Phentermine

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Phentermine
Adult Indications & Dosage
Pediatric Indications & Dosage
Contraindications
Warnings & Precautions
Adverse Reactions
Drug Interactions
Use in Specific Populations
Administration & Monitoring
Overdosage
Pharmacology
Clinical Studies
How Supplied
Images
Patient Counseling Information
Precautions with Alcohol
Brand Names
Look-Alike Names

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Adeel Jamil, M.D. [2]

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Overview

Phentermine is a sympathomimetic that is FDA approved for the treatment of exogenous obesity. Common adverse reactions include xerostomia, nervousness, insomnia, and irritability.

Adult Indications and Dosage

FDA-Labeled Indications and Dosage (Adult)

Exogenous Obesity=

  • Phentermine hydrochloride tablets USP are indicated as a short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification and caloric restriction in the management of exogenous obesity for patients with an initial body mass index greater than or equal to 30 kg/m2, or greater than or equal to 27 kg/m2 in the presence of other risk factors (e.g., controlled hypertension, diabetes, hyperlipidemia).
  • Dosage should be individualized to obtain an adequate response with the lowest effective dose.
  • The usual adult dose is one tablet as prescribed by the physician, administered in the morning, with or without food. Phentermine is not recommended for use in pediatric patients less than or equal to 16 years of age.
  • Late evening medication should be avoided because of the possibility of resulting insomnia.

Off-Label Use and Dosage (Adult)

Guideline-Supported Use

There is limited information regarding Off-Label Guideline-Supported Use of Phentermine in adult patients.

Non–Guideline-Supported Use

There is limited information regarding Off-Label Non–Guideline-Supported Use of Phentermine in adult patients.

Pediatric Indications and Dosage

FDA-Labeled Indications and Dosage (Pediatric)

There is limited information regarding FDA-Labeled Use of Phentermine in pediatric patients.

Off-Label Use and Dosage (Pediatric)

Guideline-Supported Use

There is limited information regarding Off-Label Guideline-Supported Use of Phentermine in pediatric patients.

Non–Guideline-Supported Use

There is limited information regarding Off-Label Non–Guideline-Supported Use of Phentermine in pediatric patients.

Contraindications

Warnings

Precautions

  • Coadministration with Other Drug Products for Weight Loss
  • Primary Pulmonary Hypertension
  • Primary Pulmonary Hypertension (PPH) - a rare, frequently fatal disease of the lungs - has been reported to occur in patients receiving a combination of phentermine with fenfluramine or dexfenfluramine. The possibility of an association between PPH and the use of phentermine alone cannot be ruled out; there have been rare cases of PPH in patients who reportedly have taken phentermine alone. The initial symptom of PPH is usually dyspnea. Other initial symptoms may include angina pectoris, syncope or lower extremity edema. Patients should be advised to report immediately any deterioration in exercise tolerance. Treatment should be discontinued in patients who develop new, unexplained symptoms of dyspnea, angina pectoris, syncope or lower extremity edema, and patients should be evaluated for the possible presence of pulmonary hypertension.
  • Valvular Heart Disease
  • Serious regurgitant cardiac valvular disease, primarily affecting the mitral, aortic and/or tricuspid valves, has been reported in otherwise healthy persons who had taken a combination of phentermine with fenfluramine or dexfenfluramine for weight loss. The possible role of phentermine in the etiology of these valvulopathies has not been established and their course in individuals after the drugs are stopped is not known. The possibility of an association between valvular heart disease and the use of phentermine alone cannot be ruled out; there have been rare cases of valvular heart disease in patients who reportedly have taken phentermine alone.
  • Development of Tolerance, Discontinuation in Case of Tolerance
  • When tolerance to the anorectant effect develops, the recommended dose should not be exceeded in an attempt to increase the effect; rather, the drug should be discontinued.
  • Effect on the Ability to Engage in Potentially Hazardous Tasks
  • Phentermine may impair the ability of the patient to engage in potentially hazardous activities such as operating machinery or driving a motor vehicle; the patient should therefore be cautioned accordingly.
  • Risk of Abuse and Dependence
  • Phentermine is related chemically and pharmacologically to amphetamine (d- and d/l-amphetamine) and other related stimulant drugs that have been extensively abused. The possibility of abuse of phentermine should be kept in mind when evaluating the desirability of including a drug as part of a weight reduction program.
  • The least amount feasible should be prescribed or dispensed at one time in order to minimize the possibility of overdosage.
  • Usage with Alcohol
  • Concomitant use of alcohol with phentermine may result in an adverse drug reaction.
  • Use in Patients with Hypertension
  • Use caution in prescribing phentermine for patients with even mild hypertension (risk of increase in blood pressure).
  • Use in Patients on Insulin or Oral Hypoglycemic Medications for Diabetes Mellitus

