride: Administer before breakfast or 1-2 hours after breakfast. Tablets may be divided in half and dose may be given in 2 divided doses. Avoid late evening administration.
Phentermine resin: Administer before breakfast or 10-14 hours before retiring. Swallow capsules whole.
Monitoring Parameters
Weight, waist circumference; blood pressure
Reference Range
Adult classification of weight by BMI (kg/m2):
Underweight: <18.5
Normal: 18.5-24.9
Overweight: 25-29.9
Obese, class I: 30-34.9
Obese, class II: 35-39.9
Extreme obesity (class III): ?40
Waist circumference: In adults with a BMI of 25-34.9 kg/m2, high-risk waist circumference is defined as:
Men >102 cm (>40 in)
Women >88 cm (>35 in)
Dietary Considerations
Most effective when combined with a low-calorie diet and behavior modification counseling.
Cardiovascular Considerations
Phentermine should be avoid in patients with cardiovascular disease. The combination with fenfluramine (Phen-Fen® - no longer available) was associated with mitral valve fibrosis and mitral regurgitation. Primary pulmonary hypertension has also been reported in patients receiving Phen-Fen®.
Dental Health: Effects on Dental Treatment
Key adverse event(s) related to dental treatment: Xerostomia (normal salivary flow resumes upon discontinuation) and unpleasant taste. Up to 10% of patients may present with hypertension. The use of local anesthetic without vasoconstrictor is recommended in these patients. See Dental Health Professional Considerations.
Dental Health: Vasoconstrictor/Local Anesthetic Precautions
Use vasoconstrictor with caution in patients taking phentermine. Amphetamines enhance the sympathomimetic response of epinephrine and norepinephrine leading to potential hypertension and cardiotoxicity.
Dental Comment
Many diet physicians have prescribed fenfluramine (“fen”) and phentermine (“phen”). When taken together the combination is known as “fen-phen”. The diet drug dexfenfluramine (Redux®) is chemically similar to fenfluramine (Pondimin®) and was also used in combination with phentermine called “Redux-phen”. While each of the three drugs alone had approval from the FDA for sale in the treatment of obesity, neither combination had an official approval. The use of the combinations in the treatment of obesity was considered an “off-label” use. Reports in medical literature have been accumulating for some years about significant side effects associated with fenfluramine and dexfenfluramine. In 1997, the manufacturers, at the urging of the FDA, agreed to voluntarily withdraw the drugs from the market. The action was based on findings from physicians who evaluated patients taking fenfluramine and dexfenfluramine with echocardiograms. The findings indicated that approximately 30% of patients had abnormal echocardiograms, even though they had no symptoms. This was a much higher than expected percentage of abnormal test results. This conclusion was based on a sample of 291 patients examined by five different physicians. Under normal conditions, fewer than 1% of patients would be expected to show signs of heart valve disease. The findings suggested that fenfluramine and dexfenfluramine were the likely cause of heart valve problems of the type that promoted FDA's earlier warnings concerning “fen-phen”. The earlier warning included the following: The mitral valve and other valves in the heart are damaged by a strange white coating and allow blood to flow back, causing heart muscle damage. In several cases, valve replacement surgery has been done. As a rule, the person must, thereafter for life, be on a blood thinner to prevent clots from the mechanical valve. This type of valve damage had only been seen before in persons who were exposed to large amounts of serotonin. The fenfluramine increases the availability of serotonin.
Dosage Forms
Excipient information presented when available (limited, particularly for generics); consult specific product labeling.
Capsule, as hydrochloride: 15 mg, 30 mg, 37.5 mg
Adipex-P®: 37.5 mg
Tablet, as hydrochloride: 37.5 mg
Adipex-P®: 37.5 mg
Pricing: U.S. (www.drugstore.com)
Capsules (Adipex-P)
37.5 mg (30): $66.14
Capsules (Phentermine HCl)
15 mg (30): $40.99
30 mg (30): $34.99
37.5 mg (30): $29.99
Tablets (Adipex-P)
37.5 mg (30): $66.14
Tablets (Phentermine HCl)
37.5 mg (30): $29.99
References
Abenhaim L, Moride Y, Brenot F, et al, “Appetite-Suppressant Drugs and the Risk of Primary Pulmonary Hypertension. International Primary Pulmonary Hypertension Study Group,” N Engl J Med, 1996, 335(9):609-16.
“Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: Executive Summary. Expert Panel on the Identification, Evaluation, and Treatment of Overweight in Adults,” Am J Clin Nutr, 1998, 68(4):899-917.
Devan GS, “Phentermine and Psychosis,” Br J Psychiatry, 1990, 156:442-3.
Hamer R and Phelps D, “Inadvertent Intra-arterial Injection of Phentermine: A Complication of Drug Abuse,” Ann Emerg Med, 1981, 10:148-50.
Kokkinos J and Levine SR, “Possible Association of Ischemic Stroke With Phentermine,” Stroke, 1993, 24(2):310-3.
Levine B, Caplan YH, and Dixon AM, “A Fatality Involving Phentermine,” J Forensic Sci, 1984, 29(4):1242-5.
Mokhlesi B, Leikin JB, Murray P, et al, “Adult Toxicology in Critical Care: Part II: Specific Poisonings,” Chest, 2003, 123(3):897-922.
Snow V, Barry P, Fitterman N, et al, “Pharmacologic and Surgical Management of Obesity in Primary Care: A Clinical Practice Guideline from the American College of Physicians,” Ann Intern Med, 2005, 142(7):525-31.
“U.S. Preventative Services Task Force. Screening for Obesity in Adults: Recommendations and Rationale,” Ann Intern Med, 2003, 139(11):933-49
International Brand Names
Lexi-Comp.com
Last full review/revision August 2009
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