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Phentermine

Pronunciation

Pronunciation

(FEN ter meen)

Index Terms

  • Phentermine HCl
  • Phentermine Hydrochloride

Dosage Forms

Excipient information presented when available (limited, particularly for generics); consult specific product labeling. [DSC] = Discontinued product

Capsule, Oral, as hydrochloride:

Adipex-P: 37.5 mg

Generic: 15 mg, 30 mg, 37.5 mg

Tablet, Oral, as hydrochloride:

Adipex-P: 37.5 mg [scored; contains brilliant blue fcf (fd&c blue #1)]

Lomaira: 8 mg [scored; contains brilliant blue fcf (fd&c blue #1), corn starch]

Generic: 37.5 mg

Tablet Disintegrating, Oral, as hydrochloride:

Suprenza: 15 mg [DSC] [contains fd&c blue #1 aluminum lake, fd&c yellow #5 aluminum lake]

Suprenza: 30 mg [DSC] [contains fd&c yellow #5 aluminum lake]

Suprenza: 37.5 mg [DSC] [contains fd&c blue #1 aluminum lake]

Brand Names: U.S.

  • Adipex-P
  • Lomaira
  • Suprenza [DSC]

Pharmacologic Category

  • Anorexiant
  • Central Nervous System Stimulant
  • Sympathomimetic

Pharmacology

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.

Absorption

Well absorbed. Rate and extent of exposure of orally disintegrating tablets (ODT) are equivalent to capsules and tablets administered under fasting conditions. Administration of the ODT after a high-fat/high-calorie breakfast decreased Cmax by ~5% and AUC by ~12%.

Distribution

Vd: 348 L

Metabolism

Hepatic via p-hydroxylation (aromatic ring) and N-oxidation (alipthatic side chain); primarily metabolized by CYP3A4 (but does not show extensive metabolism).

Excretion

Primarily urine (62% to 85% as unchanged drug)

Time to Peak

3 to 4.4 hours

Half-Life Elimination

~20 hours

Protein Binding

17.5%

Special Populations: Renal Function Impairment

Exposure increases can be expected in patients with renal impairment.

Use: Labeled Indications

Obesity (short-term adjunct): Short-term (few weeks) adjunct in a regimen of weight reduction based on exercise, behavioral modification and caloric restriction in the management of exogenous obesity with an initial body mass index (BMI) ≥30 kg/m2 or ≥27 kg/m2 in the presence of other risk factors (eg, diabetes, hyperlipidemia, controlled hypertension).

Contraindications

Hypersensitivity or idiosyncrasy to phentermine, other sympathomimetic amines or any component of the formulation; history of cardiovascular disease (eg, arrhythmias, heart failure, coronary artery disease, stroke, uncontrolled hypertension); hyperthyroidism; glaucoma; agitated states; history of drug abuse; use during or within 14 days following MAO inhibitor therapy; pregnancy; breast-feeding

Dosing: Adult

Note: Dosing is presented in terms of the salt, phentermine hydrochloride (not as phentermine base).

Obesity (short-term adjunct): Oral:

Capsule, tablet (excluding Lomaira): 15 to 37.5 mg/day in 1 to 2 divided doses. Individualize to achieve adequate response with lowest effective dose.

Tablet (Lomaira only): 8 mg 3 times daily. Individualize to achieve adequate response with lowest effective dose.

Orally disintegrating tablet (ODT): One tablet (15 to 37.5 mg daily) every morning. Individualize to achieve adequate response with lowest effective dose.

Dosing: Geriatric

Refer to adult dosing.

Dosing: Pediatric

Obesity (short-term adjunct): Adolescents >16 years: Refer to adult dosing.

Dosing: Renal Impairment

Capsule, tablet (excluding Lomaira):

eGFR ≥30 mL/minute/1.73 m2: There are no dosage adjustments provided in the manufacturer's labeling; systemic exposure may be increased; use with caution.

eGFR 15 to 29 mL/minute/1.73 m2: Maximum dose: 15 mg/day.

eGFR <15 mL/minute/1.73 m2: Avoid use (has not been studied).

