Indications for: Phentermine

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).

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.

Adult Dosage:

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

Children Dosage:

≤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 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.

Phentermine Classification:


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.

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.


Formerly known under the brand name Fastin.

Drug Elimination:

Renal. Half-life: ~20 hours.

How Supplied:

Contact supplier.