Phentermine is a prescription appetite suppressant from the amphetamine family. It was first approved in the US in 1959 and is still sold under brand names such as Adipex-P and Lomaira and as many generics.
The single most important fact about phentermine on its own is how narrow its approval is. The FDA labels allow it only as a short-term (a few weeks) add-on to diet, exercise and behavior change in adults with obesity. It is a Schedule IV controlled substance in the US and a non-specified stimulant (S6.A) prohibited in competition by WADA. A low-dose, extended-release combination with topiramate (Qsymia) is approved for long-term use.
This page summarizes what the FDA labels, clinical trials and regulators report. It is for education only and is not medical advice.
| Property | Detail |
|---|---|
| Drug class | Sympathomimetic amine anorectic (amphetamine-related stimulant) |
| Brands | Adipex-P (Teva), Lomaira (KVK-Tech), many generics; combined with topiramate as Qsymia |
| Route | Oral tablet or capsule |
| Labeled strengths | 37.5 mg phentermine HCl (Adipex-P; = 30 mg base); 8 mg (Lomaira; = 6.4 mg base) |
| Approved use | Short-term (a few weeks) adjunct for obesity: BMI ≥ 30, or ≥ 27 with other risk factors |
| Half-life | About 20 h (phentermine component, Qsymia label) |
| Elimination | Mostly unchanged in urine (62–85% under uncontrolled urine pH) |
| US status | Prescription only; DEA Schedule IV (21 CFR 1308.14) |
| WADA status | S6.A non-specified stimulant, prohibited in competition (2026 List) |
What it is
Phentermine hydrochloride is α,α-dimethylphenethylamine hydrochloride: formula C₁₀H₁₅N·HCl, molecular weight 185.7 (Adipex-P label). Structurally it is amphetamine with a second methyl group on the carbon next to the amine. The labels describe it as “related chemically and pharmacologically” to amphetamine. The Adipex-P label lists an initial US approval in 1959.
The labels state that it has not been established that phentermine works mainly by suppressing appetite, since other central nervous system or metabolic effects may contribute. In laboratory (in vitro) tests, phentermine and other amphetamine-type stimulants released norepinephrine more potently than dopamine or serotonin (Rothman et al., Synapse 2001). Like other drugs of this class, it stimulates the central nervous system and can raise blood pressure. The labels note that tolerance (tachyphylaxis) has been shown with every drug of this class in which it was looked for.
The products
| Product | What it contains | Labeled dosing | Labeled duration |
|---|---|---|---|
| Adipex-P tablet or capsule | 37.5 mg phentermine HCl | 37.5 mg once daily before breakfast or 1–2 h after; some patients 18.75 mg once or twice daily | Short term (a few weeks) |
| Lomaira tablet | 8 mg phentermine HCl | 8 mg three times daily, 30 minutes before meals; half tablet for patients not needing the full dose | Short term (a few weeks) |
| Qsymia extended-release capsule | Phentermine 3.75–15 mg plus topiramate 23–92 mg | Once daily in the morning, titrated (see below) | Long term |
Sources: Adipex-P label, Lomaira label, Qsymia label. FDA approved Qsymia in July 2012 (Drugs@FDA: Qsymia).
Pharmacokinetics
The Adipex-P label reports that phentermine reaches peak blood levels 3–4.4 hours after a dose. The single-ingredient labels do not give a half-life. The more recent Qsymia label has the most detailed data on the phentermine component:
| Parameter | Value (phentermine component, Qsymia label) |
|---|---|
| Time to peak (extended-release capsule, single 15 mg dose) | 6 h |
| Effect of a high-fat meal | None |
| Plasma protein binding | 17.5% |
| Metabolism | Mainly CYP3A4, but not extensive; not metabolized by MAO-A or MAO-B |
| Excreted unchanged in urine | 70–80% of a dose |
| Terminal half-life | About 20 h |
| Accumulation at steady state | About 2.5-fold (AUC and Cmax) |
Because the kidneys clear most of the drug, the Adipex-P label advises caution in renal impairment and older adults. In the Qsymia renal study, phentermine exposure (AUC) was 22%, 45% and 91% higher in mild, moderate and severe renal impairment than in healthy volunteers. The labels also note that making the urine more acidic increases phentermine excretion.
