Phentermine won FDA approval in 1959 and is still labeled for short-term weight loss

The prescribing information for Adipex-P, the brand form of phentermine hydrochloride, lists an initial U.S. approval date of 1959 ^[1]. The drug was cleared as a short-term adjunct — a few weeks — to a weight-reduction regimen built on exercise, behavioral modification and caloric restriction for people with exogenous obesity ^[1].

The label states plainly that "the limited usefulness of agents of this class, including ADIPEX-P®, should be measured against possible risk factors inherent in their use" ^[1].

What the label allows

Phentermine is a sympathomimetic amine anorectic, a stimulant-type appetite suppressant ^[1]. The approved indication covers adults with an initial body mass index of 30 kg/m² or higher, or 27 kg/m² or higher when other risk factors are present, such as controlled hypertension, diabetes or hyperlipidemia ^[1]. An ASHP monograph on the drug classifies it as an amphetamine derivative and says it is for short-term monotherapy only ^[2].

Adipex-P is supplied as 37.5 mg phentermine hydrochloride capsules and tablets, equivalent to 30 mg of phentermine base, and is a Schedule IV controlled substance ^[1]. The label directs that dosage be individualized to the lowest effective dose and that late-evening administration be avoided because of the risk of insomnia ^[1][2]. The label also states phentermine is not recommended for children 16 years of age and younger ^[1].

The safety record in the label

The most serious warnings involve the heart and lungs. Rare cases of primary pulmonary hypertension, described in the label as a rare and frequently fatal disease of the lungs, have been reported in patients taking phentermine in combination with fenfluramine or dexfenfluramine, and the label says an association with phentermine alone cannot be ruled out ^[1][2]. Serious regurgitant cardiac valvular disease has also been reported with those combinations, primarily affecting the mitral, aortic or tricuspid valves, and rare cases have been reported in people who reportedly took phentermine alone ^[1][2]. Patients are told to report new, unexplained shortness of breath, chest pain, fainting or leg swelling, and the drug should be discontinued in those cases ^[1].

The label also states that tolerance to the appetite-suppressing effect usually develops within a few weeks, and that the dose should not be raised to try to restore it — the drug should be stopped instead ^[1][2]. Phentermine is related chemically and pharmacologically to amphetamine, carries a risk of abuse and dependence, and may impair driving or the operation of machinery ^[1]. Alcohol use with the drug may cause an adverse reaction ^[1].

Contraindications include a history of cardiovascular disease, hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, and use within 14 days of a monoamine oxidase inhibitor ^[1]. The label advises caution even in mild hypertension and notes that insulin or oral diabetes medication requirements may change ^[1]. Combining phentermine with any other weight-loss product, including SSRIs and MAO inhibitors, is not recommended because safety and efficacy have not been established ^[1][2].

Why it matters for patients

Phentermine's approved role is narrow. It is not framed as an open-ended treatment: the label limits it to a few weeks, warns that its effect fades with tolerance, and directs that it be stopped rather than increased when that happens ^[1][2]. The contraindication list rules out many people who might otherwise ask about it, including anyone with a history of cardiovascular disease, glaucoma, hyperthyroidism, uncontrolled hypertension or pregnancy ^[1].

Because it is a Schedule IV controlled substance, the label instructs that the smallest feasible amount be prescribed or dispensed at one time to reduce the chance of overdose ^[1]. Its label does not recommend stacking it with other weight-loss drugs or with certain antidepressants ^[1][2]. Neither the labeling nor the ASHP monograph compares phentermine with semaglutide, tirzepatide or other GLP-1 medicines, so no head-to-head information appears in these documents ^[1][2].

How many phentermine prescriptions are filled in the United States today, and what patients typically pay out of pocket, are not stated in the label or the monograph ^[1][2].

What happens next

The Adipex-P label described here was revised in January 2012 and carries a notice that it may not be the latest version approved by the FDA ^[1]. The ASHP monograph was medically reviewed on Nov. 10, 2025 ^[2]. Neither document describes new indications, formulation changes or pending FDA actions for phentermine ^[1][2].