Adverse Reactions

Clinical Trials Experience

  • The following adverse reactions to phentermine have been identified:
Cardiovascular

Primary pulmonary hypertension and/or regurgitant cardiac valvular disease, palpitation, tachycardia, hypertension, ischemic events.

Central Nervous System

Overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, psychosis.

Gastrointestinal

Dry mouth, unpleasant taste, diarrhea, constipation, other gastrointestinal disturbances.

Allergic

Urticaria.

Endocrine

Impotence, changes in libido.

Postmarketing Experience

There is limited information regarding Postmarketing Experience of Phentermine in the drug label.

Drug Interactions

  • Monoamine Oxidase Inhibitors
  • Alcohol
  • Concomitant use of alcohol with phentermine may result in an adverse drug reaction.
  • Insulin and Oral Hypoglycemic Medications
  • Requirements may be altered.
  • Adrenergic Neuron Blocking Drugs

Use in Specific Populations

Pregnancy

Pregnancy Category (FDA):

  • Pregnancy Category X
  • Phentermine is contraindicated during pregnancy because weight loss offers no potential benefit to a pregnant woman and may result in fetal harm. A minimum weight gain, and no weight loss, is currently recommended for all pregnant women, including those who are already overweight or obese, due to obligatory weight gain that occurs in maternal tissues during pregnancy. Phentermine has pharmacologic activity similar to amphetamine (d- and d/l-amphetamine). Animal reproduction studies have not been conducted with phentermine. If this drug is used during pregnancy, or if the patient becomes pregnant while taking this drug, the patient should be apprised of the potential hazard to a fetus.


Pregnancy Category (AUS):

  • Australian Drug Evaluation Committee (ADEC) Pregnancy Category

There is no Australian Drug Evaluation Committee (ADEC) guidance on usage of Phentermine in women who are pregnant.

Labor and Delivery

There is no FDA guidance on use of Phentermine during labor and delivery.

Nursing Mothers

  • It is not known if phentermine is excreted in human milk; however, other amphetamines are present in human milk. Because of the potential for serious adverse reactions in nursing infants, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Pediatric Use

  • Safety and effectiveness in pediatric patients have not been established. Because pediatric obesity is a chronic condition requiring long-term treatment, the use of this product, approved for short-term therapy, is not recommended.

Geriatic Use

  • In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.
  • This drug is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function. Because elderly patients are more likely to have decreased renal function, care should be taken in dose selection, and it may be useful to monitor renal function.

Gender

There is no FDA guidance on the use of Phentermine with respect to specific gender populations.

Race

There is no FDA guidance on the use of Phentermine with respect to specific racial populations.

Renal Impairment

  • Phentermine was not studied in patients with renal impairment. Based on the reported excretion of phentermine in urine, exposure increases can be expected in patients with renal impairment. Use caution when administering phentermine to patients with renal impairment.

Hepatic Impairment

There is no FDA guidance on the use of Phentermine in patients with hepatic impairment.

Females of Reproductive Potential and Males

There is no FDA guidance on the use of Phentermine in women of reproductive potentials and males.

