• Obesity

Phentermine Generic Name & Formulations

General Description

Phentermine HCl 15mg (equivalent to 12mg phentermine), 30mg (equivalent to 24mg phentermine); powder-filled or pellet-filled caps.

Pharmacological Class


How Supplied

Contact supplier.


Store at 20° to 25° C (68° to 77° F) with excursions permitted between 15° to 30° C (59° to 86° F).

Phentermine Indications


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 ≥30kg/m2, or ≥27kg/m2 in the presence of other risk factors (eg, hypertension, diabetes, hyperlipidemia).

Phentermine Dosage and Administration


>16yrs: Individualize. Usual dose: 15–30mg at approx. 2hrs after breakfast. Severe renal impairment (eGFR 15–29mL/min/1.73m2): max 15mg daily.


≤16yrs: not recommended.

Phentermine Contraindications


History of cardiovascular disease (eg, coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension). Hyperthyroidism. Glaucoma. Agitated states. History of drug abuse. Pregnancy (Cat. X). Nursing. During or within 14 days of MAOIs.

Phentermine Boxed Warnings

Not Applicable

Phentermine Warnings/Precautions


Increased risk of primary pulmonary hypertension, valvular heart disease. Discontinue if unexplained dyspnea, angina, syncope, or lower extremity edema develops. Discontinue if tolerance to the anorectant effect develops. Mild hypertension. Diabetes. Prescribe minimal supply to avoid overdose. Tartrazine allergy (some 15mg caps may contain FD&C yellow #5 dye). Renal impairment (eGFR <15mL/min/1.73m2), including ESRD requiring dialysis: avoid. Elderly.


Primary Pulmonary Hypertension (PPH)

  • PPH has been reported when phentermine was combined with fenfluramine or dexfenfluramine and with phentermine alone.

  • Treatment should be discontinued if patients report symptoms of PPH (eg, unexplained dyspnea, angina, syncope, lower extremity pain); evaluate for possible pulmonary hypertension.

Valvular Heart Disease

  • Cardiac valvular disease has been reported in patients taking a combination of phentermine with fenfluramine or dexfenfluramine.

  • Rare cases of valvular heart disease have been reported with phentermine alone.

Development of Tolerance

  • Recommended dose should not be exceeded in an attempt to increase the effect.

  • Discontinue phentermine if tolerance develops.

Risk of Dependence

  • Consider abuse potential of phentermine when evaluating the desirability of including the drug as part of a weight reduction program.


  • Use caution in patients with mild hypertension as phentermine may increase blood pressure.

Pregnancy Considerations

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.

Nursing Mother Considerations

It is not known if phentermine is excreted in human milk; however, other amphetamines are present in human milk.

Pediatric Considerations

Safety and effectiveness in pediatric patients have not been established. Phentermine is not recommended for pediatric obesity.

Geriatric Considerations

Dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range.

Renal Impairment Considerations

Phentermine was not studied in patients with renal impairment. Use caution when administering phentermine to patients with renal impairment.

Limit phentermine dose to 15mg daily in patients with severe renal impairment (eGFR 15–29mL/min/1.73m2).

Phentermine has not been studied in patients with eGFR <15mL/min/1.73m2, including end-stage renal disease requiring dialysis; avoid use in these populations.

Phentermine Pharmacokinetics


Renal. Half-life: ~20 hours.

Phentermine Interactions


See Contraindications. Hypertensive crisis possible with MAOIs. Concomitant weight loss drugs, including SSRIs (fluoxetine, sertraline, fluvoxamine, paroxetine), OTC and herbal products: not recommended. Caution with alcohol. Insulin or oral hypoglycemic medications requirements may be altered. May decrease hypotensive effect of adrenergic neuron blocking drugs.

Phentermine Adverse Reactions

Adverse Reactions

Primary pulmonary hypertension, regurgitant cardiac valvular disease, palpitation, tachycardia, elevated blood pressure, ischemic events, overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, dry mouth, unpleasant taste, GI upset, constipation, urticaria, impotence, changes in libido; rare: psychotic episodes.

Phentermine Clinical Trials

Clinical Trials

In relatively short-term clinical trials, adult obese patients 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 patients given placebo 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 patients and tends to decrease in succeeding weeks. 

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.

Phentermine Note


Formerly known under the brand name Fastin.

Phentermine Patient Counseling

Patient Counseling

Inform patients that phentermine should be used only for a short period of time (a few weeks) as an adjunct to a weight reduction program based on exercise, behavioral modification, and caloric restriction.

Advise pregnant women and nursing mothers not to use phentermine.

Caution patients about engaging in potentially hazardous tasks as phentermine may impair the ability to operate machinery or drive a motor vehicle.

Inform patients of the risks associated with phentermine use (eg, PPH, valvular heart disease, increased BP, drug interactions); the potential for developing tolerance, and the risk of dependence.

Phentermine should be kept in a safe place to prevent theft, accidental overdose, misuse or abuse.