Approved medical use
Phentermine alone
Adipex-P and Lomaira are approved as a short-term (a few weeks) adjunct to a weight-reduction program based on exercise, behavior change and calorie restriction (Adipex-P label; Lomaira label). Eligible patients have:
- an initial body mass index (BMI) of 30 kg/m² or more, or
- a BMI of 27 kg/m² or more with other risk factors such as controlled hypertension, diabetes or high blood lipids.
Other key label points:
- dose should be the lowest effective dose; late-evening doses should be avoided because of insomnia;
- not recommended for children aged 16 or younger;
- if tolerance to the appetite effect develops, the drug should be stopped rather than the dose raised;
- it is approved only as monotherapy. Safety and efficacy with any other weight-loss drug (prescription, over-the-counter or herbal) or with serotonergic drugs such as SSRIs have not been established, and the labels do not recommend combining them.
That last point applies to combinations with newer drugs such as semaglutide, tirzepatide and liraglutide, which the phentermine labels do not cover.
Phentermine/topiramate (Qsymia)
Qsymia combines low-dose phentermine with extended-release topiramate, an anti-seizure drug. Unlike phentermine alone, it is approved for long-term weight reduction and maintenance, together with diet and exercise, in adults with obesity, adults with overweight plus at least one weight-related condition, and children aged 12 and older with obesity (Qsymia label).
The label sets out a stepwise schedule (phentermine mg/topiramate mg):
| Step | Dose | When |
|---|---|---|
| Start | 3.75/23 mg once daily | 14 days |
| Recommended dose | 7.5/46 mg once daily | Review after 12 weeks |
| If under 3% weight loss (adults) | 11.25/69 mg for 14 days, then 15/92 mg | Review after 12 weeks |
| If under 5% weight loss on 15/92 mg | Stop | Taper over at least 1 week (every-other-day dosing) to avoid seizures |
Because topiramate can cause birth defects, Qsymia is distributed through a Risk Evaluation and Mitigation Strategy (REMS) limited to certified pharmacies. The label recommends a negative pregnancy test before starting and monthly during treatment in patients who can become pregnant.
What the research shows
Phentermine alone: modest, short-term evidence
The labels themselves describe the evidence for anorectic drugs as limited. They state that the extra weight loss over placebo is “only a fraction of a pound a week” in trials lasting a few weeks, and that the total effect over diet alone “must be considered clinically limited.”
Controlled data that do exist:
| Study | Design | Result |
|---|---|---|
| Li et al., Ann Intern Med 2005 | Review citing a pooled meta-analysis of phentermine trials | 3.6 kg more weight loss than placebo at 6 months (95% CI 0.6–6.0 kg) |
| Kang et al., Diabetes Obes Metab 2010 | Randomized, double-blind, 12 weeks, n = 74; 30 mg once-daily controlled-release phentermine vs placebo | −8.1 kg vs −1.7 kg (p under 0.001); ≥ 5% weight loss in 95.8% vs 20.8% |
| Aronne et al., Obesity 2013 | Randomized, 28 weeks; phentermine 7.5 or 15 mg, topiramate, the combinations, or placebo | ≥ 5% weight loss: 43.3% (7.5 mg) and 46.2% (15 mg) vs 15.5% on placebo; combinations 62.1–66.0% |
In the Aronne trial, both phentermine/topiramate combinations produced more weight loss (−8.5% and −9.2%) than placebo (−1.7%) and more than either drug alone (p under 0.05).