End-stage renal disease (ESRD) requiring dialysis: Avoid use (has not been studied).

Tablet (Lomaira only): There are no dosage adjustments provided in the manufacturer's labeling (has not been studied); systemic exposure may be increased; use with caution.

Dosing: Hepatic Impairment

There are no dosage adjustments provided in the manufacturer's labeling (has not been studied).

Administration

Avoid late evening administration. Most effective when combined with a low-calorie diet and behavior modification counseling.

Capsules, tablets (excluding Lomaira): Administer before breakfast or 1 to 2 hours after breakfast. Tablets may be divided in half and dose may be given in 2 divided doses.

Tablet (Lomaira only): Administer 30 minutes before meals. Tablets are scored and may be divided in half.

Orally disintegrating tablets (ODT): With dry hands, place tablet on the tongue and allow to dissolve, then swallow with or without water. May administer with or without food.

Dietary Considerations

Capsules, tablets (excluding Lomaira): Should be taken before breakfast or 1 to 2 hours after breakfast.

Tablet (Lomaira only): Should be taken 30 minutes before meals.

Storage

Store at 20°C to 25°C (68°F to 77°F).

Drug Interactions

Acebrophylline: May enhance the stimulatory effect of CNS Stimulants. Avoid combination

Alcohol (Ethyl): May enhance the adverse/toxic effect of Phentermine. Monitor therapy

Alkalinizing Agents: May decrease the excretion of Amphetamines. 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. Monitor therapy

Analgesics (Opioid): Amphetamines may enhance the analgesic effect of Analgesics (Opioid). Monitor therapy

Antacids: May decrease the excretion of Amphetamines. Monitor therapy

Antihistamines: Amphetamines may diminish the sedative effect of Antihistamines. Monitor therapy

Antihypertensive Agents: Amphetamines may diminish the antihypertensive effect of Antihypertensive Agents. Monitor therapy

Antipsychotic Agents: May diminish the stimulatory effect of Amphetamines. Monitor therapy

Ascorbic Acid: May decrease the serum concentration of Amphetamines. Monitor therapy

AtoMOXetine: May enhance the hypertensive effect of Sympathomimetics. AtoMOXetine may enhance the tachycardic effect of Sympathomimetics. Monitor therapy

Cannabinoid-Containing Products: May enhance the tachycardic effect of Sympathomimetics. Exceptions: Cannabidiol. Monitor therapy

Carbonic Anhydrase Inhibitors: May decrease the excretion of Amphetamines. Exceptions: Brinzolamide; Dorzolamide. Monitor therapy

Cocaine: May enhance the hypertensive effect of Sympathomimetics. Management: Consider alternatives to use of this combination when possible. Monitor closely for substantially increased blood pressure or heart rate and for any evidence of myocardial ischemia with concurrent use. Consider therapy modification

Doxofylline: Sympathomimetics may enhance the adverse/toxic effect of Doxofylline. Monitor therapy

Ethosuximide: Amphetamines may diminish the therapeutic effect of Ethosuximide. Amphetamines may decrease the serum concentration of Ethosuximide. Monitor therapy

Gastrointestinal Acidifying Agents: May decrease the serum concentration of Amphetamines. Monitor therapy

Guanethidine: May enhance the arrhythmogenic effect of Sympathomimetics. Guanethidine may enhance the hypertensive effect of Sympathomimetics. Monitor therapy

Iobenguane I 123: Sympathomimetics may diminish the therapeutic effect of Iobenguane I 123. Avoid combination

Ioflupane I 123: Amphetamines may diminish the diagnostic effect of Ioflupane I 123. Monitor therapy

Iohexol: Agents With Seizure Threshold Lowering Potential may enhance the adverse/toxic effect of Iohexol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iohexol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic anticonvulsants. Consider therapy modification