Immunocompromised Patients

There is no FDA guidance one the use of Phentermine in patients who are immunocompromised.

Administration and Monitoring

Administration

  • Oral

Monitoring

There is limited information regarding Monitoring of Phentermine in the drug label.

IV Compatibility

There is limited information regarding IV Compatibility of Phentermine in the drug label.

Overdosage

Acute Overdose

Signs and Symptoms

Management

  • Management of acute phentermine hydrochloride intoxication is largely symptomatic and includes lavage and sedation with a barbiturate. Experience with hemodialysis or peritoneal dialysis is inadequate to permit recommendations in this regard. Acidification of the urine increases phentermine excretion. Intravenous phentolamine (Regitine®, CIBA) has been suggested on pharmacologic grounds for possible acute, severe hypertension, if this complicates overdosage.

Chronic Overdose

  • Manifestations of chronic intoxication with anorectic drugs include severe dermatoses, marked insomnia, irritability, hyperactivity and personality changes. The most severe manifestation of chronic intoxications is psychosis, often clinically indistinguishable from schizophrenia.

Pharmacology

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Phentermine
Systematic (IUPAC) name
2-methyl-1-phenylpropan-2-amine
Identifiers
CAS number 122-09-8
ATC code A08AA01
PubChem 4771
DrugBank DB00191
Chemical data
Formula C10H15N 
Mol. mass 149.233 g/mol
SMILES eMolecules & PubChem
Synonyms α-methyl-amphetamine
α,α-dimethylphenethylamine
Pharmacokinetic data
Bioavailability High (almost complete)
Protein binding Approximately 96.3%
Metabolism Hepatic
Half life 25 hours, urinary pH-dependent
Excretion Urinary (62-85% unchanged)
Therapeutic considerations
Pregnancy cat.

B3(AU) X(US)

Legal status

Prescription Only (S4)(AU) Schedule IV(CA) Schedule IV(US)

Routes Medical: oral,
Recreational: oral, insufflation, intravenous

Mechanism of Action

  • Phentermine is a sympathomimetic amine with pharmacologic activity similar to the prototype drugs of this class used in obesity, amphetamine (d- and d/l-amphetamine). Drugs of this class used in obesity are commonly known as "anorectics" or "anorexigenics." It has not been established that the primary action of such drugs in treating obesity is one of appetite suppression since other central nervous system actions, or metabolic effects, may also be involved.

Structure

  • Phentermine hydrochloride USP is a sympathomimetic amine anorectic. Its chemical name is a,a-dimethylphenethylamine hydrochloride. The structural formula is as follows:
This image is provided by the National Library of Medicine.
  • Phentermine hydrochloride USP is a white, odorless, hygroscopic, crystalline powder which is soluble in water and lower alcohols, slightly soluble in chloroform and insoluble in ether.
  • Phentermine hydrochloride tablets USP are available as an oral tablet containing 37.5 mg of phentermine hydrochloride USP (equivalent to 30 mg of phentermine base). Each phentermine hydrochloride tablet USP also contains the inactive ingredients microcrystalline cellulose, pregelatinized starch, anhydrous lactose, crospovidone, colloidal silicon dioxide, magnesium stearate, sucrose, corn starch and FD&C Blue #1.

Pharmacodynamics

  • Typical actions of amphetamines include central nervous system stimulation and elevation of blood pressure. Tachyphylaxis and tolerance have been demonstrated with all drugs of this class in which these phenomena have been looked for.

Pharmacokinetics

  • Following the administration of phentermine, phentermine reaches peak concentrations (Cmax) after 3 to 4.4 hours.
  • Drug Interactions
  • In a single-dose study comparing the exposures after oral administration of a combination capsule of 15 mg phentermine and 92 mg topiramate to the exposures after oral administration of a 15 mg phentermine capsule or a 92 mg topiramate capsule, there is no significant topiramate exposure change in the presence of phentermine. However in the presence of topiramate, phentermine Cmax and AUC increase 13% and 42%, respectively.
  • Specific Populations
  • Renal Impairment
  • Phentermine was not studied in patients with renal impairment. The literature reported cumulative urinary excretion of phentermine under uncontrolled urinary pH conditions is 62% to 85%. Exposure increases can be expected in patients with renal impairment. Use caution when administering phentermine to patients with renal impairment.