Longer-term phentermine use
Although phentermine alone is labeled for a few weeks, it is often prescribed for longer. A US electronic health record study followed 13,972 adults who started phentermine in 2010–2015 (Lewis et al., Obesity 2019):
- people who used it continuously for more than 12 months lost 7.4% more body weight at 24 months than those who used it for 3 months or less (p under 0.001);
- the combined outcome of cardiovascular disease or death was rare (0.3%, 41 events), with no significant difference between duration groups.
This was an observational study of a mostly younger (mean age 43.5), lower-risk population. It cannot prove safety in people with heart disease, who are excluded by the label in any case.
Phentermine/topiramate (Qsymia) trials
Two 1-year (56-week) randomized, double-blind trials supported approval: EQUIP (Study 1, BMI ≥ 35) and CONQUER (Study 2, BMI 27–45 with two or more weight-related conditions) (Qsymia label; Gadde et al., Lancet 2011). Results in the intention-to-treat analysis:
| Outcome at 1 year | Study 1: placebo | Study 1: 3.75/23 mg | Study 1: 15/92 mg | Study 2: placebo | Study 2: 7.5/46 mg | Study 2: 15/92 mg |
|---|---|---|---|---|---|---|
| Mean weight change | −1.6% | −5.1% | −10.9% | −1.2% | −7.8% | −9.8% |
| Difference from placebo (95% CI) | — | 3.5% (2.4–4.7) | 9.4% (8.4–10.3) | — | 6.6% (5.8–7.4) | 8.6% (8.0–9.3) |
| Lost ≥ 5% of body weight | 17% | 45% | 67% | 21% | 62% | 70% |
| Lost ≥ 10% of body weight | 7% | 19% | 47% | 7% | 37% | 48% |
Withdrawal before week 56 was high: 40% in Study 1 and 31% in Study 2.
How it compares with newer drugs
A 2026 network meta-analysis of 262 randomized trials (99,791 participants) compared obesity drugs with lifestyle treatment alone at one year (Nong et al., BMJ 2026):
| Drug | Mean extra weight loss at 1 year (95% CI) |
|---|---|
| Tirzepatide | −14.9% (−16.0 to −13.9) |
| Subcutaneous semaglutide | −9.8% (−10.6 to −9.1) |
| Phentermine/topiramate | −8.1% (−9.7 to −6.5) |
The same analysis found that phentermine/topiramate was among the drugs with the highest rates of stopping because of side effects. Subcutaneous semaglutide was the only drug linked to lower all-cause mortality and fewer heart attacks, mainly based on outcome trials in high-risk patients. For phentermine alone, the labels note that the trials they rely on lasted only a few weeks.
History: fen-phen and heart valves
In 1992, a 34-week double-blind trial in 121 people tested extended-release fenfluramine 60 mg plus phentermine resin 15 mg against placebo, added to diet, exercise and behavior therapy. The combination group lost 14.2 kg (15.9%) vs 4.6 kg (4.9%) on placebo (Weintraub et al., Clin Pharmacol Ther 1992). The combination, never FDA-approved as such, became known as “fen-phen.” By 1996, US prescriptions for fenfluramine and phentermine exceeded 18 million (Connolly et al., NEJM 1997).
In 1997, Mayo Clinic doctors described 24 women with unusual valve disease after fen-phen, with valve tissue that looked like the valve damage seen in carcinoid syndrome and with ergot drugs (Connolly et al., NEJM 1997). FDA then requested voluntary withdrawal of fenfluramine and dexfenfluramine from the market; phentermine remained approved (CDC, MMWR 1997).
Later evidence points to fenfluramine, not phentermine, as the cause:
- Population data. A population-based study followed 6,532 dexfenfluramine users, 2,371 fenfluramine users, 862 phentermine-only users and 9,281 untreated obese controls for about four years. All 11 new idiopathic valve cases occurred after fenfluramine or dexfenfluramine; there were none in people who took phentermine alone (Jick et al., NEJM 1998). Because the phentermine group was small, the upper 95% CI limit was still 76.6 cases per 10,000.