Iomeprol: Agents With Seizure Threshold Lowering Potential may enhance the adverse/toxic effect of Iomeprol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iomeprol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic anticonvulsants. Consider therapy modification

Iopamidol: Agents With Seizure Threshold Lowering Potential may enhance the adverse/toxic effect of Iopamidol. Specifically, the risk for seizures may be increased. Management: Discontinue agents that may lower the seizure threshold 48 hours prior to intrathecal use of iopamidol. Wait at least 24 hours after the procedure to resume such agents. In nonelective procedures, consider use of prophylactic anticonvulsants. Consider therapy modification

Linezolid: May enhance the hypertensive effect of Sympathomimetics. Management: Reduce initial doses of sympathomimetic agents, and closely monitor for enhanced pressor response, in patients receiving linezolid. Specific dose adjustment recommendations are not presently available. Consider therapy modification

Lithium: May diminish the stimulatory effect of Amphetamines. Monitor therapy

MAO Inhibitors: May enhance the hypertensive effect of Amphetamines. While linezolid and tedizolid may interact via this mechanism, management recommendations differ from other monoamine oxidase inhibitors. Refer to monographs specific to those agents for details. Exceptions: Linezolid; Tedizolid. Avoid combination

Methenamine: May decrease the serum concentration of Amphetamines. This effect is likely due to an enhanced excretion of amphetamines in the urine. Monitor therapy

Multivitamins/Fluoride (with ADE): May decrease the serum concentration of Amphetamines. More specifically, the ascorbic acid (vitamin C) in many multivitamins may decrease amphetamine concentrations. Monitor therapy

Multivitamins/Minerals (with ADEK, Folate, Iron): May decrease the serum concentration of Amphetamines. Monitor therapy

Multivitamins/Minerals (with AE, No Iron): May decrease the serum concentration of Amphetamines. Specifically, vitamin C may impair absorption of amphetamines. Monitor therapy

PHENobarbital: Amphetamines may decrease the serum concentration of PHENobarbital. Monitor therapy

Phenytoin: Amphetamines may decrease the serum concentration of Phenytoin. Monitor therapy

Sympathomimetics: May enhance the adverse/toxic effect of other Sympathomimetics. Monitor therapy

Tedizolid: May enhance the hypertensive effect of Sympathomimetics. Tedizolid may enhance the tachycardic effect of Sympathomimetics. Monitor therapy

Tricyclic Antidepressants: May enhance the stimulatory effect of Amphetamines. Tricyclic Antidepressants may also potentiate the cardiovascular effects of Amphetamines. Monitor therapy

Urinary Acidifying Agents: May decrease the serum concentration of Amphetamines. Monitor therapy

Test Interactions

May interfere with urine detection of amphetamines/methamphetamines (false-positive).

Adverse Reactions

Frequency not defined.

Cardiovascular: Hypertension, ischemia, palpitations, tachycardia

Central nervous system: Dizziness, dysphoria, euphoria, headache, insomnia, overstimulation, psychosis, restlessness

Dermatologic: Urticaria

Endocrine & metabolic: Change in libido

Gastrointestinal: Constipation, diarrhea, gastrointestinal distress, unpleasant taste, xerostomia

Genitourinary: Impotence

Neuromuscular & skeletal: Tremor

<1% (Limited to important or life-threatening): Acquired valvular heart disease (regurgitant), primary pulmonary hypertension

Warnings/Precautions

Concerns related to adverse effects:

• CNS effects: May cause CNS depression, which may impair physical or mental abilities; patients must be cautioned about performing tasks that require mental alertness (eg, operating machinery or driving).

• Heart failure: In a scientific statement from the American Heart Association, phentermine has been determined to be an agent that may cause direct myocardial toxicity (magnitude: major) (AHA [Page 2016]).

• 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; rare cases of PPH have been reported in patients taking phentermine alone. Discontinue in patients experiencing new-onset dyspnea, chest pain, syncope, or lower extremity edema.

