Phentermine is the oldest and cheapest of the prescription weight-loss drugs, and its own label is unusually blunt about its limits. It is approved only as a short-term aid, "a few weeks", and the label says the extra weight lost over placebo in its trials is a fraction of a pound a week. Newer data measured more than that: a 28-week trial and a 13,972-person health-record study both found larger effects with longer use. None of it approaches the placebo-adjusted results in the GLP-1 trials, and none of those trials compared the two directly.
This page reads the US labels for generic phentermine 37.5 mg (Aurolife, effective 10 June 2026) and Lomaira 8 mg (effective 21 May 2026), read on 3 November 2026, with the trials and safety records behind them. It sets the GLP-1 figures this site already reports beside phentermine's, each with its population, and does not rank them. The other medicines people compare with GLP-1s are on GLP-1 alternatives.
What phentermine is
Phentermine hydrochloride is a sympathomimetic amine — a stimulant chemically and pharmacologically related to amphetamine. The label says its class is known as "anorectics" and that "it has not been established" that its main action in obesity is appetite suppression; other central nervous system or metabolic effects may be involved.
| Product | Strength | What Drugs@FDA records |
|---|---|---|
| Generic phentermine tablets and capsules | 37.5 mg (also 15 mg and 30 mg capsules) | many abbreviated applications; 37.5 mg phentermine hydrochloride equals 30 mg of phentermine base |
| Adipex-P (Teva) | 37.5 mg tablet and capsule | ANDA 085128, approved 22 October 1980 |
| Lomaira | 8 mg tablet | ANDA 203495, approved 12 September 2016 |
| Qsymia (with topiramate) | 3.75 to 15 mg phentermine | NDA 022580, approved 17 July 2012 — a different, long-term product with its own page: Qsymia |
Strengths are listed as product facts, not as doses for anyone.
Who the label covers, and for how long
The indication is a short-term (a few weeks) adjunct to a programme of exercise, behaviour change and calorie restriction, in people with:
- a BMI of 30 or more, or
- a BMI of 27 or more with another risk factor (the label's examples: controlled hypertension, diabetes, high cholesterol).
The label adds that tolerance to the appetite effect "usually develops within a few weeks", and that when it does, the drug should be stopped rather than the dose raised. It is not recommended for children aged 16 or younger. The 37.5 mg label limits use in severe kidney impairment (eGFR 15–29) to 15 mg a day.
Who should not use it, per the label: a history of cardiovascular disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled hypertension); use of an MAO inhibitor within 14 days; hyperthyroidism; glaucoma; agitated states; a history of drug abuse; pregnancy; nursing; and hypersensitivity to sympathomimetic amines. The label also says combining it with any other weight-loss product, prescription or herbal, or with SSRIs such as fluoxetine or sertraline, "is not recommended" because its safety has not been established.
What the trials measured
The label's own clinical-studies section does not report a trial table. It says that in "relatively short-term" trials, people on anorectic drugs lost more weight than people on placebo, that the extra amount was "only a fraction of a pound a week", that the effect was largest in the first weeks and tapered, and that because obesity runs over years and the trials over weeks, the drugs' total impact over diet alone "must be considered clinically limited". That wording dates from an older generation of evidence, and two later studies measured more:
| Study | Design and population | Length | Result |
|---|---|---|---|
| Aronne et al., Obesity 2013 | Randomised, double-blind; adults with obesity; phentermine alone, topiramate alone, the combination, or placebo; all given lifestyle counselling | 28 weeks | Lost ≥5% of weight: 46.2% on phentermine 15 mg, 43.3% on 7.5 mg, 15.5% on placebo; the 15 mg/92 mg combination −9.2% vs −1.7% on placebo |
| Lewis et al., Obesity 2019 | Health-record cohort, 13,972 adults starting phentermine in 2010–2015 at Kaiser Permanente and partner systems; 84% women; mean BMI 37.8 | up to 3 years | Continuous use >12 months: 7.4 percentage points more weight lost at 24 months than use ≤3 months; heart disease or death in 0.3% (41 events), no difference between groups |
The two studies answer different questions. The trial shows phentermine alone moved more people past the 5% mark than placebo over about six months. The cohort shows that people who stayed on it longer lost more, but in an observational study the people who stay on a drug are the ones it is working for, so the 7.4-point gap cannot be read as the effect of the extra months. The authors acknowledge "the limitations of the observational design".
