FDA approves phentermine for short-term weight management

Phentermine hydrochloride first received FDA approval in 1959, making it one of the oldest weight-loss drugs still sold in the United States.^[1] The drug is sold under the brand name Adipex-P and generically, and it is classified as a sympathomimetic amine anorectic, a chemical class related to amphetamine.^[1][2]

The FDA label states that phentermine is indicated only as a short-term adjunct, described as "a few weeks," to a weight-reduction plan built around exercise, behavioral changes, and calorie restriction.^[1] It is meant for adults with a body mass index of 30 or higher, or 27 or higher if they also have a risk factor such as controlled high blood pressure, diabetes, or high cholesterol.^[1] The usual adult dose is one 37.5 mg capsule or tablet daily, taken before breakfast or one to two hours after, since taking it late in the day can cause insomnia.^[1][2]

The label carries several specific safety limits. Phentermine is contraindicated in people with a history of cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, agitated states, or drug abuse, and in anyone who used a monoamine oxidase inhibitor in the prior 14 days.^[1] It is not recommended in combination with other weight-loss drugs, including SSRIs such as fluoxetine, sertraline, fluvoxamine, or paroxetine, because that combination's safety and effectiveness have not been established.^[1][2] The label also notes tolerance to phentermine's appetite-suppressing effect usually develops within a few weeks; when that happens, the label says the drug should be stopped rather than the dose increased.^[1][2]

Because of its chemical relationship to amphetamine, phentermine carries a documented risk of abuse and dependence, and prescribers are told to dispense the smallest amount feasible to limit the chance of overdose.^[1][2] The label also flags rare reports of primary pulmonary hypertension and serious valvular heart disease, conditions first tied to the combination of phentermine with fenfluramine or dexfenfluramine in the 1990s, though the label states an association with phentermine used alone cannot be ruled out.^[1][2]

Why it matters for patients

Phentermine's age and mechanism set it apart from newer GLP-1 medicines like semaglutide (Ozempic, Wegovy, Rybelsus) or tirzepatide (Mounjaro, Zepbound). It works on the central nervous system to suppress appetite rather than mimicking gut hormones, and its own prescribing information limits use to a short course of a few weeks, not the ongoing, often years-long treatment typical of GLP-1 drugs.^[1][2] Patients considering phentermine should know it is a controlled substance with documented abuse potential, and that combining it with other weight-loss drugs, including SSRIs, is specifically not recommended on the label.^[1][2] People with any history of heart disease, uncontrolled high blood pressure, hyperthyroidism, or glaucoma are excluded from use under the label's contraindications.^[1]

The sources reviewed for this article, phentermine's FDA label and a clinical monograph, do not include current U.S. retail pricing or data on how phentermine ranks among prescribed weight-loss drugs today, so those specific figures are not confirmed here and should be checked against current pharmacy and market data.

What happens next

Phentermine's label has been revised multiple times since 1959, most recently reflected in a 2012 update on file with the FDA, and it continues to carry warnings about cardiovascular risk, tolerance, and abuse potential that shape how it is prescribed alongside newer obesity medicines.^[1]