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Pronunciation:

(FEN ter meen)

U.S. Brand Names:

Adipex-P®; Ionamin®

Synonyms:

Phentermine Hydrochloride

Generic Available:

Yes: Capsule (excludes resin complex capsule), tablet

Canadian Brand Names:

Ionamin®

Use:

Short-term adjunct in a regimen of weight reduction based on exercise, behavioral modification, and caloric reduction in the management of exogenous obesity for patients with an initial body mass index 30 kg/m² or 27 kg/m² in the presence of other risk factors (diabetes, hypertension)

Restrictions:

C-IV

Pregnancy Risk Factor:

Contraindications:

Hypersensitivity or idiosyncrasy to sympathomimetic amines or any component of the formulation; patients with advanced arteriosclerosis, symptomatic cardiovascular disease, moderate to severe hypertension (stage II or III), hyperthyroidism, glaucoma, agitated states; patients with a history of drug abuse; use during or within 14 days following MAO inhibitor therapy; children <16 years of age (per manufacturer)

Warnings/Precautions:

Use with caution in patients with bipolar disorder, diabetes mellitus, cardiovascular disease, seizure disorders, insomnia, porphyria, or mild hypertension (stage I). May exacerbate symptoms of behavior and thought disorder in psychotic patients. Stimulants may unmask tics in individuals with coexisting Tourette's syndrome. Potential for drug dependency exists; avoid abrupt discontinuation in patients who have received for prolonged periods. Stimulant use has been associated with growth suppression, and careful monitoring is recommended.

Primary pulmonary hypertension (PPH), a rare and frequently fatal pulmonary disease, 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.

Use in weight reduction programs only when alternative therapy has been ineffective. Serious, potentially life-threatening toxicities may occur when thyroid hormones (at dosages above usual daily hormonal requirements) are used in combination with sympathomimetic amines to induce weight loss. Treatment of obesity is not an approved use for thyroid hormone.

Adverse Reactions:

Frequency not defined.

Cardiovascular: Hypertension, palpitation, tachycardia, primary pulmonary hypertension and/or regurgitant cardiac valvular disease

Central nervous system: Euphoria, insomnia, overstimulation, dizziness, dysphoria, headache, restlessness, psychosis

Dermatologic: Urticaria

Endocrine & metabolic: Changes in libido, impotence

Gastrointestinal: Nausea, constipation, xerostomia, unpleasant taste, diarrhea

Hematologic: Blood dyscrasias

Neuromuscular & skeletal: Tremor

Ocular: Blurred vision

Overdosage/Toxicology:

Symptoms of overdose include hyperactivity, agitation, hyperthermia, hypertension, seizures

There is no specific antidote for phentermine intoxication and the bulk of the treatment is supportive. Hyperactivity and agitation usually respond to reduced sensory input, however, with extreme agitation haloperidol (2-5 mg I.M. for adults) may be required. Hyperthermia is best treated with external cooling measures, or when severe or unresponsive, muscle paralysis with pancuronium may be needed. Hypertension is usually transient and generally does not require treatment unless severe. For diastolic blood pressures >110 mm Hg, a nitroprusside infusion should be initiated. Seizures usually respond to diazepam IVP and/or phenytoin maintenance regimens.

Drug Interactions:

Antihypertensives: Phentermine may decrease the effect of antihypertensive medications

Antipsychotics: Efficacy of anorexiants may be decreased by antipsychotics; in addition, amphetamines or related compounds may induce an increase in psychotic symptoms in some patients

Furazolidone: Amphetamines (and related compounds) may induce hypertensive episodes in patients receiving furazolidone

Guanethidine: Amphetamines (and related compounds) inhibit the antihypertensive response to guanethidine; probably also may occur with guanadrel

Hypoglycemic agents: Dosage may need to be adjusted when phentermine is used in a diabetic receiving a special diet

Linezolid: Due to MAO inhibition (see note on MAO inhibitors), this combination should generally be avoided

MAO inhibitors: Concurrent use may be associated with hypertensive episodes

SSRIs: Concurrent use may be associated with a risk of serotonin syndrome

Mechanism of Action:

Phentermine is structurally similar to dextroamphetamine and is comparable to dextroamphetamine as an appetite suppressant, but is generally associated with a lower incidence and severity of CNS side effects. Phentermine, like other anorexiants, stimulates the hypothalamus to result in decreased appetite; anorexiant effects are most likely mediated via norepinephrine and dopamine metabolism. However, other CNS effects or metabolic effects may be involved.

Pharmacodynamics/Kinetics:

Duration: Resin produces more prolonged clinical effects

Absorption: Well absorbed; resin absorbed slower

Half-life elimination: 20 hours

Excretion: Primarily urine (as unchanged drug)

Dosage:

Oral: Adults: Obesity: 8 mg 3 times/day 30 minutes before meals or food or 15-37.5 mg/day before breakfast or 10-14 hours before retiring

Monitoring Parameters:

CNS

Patient Education:

Take during day to avoid insomnia; do not discontinue abruptly, may cause physical and psychological dependence with prolonged use

Nursing Implications:

Dose should not be given in evening or at bedtime

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: Up to 10% of patients may present with hypertension. The use of local anesthetic without vasoconstrictor is recommended in these patients.

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:

Capsule, as hydrochloride: 15 mg, 30 mg

Adipex-P®: 37.5 mg

Capsule, resin complex (Ionamin®): 15 mg, 30 mg

Tablet, as hydrochloride: 37.5 mg

Adipex-P®: 37.5 mg

International Brand Names:

Adipex® (CH, CZ, SI); Duromine® (AU, CR, DO, GT, HK, HN, NZ, PA, SG, SV, TH, ZA); Ifa Reduccing "S"® (MX); Inosie F® (CL); Ionamin® (CA, LU, SG); Ionamine® (CH); Minobese® (ZA); Novirasin® (IL); Phentermine HCl SR Osmopharm® (TH); Phentermine Trenker® (TH); Redusa® (HK); Umine® (SG); Umine Timedcaps® (NZ)

References

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.

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