What is missing is as telling: no randomised trial of phentermine alone runs beyond about six months, and its US label carries no trial table at all. The label's wording ("a few weeks") and the way it is sometimes used (months or longer) do not line up, and the label has not been changed to reflect the longer data.
Beside the GLP-1 and Qsymia figures
No trial compares phentermine with a GLP-1 medicine. These are the closest numbers each has, from separate trials:
| Medicine (trial) | Population | Weeks | Medicine | Placebo |
|---|---|---|---|---|
| Phentermine 15 mg alone (Aronne 2013) | Adults with obesity | 28 | 46.2% lost ≥5% | 15.5% lost ≥5% |
| Qsymia 15 mg/92 mg (label Study 1) | Adults with BMI ≥35, no diabetes | 56 | −10.9% | −1.6% |
| Semaglutide 2.4 mg (STEP 1) | Adults with BMI ≥30, or ≥27 with a related condition, no diabetes; 1,961 people | 68 | −14.9% | −2.4% |
| Tirzepatide 5 / 10 / 15 mg (SURMOUNT-1) | Adults with obesity or overweight plus a related condition, no diabetes; 2,539 people | 72 | −15.0% / −19.5% / −20.9% | −3.1% |
Sources: Aronne 2013; Qsymia label section 14; STEP 1, NEJM 2021; SURMOUNT-1, NEJM 2022. Phentermine's published result is a share of people past 5%, not a mean percentage, so it cannot be placed on the same scale; the trials also differ in length, starting weight and how dropouts were counted. The head-to-head record for the two GLP-1 molecules is on tirzepatide vs semaglutide.
The fen-phen history
Phentermine was the "phen" in fen-phen. In 1997, Mayo Clinic cardiologists reported 24 women with unusual heart-valve disease after fenfluramine–phentermine therapy; eight also had pulmonary hypertension, and five had needed heart surgery (Connolly et al., NEJM 1997). The paper notes the combination was never FDA-approved, yet US prescriptions for the two drugs exceeded 18 million in 1996.
On 15 September 1997, FDA asked for fenfluramine (Pondimin) and dexfenfluramine (Redux) to be withdrawn because of valvular heart disease, as FDA's later Federal Register notice records (Federal Register, 29 September 2015). Phentermine was not withdrawn. Its label today says the role of phentermine in those valve problems "has not been established", but that a link between phentermine alone and valvular heart disease or primary pulmonary hypertension "cannot be ruled out", with rare cases reported in people who said they had used phentermine alone. The full side-effect record is on phentermine side effects.
Dependence, blood pressure and other warnings
- Abuse and dependence. Schedule IV. The label notes that amphetamines and related stimulants "have been extensively abused" and that stopping abruptly after prolonged high-dose use causes extreme fatigue and depression.
- Blood pressure and heart rate. Palpitations, a fast heart rate, raised blood pressure and ischaemic events are on the label's adverse-reaction list. How GLP-1 medicines affect blood pressure is on GLP-1 and blood pressure.
- Driving and machinery. The label warns phentermine may impair the ability to do hazardous tasks.
- Alcohol, insulin and diabetes pills. Alcohol may cause an adverse reaction; insulin and oral diabetes-drug needs may change.
- Pregnancy. Contraindicated; the label says weight loss offers no benefit in pregnancy and may harm a fetus.
Is it the same as Adderall?
No. Both belong to the amphetamine family of stimulants, which is why the comparison comes up. Adderall contains amphetamine salts, is approved for ADHD, and is a Schedule II controlled substance (Adderall XR label, effective 30 April 2026). Phentermine is a different molecule, approved only for short-term weight loss, and is Schedule IV, a less tightly controlled category than Schedule II.