Nonclinical Toxicology

  • Carcinogenesis, Mutagenesis, Impairment of Fertility
  • Studies have not been performed with phentermine to determine the potential for carcinogenesis, mutagenesis or impairment of fertility.

Clinical Studies

  • In relatively short-term clinical trials, adult obese subjects instructed in dietary management and treated with “anorectic” drugs lost more weight on the average than those treated with placebo and diet.
  • The magnitude of increased weight loss of drug-treated patients over placebo-treated patients is only a fraction of a pound a week. The rate of weight loss is greatest in the first weeks of therapy for both drug and placebo subjects and tends to decrease in succeeding weeks. The possible origins of the increased weight loss due to the various drug effects are not established. The amount of weight loss associated with the use of an “anorectic” drug varies from trial to trial, and the increased weight loss appears to be related in part to variables other than the drugs prescribed, such as the physician-investigator, the population treated and the diet prescribed. Studies do not permit conclusions as to the relative importance of the drug and non-drug factors on weight loss.
  • The natural history of obesity is measured over several years, whereas the studies cited are restricted to a few weeks’ duration; thus, the total impact of drug-induced weight loss over that of diet alone must be considered clinically limited.

How Supplied

  • Available as tablets containing 37.5 mg phentermine hydrochloride USP (equivalent to 30 mg phentermine base).
  • Phentermine hydrochloride tablets, USP are supplied as white to off-white with blue specks, capsule shaped, uncoated tablets, debossed with “U40” on one side and break line on the other side.
  • Bottles of 30 NDC 13107-061-30
  • Bottles of 100 NDC 13107-061-01
  • Bottles of 500 NDC 13107-061-05
  • Bottles of 1000 NDC 13107-061-99
  • Store at 20° to 25°C (68° to 77°F).
  • Dispense in a tight container as defined in the USP, with a child-resistant closure (as required).
  • Keep out of the reach of children.

Storage

There is limited information regarding Phentermine Storage in the drug label.

Images

Drug Images

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This pill image is provided by the National Library of Medicine's PillBox.

Package and Label Display Panel

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This image of the FDA label is provided by the National Library of Medicine.
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This image of the FDA label is provided by the National Library of Medicine.

Patient Counseling Information

  • Patients must be informed that phentermine hydrochloride is a short-term (a few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification and caloric restriction in the management of exogenous obesity, and that coadministration of phentermine with other drugs for weight loss is not recommended.
  • Patients must be instructed on how much phentermine to take, and when and how to take it.
  • Advise pregnant women and nursing mothers not to use phentermine.
  • Patients must be informed about the risks of use of phentermine, about the symptoms of potential adverse reactions and when to contact a physician and/or take other action. The risks include, but are not limited to:
  • The patients must also be informed about
  • The potential for developing tolerance and actions if they suspect development of tolerance and
  • The risk of dependence and the potential consequences of abuse.
  • Tell patients to keep phentermine in a safe place to prevent theft, accidental overdose, misuse or abuse. Selling or giving away phentermine may harm others and is against the law.

Precautions with Alcohol

  • Alcohol-Phentermine interaction has not been established. Talk to your doctor about the effects of taking alcohol with this medication.

Brand Names

  • PHENTERMINE HYDROCHLORIDE®[1]

Look-Alike Drug Names

There is limited information regarding Phentermine Look-Alike Drug Names in the drug label.

Drug Shortage Status

Price

References

The contents of this FDA label are provided by the National Library of Medicine.

  1. "PHENTERMINE HYDROCHLORIDE phentermine hydrochloride tablet".

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