• Valvular heart disease: Serious regurgitant cardiac valvular disease (primarily affecting the mitral, aortic, and/or tricuspid valves) has been reported to occur in patients receiving a combination of phentermine and fenfluramine or dexfenfluramine. The possibility of an association between valvular heart disease and the use of phentermine alone cannot be ruled out; rare cases of valvular heart disease have been reported in patients taking phentermine alone.

Disease-related concerns:

• Cardiovascular 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. Use with caution in patients with mild hypertension and other cardiovascular conditions that might be exacerbated by increases in blood pressure or heart rate.

• Diabetes: Use with caution in patients with diabetes mellitus; antidiabetic agent requirements (eg, insulin or oral hypoglycemic agents) may be decreased with anorexigens and concomitant dietary restrictions.

• Renal impairment: Use caution in patients with renal impairment; an increase in exposure is expected. Avoid use in patients with eGFR <15 mL/minute/1.73 m2, including end-stage renal disease (ESRD) requiring dialysis (has not been studied).

• Seizure disorders: Avoid or use with caution in patients with history of seizures (Apovian 2015).

• Tourette syndrome: Use with caution in patients with Tourette syndrome; stimulants may unmask tics.

Concurrent drug therapy issues:

• Drug-drug interactions: Potentially significant interactions may exist, requiring dose or frequency adjustment, additional monitoring, and/or selection of alternative therapy. Consult drug interactions database for more detailed information.

Special populations:

• Elderly: Use caution in this age group due to the risk for causing dependence, hypertension, angina, and myocardial infarction.

Dosage form specific issues:

• Tartrazine: Some products may contain tartrazine Some products may contain tartrazine (FDC yellow #5), which may cause allergic reactions in patients with sensitivity (caution in patients with asthma or aspirin hypersensitivity).

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.

• Appropriate use: Phentermine is not approved for long-term use. Clinicians should carefully examine the potentially benefits against potential risks associated with use of medications in this class. Consult weight loss guidelines for current pharmacotherapy recommendations. Therapy should be used in conjunction with a comprehensive weight management program.

• Tolerance: Tolerance to the anorectic effect usually develops within a few weeks; discontinue use when tolerance develops, do not exceed recommended dosage in an attempt to overcome tolerance.

Monitoring Parameters

Weight and waist circumference every month for the first 3 months, then at 3-month intervals (Apovian 2015); blood pressure

Pregnancy Risk Factor

X

Pregnancy Considerations

Use of phentermine is contraindicated during pregnancy (lack of potential benefit and possible fetal harm). Limited information is available about the use of phentermine in pregnancy (Jones 2002; McElhatton 2006). An increased risk of adverse maternal and fetal outcomes is associated with obesity; however, medications for weight loss therapy are not recommended at conception or during pregnancy (ACOG 156 2015).

Patient Education

• Discuss specific use of drug and side effects with patient as it relates to treatment. (HCAHPS: During this hospital stay, were you given any medicine that you had not taken before? Before giving you any new medicine, how often did hospital staff tell you what the medicine was for? How often did hospital staff describe possible side effects in a way you could understand?)

• Patient may experience anxiety, constipation, diarrhea, dry mouth, insomnia, bad taste, decreased libido, sexual dysfunction, or agitation. Have patient report immediately to prescriber behavioral changes, angina, tachycardia, abnormal heartbeat, mood changes, tremors, shortness of breath, swelling of arms or legs, vision changes, severe dizziness, passing out, or severe headache (HCAHPS).

• Educate patient about signs of a significant reaction (eg, wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat). Note: This is not a comprehensive list of all side effects. Patient should consult prescriber for additional questions.

Intended Use and Disclaimer: Should not be printed and given to patients. This information is intended to serve as a concise initial reference for health care professionals to use when discussing medications with a patient. You must ultimately rely on your own discretion, experience, and judgment in diagnosing, treating, and advising patients.

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