What it costs
Phentermine is a cheap generic. The National Average Drug Acquisition Cost survey (NADAC, Centers for Medicare & Medicaid Services), which records what US pharmacies pay, listed for the week of 21 October 2026:
| Product | Pharmacy cost per unit | 30 units |
|---|---|---|
| Generic phentermine 37.5 mg tablet | $0.074 | $2.21 |
| Generic phentermine 37.5 mg capsule | $0.105 | $3.16 |
| Generic phentermine 15 mg capsule | $0.103 | $3.10 |
| Lomaira 8 mg tablet | $0.628 | $18.83 |
| Generic phentermine/topiramate ER 15 mg/92 mg (Qsymia's generic) | $2.696 | $80.89 |
Our arithmetic from NADAC unit costs. These are acquisition costs, not what a pharmacy charges; counter prices are higher and differ by pharmacy, and a prescriber visit is extra. The GLP-1 self-pay ladders, for comparison, are on GLP-1 cost.
What the record does not show
- A randomised trial of phentermine alone beyond about six months. The longer evidence is observational.
- Phentermine's effect on heart attacks, strokes or deaths in a trial. None has been run; the Lewis cohort found no difference, but cohorts cannot exclude one.
- Whether combining it with a GLP-1 medicine adds anything, or is safe. No trial is on either label, and phentermine's label advises against combining it with other weight-loss drugs.
Phentermine is a controlled prescription medicine. Whether it, a GLP-1 medicine or anything else suits a person's heart, blood pressure and history is a decision for their prescriber. The older off-label pills people weigh alongside it are on metformin for weight loss and new weight-loss drugs.
Sources
- Phentermine hydrochloride tablets USP 37.5 mg, US prescribing information, Aurolife Pharma, effective 10 June 2026, set ID 5927e0f6-3de3-4885-a633-5b92003f6fe4 (openFDA drug label record), read 3 November 2026 — sections 1, 2, 4, 5, 6, 9.
- LOMAIRA (phentermine hydrochloride) 8 mg, US prescribing information, effective 21 May 2026, set ID 0718f0f3-77f2-45d7-be12-1ed718c8b806 (openFDA), read 3 November 2026 — Indications, Clinical Studies, Warnings, Drug Abuse and Dependence.
- Drugs@FDA (openFDA): ANDA 085128 (Adipex-P, Teva, approved 22 October 1980); ANDA 203495 (Lomaira, approved 12 September 2016); NDA 022580 (Qsymia, approved 17 July 2012). Read 3 November 2026.
- Aronne LJ et al. "Evaluation of phentermine and topiramate versus phentermine/topiramate extended-release in obese adults." Obesity 2013;21(11):2163–71, doi:10.1002/oby.20584, PMID 24136928.
- Lewis KH et al. "Safety and Effectiveness of Longer-Term Phentermine Use: Clinical Outcomes from an Electronic Health Record Cohort." Obesity 2019;27(4):591–602, doi:10.1002/oby.22430, PMID 30900410.
- Connolly HM et al. "Valvular heart disease associated with fenfluramine-phentermine." NEJM 1997;337(9):581–8, doi:10.1056/NEJM199708283370901, PMID 9271479.
- Federal Register, 29 September 2015, "Determination That PONDIMIN (Fenfluramine Hydrochloride) Tablets… Were Withdrawn From Sale for Reasons of Safety or Effectiveness," document 2015-24619 (govinfo), read 3 November 2026.
- Wilding JPH et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." NEJM 2021, PMID 33567185. Jastreboff AM et al. "Tirzepatide Once Weekly for the Treatment of Obesity." NEJM 2022, PMID 35658024.
- QSYMIA prescribing information, Vivus, effective 22 April 2026, set ID 40dd5602-53da-45ac-bb4b-15789aba40f9 — section 14.
- ADDERALL XR prescribing information, effective 30 April 2026, set ID aff45863-ffe1-4d4f-8acf-c7081512a6c0 — section 9.1.
- NADAC (National Average Drug Acquisition Cost) 2026, Centers for Medicare & Medicaid Services, data.medicaid.gov, as-of date 21 October 2026, read 3 November 2026.