- Mechanism. Fenfluramine’s metabolite norfenfluramine has high affinity for, and activates, the serotonin 5-HT2B receptor, as do ergot drugs known to damage valves. The authors concluded that 5-HT2B activation is likely necessary for this valve disease. In the same screen, phentermine was used as a negative control, a drug not associated with valve disease (Rothman et al., Circulation 2000).
The labels remain cautious. They state that the role of phentermine in these valve lesions “has not been established” and that a link with phentermine alone “cannot be ruled out,” citing rare reports in people who said they took only phentermine (Adipex-P label).
Fen-phen era drugs were also linked to primary pulmonary hypertension. In a European case-control study of 95 patients, appetite-suppressant use (mainly fenfluramine derivatives) raised the odds of the disease (OR 6.3, 95% CI 3.0–13.2), and use for more than 3 months raised them further (OR 23.1, 95% CI 6.9–77.7) (Abenhaim et al., NEJM 1996).
Side effects and risks
Cardiovascular
The labels list palpitations, fast heart rate, increased blood pressure and ischemic events among phentermine’s adverse reactions (Adipex-P label). They advise caution even in mild hypertension. Phentermine is contraindicated in people with a history of cardiovascular disease, including coronary artery disease, stroke, arrhythmias, heart failure and uncontrolled hypertension.
Two warnings date from the fen-phen era:
- Primary pulmonary hypertension, a rare and often fatal lung disease. Mostly reported with phentermine plus fenfluramine or dexfenfluramine, with rare reports on phentermine alone. The first symptom is usually shortness of breath; the label says to stop the drug if new unexplained breathlessness, chest pain, fainting or leg swelling appear.
- Valvular heart disease, discussed above.
In the Qsymia program, heart rate rose more than on placebo. In an 8-week ambulatory study, 24-hour average heart rate was 3.6 beats per minute higher (95% CI 2.1–5.2). A rise of more than 20 bpm at any single visit occurred in 19.6% of adults on 15/92 mg vs 11.9% on placebo (Qsymia label). The label states that Qsymia’s effect on cardiovascular events and death has not been established.
Abuse and dependence
Phentermine is a Schedule IV controlled substance (21 CFR 1308.14). The labels warn that amphetamines and related stimulants “have been extensively abused” and contraindicate phentermine in people with a history of drug abuse (Adipex-P label). They describe:
- reports of people raising the dose to many times the recommended amount;
- extreme fatigue, depression and sleep EEG changes after abrupt stopping of prolonged high doses;
- chronic intoxication with severe skin disorders (dermatoses), marked insomnia, irritability, hyperactivity and personality change; the most severe form is a psychosis that can look like schizophrenia.
Evidence on how often this happens in medically supervised use is limited. In one study at a weight-management practice, 269 patients on short- or long-term phentermine (up to 21 years) were interviewed with standard addiction and craving questionnaires. None met criteria for phentermine abuse or psychological dependence, and apart from increased hunger, amphetamine-like withdrawal symptoms were not seen after abrupt stopping (Hendricks et al., Int J Obes 2014). This was a single-practice study with no placebo group, and it does not override the label warnings.
Nervous system and other effects
Listed adverse reactions include overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache and psychosis; dry mouth, unpleasant taste, diarrhea and constipation; hives; and impotence or changes in libido (Adipex-P label). The labels warn that phentermine may impair driving or operating machinery.
With Qsymia, the most common adverse reactions in adults (at least 5% and at least 1.5 times placebo) were tingling (paresthesia), dizziness, altered taste, insomnia, constipation and dry mouth. Paresthesia occurred in 20% on 15/92 mg vs 2% on placebo. Topiramate adds its own warnings: birth defects (oral clefts), suicidal thoughts, acute myopia and angle-closure glaucoma, mood and sleep problems, cognitive slowing, metabolic acidosis, kidney stones, low potassium and seizures on abrupt withdrawal (Qsymia label).
