What Is Phentermine? Uses, Label Limits and How It Compares

Phentermine is a prescription stimulant, chemically related to amphetamine, used for short-term weight loss. FDA first approved it in 1959, and its label limits it to “a few weeks” alongside diet, exercise and behavior change, for adults whose BMI is 30 and up, or 27 and up when a risk factor such as high blood pressure is present.^[1] It is a Schedule IV controlled substance and, by a 2024 review, still the most commonly prescribed obesity drug in the US, even though little rigorous research supports using it for long periods.^[1][7]

What is phentermine used for?

Phentermine has one approved use, the management of obesity, and only for a short stretch of a larger weight-loss plan. The label calls it an adjunct, meaning it is meant to sit on top of calorie reduction, exercise and behavior change. Treating a BMI of 27 to 29.9 requires another risk factor, and the label’s examples are controlled high blood pressure, diabetes and high cholesterol.^[1] Even within that use, the label asks prescribers to weigh what it calls the drug’s “limited usefulness” against its risks.^[1]

It is a daily pill. It is not an injection, and it is not a GLP‑1. People often group it with Ozempic and Zepbound because all three are prescribed for weight, but the drugs belong to different classes and were approved on very different evidence. The two families are set side by side in phentermine vs a GLP-1.

Is phentermine a stimulant or a controlled substance?

It is both. The label describes phentermine as a sympathomimetic amine, a drug that mimics the body’s adrenaline-type signals, and says it is related chemically and pharmacologically to amphetamine.^[1] Federal law lists it among Schedule IV stimulants.^[6] That is why a prescription is required and the label tells prescribers to dispense the smallest workable amount.^[1]

The label warns that the possibility of abuse should be kept in mind.^[1] One direct study found no abuse or psychological dependence among 117 people who had taken it long term, though it had no comparison group of people never given the drug.^[12] Being related to amphetamine also explains why it can complicate a urine screen, a question that turns on how long phentermine stays in your system.

What are the brand names and doses?

Most phentermine is sold as a generic. Lomaira is a lower-strength branded tablet. Adipex-P is the name many people still search for, but as of November 2026 DailyMed lists no label under it, so the labels cited here are the generic and Lomaira ones.

The label says to use the lowest dose that works and to avoid late-evening doses because of insomnia. It sets a 15 mg daily ceiling for people with severe kidney impairment, and it does not recommend phentermine for anyone 16 or younger.^[1] The label also says what to do when the effect fades. Tolerance usually develops within a few weeks, and at that point the drug should be stopped rather than the dose raised.^[1]

How does phentermine work, in brief?

Nobody has pinned this down exactly. Qsymia’s label says phentermine’s effect is likely mediated by releasing signaling chemicals in the hypothalamus, which lowers appetite, while the older phentermine label says appetite suppression has not been established as its main action.^[3][1] The evidence behind each statement, and whether the drug burns fat, belong to the longer question of how phentermine works.

How much weight do people lose on phentermine?

Modest amounts over short periods. The label says the extra loss over placebo in the older trials was only a fraction of a pound a week.^[1] In a larger 28-week trial, 46.2% of people on phentermine 15 mg reached 5% weight loss, compared with 15.5% on placebo.^[11] Real-world phentermine results and the long-term cohort are a separate subject.

Who should not take phentermine?

The list of contraindications on the label is long, and several items are common in the people most likely to be offered a weight-loss drug.^[1]

• A history of heart disease: coronary artery disease, stroke, arrhythmia, congestive heart failure or uncontrolled high blood pressure.
• An MAO inhibitor taken now or within the past 14 days.
• Hyperthyroidism or glaucoma.
• Agitated states or a history of drug abuse.
• Pregnancy or nursing.
• A known allergy or unusual reaction to this type of drug.

The label separately urges caution in anyone with even mild high blood pressure, says insulin or diabetes pills may need to be reduced, warns that alcohol can cause an adverse reaction, and says the drug can impair driving or operating machinery.^[1]

What are the side effects of phentermine?

Two of those, pulmonary hypertension and valve disease, trace to the fen-phen era. They were reported in people taking phentermine with fenfluramine or dexfenfluramine, and the label adds that rare cases have occurred on phentermine alone, so a link cannot be ruled out. New breathlessness, chest pain, fainting or leg swelling is the label’s signal to stop and be evaluated.^[1]

Why is phentermine approved for only a few weeks?

Its approval dates from an era that did not require long trials. The label says the studies behind it lasted a few weeks while obesity runs over years, and therefore calls the overall impact clinically limited.^[1] Authors of a 2026 trial protocol put it directly: phentermine’s long-term efficacy and safety have never been tested in a randomized trial because its approval predates that requirement.^[13]

That gap may close soon. The LEAP trial has randomized 870 adults to phentermine 24 mg or placebo for 24 months.^[13] As of November 10, 2026, ClinicalTrials.gov lists it as completed, with a primary completion date of September 30, 2026, and no results posted.^[14] Until its results are published, doctors who prescribe phentermine for longer are going beyond the label on observational data, a practice the 2024 review describes as an option in selected patients where local rules allow it.^[7]

How does phentermine compare with other weight-loss drugs?

Guidelines reflect that ordering. The American College of Physicians’ 2026 guideline ranks semaglutide and tirzepatide first, phentermine–topiramate second, liraglutide third and naltrexone–bupropion fourth, and it does not list phentermine alone.^[8] A 2026 statement from three US obesity groups did address phentermine, placing it among the drugs with conditional rather than strong recommendations.^[9] Every appetite-suppressing drug on the US market is compared in FDA-approved appetite suppressants. Phentermine’s label advises against adding it to any other weight-loss drug, and what is known about that pairing is in phentermine and a GLP-1 together.^[1]