Overdose
Signs of acute overdose include restlessness, tremor, rapid breathing, confusion, aggression, hallucinations and panic, followed by fatigue and depression. Cardiovascular effects include arrhythmia, high or low blood pressure and circulatory collapse. The labels note that overdose of similar drugs has caused fatal poisoning ending in convulsions and coma.
Interactions
| Interacting drug | Label guidance |
|---|---|
| MAO inhibitors | Contraindicated during or within 14 days of MAOI use (risk of hypertensive crisis) |
| Other weight-loss drugs, including OTC and herbal products | Not recommended; safety and efficacy not established |
| SSRIs and other serotonergic drugs | Combination not recommended; safety not established |
| Alcohol | May cause an adverse reaction |
| Insulin and oral diabetes drugs | Dose may need to be reduced as weight falls |
| Adrenergic neuron blocking drugs | Phentermine may reduce their blood-pressure-lowering effect |
Who should not take it
Per the labels, phentermine is contraindicated with a history of cardiovascular disease, hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, breastfeeding, recent MAOI use, or known sensitivity to sympathomimetic amines. The 2012 Adipex-P label lists pregnancy category X: weight loss offers no benefit in pregnancy and may harm the fetus.
Legal status and use outside medicine
United States
Phentermine is listed among the stimulants in Schedule IV of the federal Controlled Substances Act (21 CFR 1308.14). It requires a prescription. The Qsymia label notes that its phentermine component is Schedule IV while topiramate is not controlled.
Sport (WADA)
Phentermine is named in S6.A Non-Specified Stimulants on the WADA 2026 Prohibited List. S6 stimulants are prohibited in competition only. That period runs, by default, from just before midnight on the day before a competition until the end of the competition and sample collection.
A positive test does not always mean phentermine was taken directly. In one reported case, an athlete testing positive for mephentermine and phentermine reported taking an antacid containing oxethazaine, a local anesthetic. A volunteer study confirmed that oxethazaine produces both substances in urine (Huang et al., J Anal Toxicol 2010).
Use in physique sports
Phentermine’s appetite-suppressing effect makes it of interest for dieting phases. No controlled trials have studied it in lean people or athletes. Its approval is limited to people with a BMI of 30 or more (or 27 with risk factors), and the labels do not recommend combining it with any other weight-loss product, including over-the-counter and herbal ones.
Key takeaways
- An amphetamine-related stimulant. Phentermine is a Schedule IV sympathomimetic amine approved in 1959. In lab tests it releases norepinephrine most potently, and its half-life is about 20 hours.
- Approved only for a few weeks on its own. Adipex-P and Lomaira are labeled as short-term add-ons for adults with obesity. The labels call the extra weight loss over diet alone “clinically limited.”
- The combination with topiramate is a long-term drug. Qsymia produced 8.6–9.4% more weight loss than placebo at one year at the top dose, but it requires a pregnancy-prevention program and carries topiramate’s risks.
- Phentermine was not the cause of fen-phen valve disease. Population and receptor studies point to fenfluramine’s 5-HT2B activity. The labels still warn about rare valve and pulmonary hypertension reports with phentermine alone.
- Heart disease and drug abuse history are contraindications. Phentermine raises heart rate and can raise blood pressure, and abuse, dependence and psychosis are labeled risks.
- Banned in competition. WADA lists phentermine as an S6.A non-specified stimulant, prohibited in competition.
Sources
- Teva Pharmaceuticals USA. Adipex-P (phentermine hydrochloride USP) tablets and capsules prescribing information, Rev. 1/2012 (ANDA 085128/S-065). FDA.
- KVK-Tech, Inc. Lomaira (phentermine hydrochloride) tablets 8 mg prescribing information (ANDA 203495). DailyMed.
- Vivus LLC. Qsymia (phentermine and topiramate extended-release capsules) prescribing information, 2026 (NDA 022580). DailyMed.
- Food and Drug Administration. Drugs@FDA: Qsymia, NDA 022580 (original approval July 17, 2012). FDA.
- Drug Enforcement Administration. Schedule IV. 21 CFR 1308.14. eCFR.
- World Anti-Doping Agency. The 2026 Prohibited List, International Standard (in effect January 1, 2026). WADA 2026 Prohibited List.
- Rothman RB, Baumann MH, Dersch CM, et al. Amphetamine-type central nervous system stimulants release norepinephrine more potently than they release dopamine and serotonin. Synapse. 2001;39(1):32–41. PubMed.
- Li Z, Maglione M, Tu W, et al. Meta-analysis: pharmacologic treatment of obesity. Ann Intern Med. 2005;142(7):532–546. PubMed.
- Kang JG, Park CY, Kang JH, et al. Randomized controlled trial to investigate the effects of a newly developed formulation of phentermine diffuse-controlled release for obesity. Diabetes Obes Metab. 2010;12(10):876–882. PubMed.
- Aronne LJ, Wadden TA, Peterson C, et al. Evaluation of phentermine and topiramate versus phentermine/topiramate extended-release in obese adults. Obesity (Silver Spring). 2013;21(11):2163–2171. PubMed.
- Lewis KH, Fischer H, Ard J, et al. Safety and effectiveness of longer-term phentermine use: clinical outcomes from an electronic health record cohort. Obesity (Silver Spring). 2019;27(4):591–602. PubMed.
- Gadde KM, Allison DB, Ryan DH, et al. Effects of low-dose, controlled-release, phentermine plus topiramate combination on weight and associated comorbidities in overweight and obese adults (CONQUER): a randomised, placebo-controlled, phase 3 trial. Lancet. 2011;377(9774):1341–1352. PubMed.
- Nong K, Shi Q, Xie X, et al. Comparative effects of drugs for adults with overweight or obesity: systematic review and network meta-analysis. BMJ. 2026;394:e372161. PubMed.
- Weintraub M, Sundaresan PR, Madan M, et al. Long-term weight control study. I (weeks 0 to 34). The enhancement of behavior modification, caloric restriction, and exercise by fenfluramine plus phentermine versus placebo. Clin Pharmacol Ther. 1992;51(5):586–594. PubMed.
- Connolly HM, Crary JL, McGoon MD, et al. Valvular heart disease associated with fenfluramine-phentermine. N Engl J Med. 1997;337(9):581–588. PubMed.
- Centers for Disease Control and Prevention. Cardiac valvulopathy associated with exposure to fenfluramine or dexfenfluramine: U.S. Department of Health and Human Services interim public health recommendations, November 1997. MMWR Morb Mortal Wkly Rep. 1997;46(45):1061–1066. PubMed.
- Jick H, Vasilakis C, Weinrauch LA, et al. A population-based study of appetite-suppressant drugs and the risk of cardiac-valve regurgitation. N Engl J Med. 1998;339(11):719–724. PubMed.
- Rothman RB, Baumann MH, Savage JE, et al. Evidence for possible involvement of 5-HT(2B) receptors in the cardiac valvulopathy associated with fenfluramine and other serotonergic medications. Circulation. 2000;102(23):2836–2841. PubMed.
- Abenhaim L, Moride Y, Brenot F, et al. Appetite-suppressant drugs and the risk of primary pulmonary hypertension. International Primary Pulmonary Hypertension Study Group. N Engl J Med. 1996;335(9):609–616. PubMed.
- Hendricks EJ, Srisurapanont M, Schmidt SL, et al. Addiction potential of phentermine prescribed during long-term treatment of obesity. Int J Obes (Lond). 2014;38(2):292–298. PubMed.
- Huang WH, Liu CH, Liu RH, Tseng YL. Confirming urinary excretion of mephentermine and phentermine following the ingestion of oxethazaine by gas chromatography-mass spectrometry analysis. J Anal Toxicol. 2010;34(2):73–77. PubMed.