Phentermine vs GLP-1 Drugs: Cost, Weight Loss, and Who Fits Which
Phentermine is a short-term oral appetite suppressant that averaged $15 out of pocket per 30-day fill in 2025, while GLP-1s like Wegovy and Zepbound cost far more — in a 72-week head-to-head trial, Wegovy produced 13.7% and Zepbound 20.2% average weight loss. Here is how they compare.
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Phentermine and GLP-1 drugs such as Wegovy and Zepbound are both prescription options for weight loss, but they sit at opposite ends on price, duration, and results. Phentermine is an oral appetite suppressant — the Adipex-P label dates its initial US approval to 1959 [5] — and it averaged $15 out of pocket per 30-day fill in 2025 [8]. Zepbound (tirzepatide) is a once-weekly injection, and Wegovy (semaglutide) is the semaglutide product prescribed for weight loss [4]. They averaged over $110 and $75 out of pocket per 30-day fill respectively in 2025 [8], and in a 72-week head-to-head trial they produced 20.2% and 13.7% average weight loss [4].
The other headline difference is time. The phentermine label describes it as a short-term adjunct — “a few weeks” — used alongside exercise, behavior change, and calorie restriction [5], and a phentermine patient information sheet puts the usual duration at 12 weeks or less [6]. Zepbound is FDA-approved for adults with obesity, or overweight plus a weight-related condition, to lose weight and keep it off, and the head-to-head trial comparing it with Wegovy ran 72 weeks [4].
How does phentermine work compared with Wegovy or Zepbound?
Phentermine is a sympathomimetic amine anorectic — a stimulant-type drug related to amphetamine [2][5]. It is described as an appetite suppressant that works in the brain [6]. It is a Schedule CIV federally controlled substance because it may be abused or lead to dependence [5][6].
Wegovy’s semaglutide is a GLP-1 receptor agonist, and Zepbound’s tirzepatide is a once-weekly dual GIP and GLP-1 receptor agonist [4]. Both GIP and GLP-1 receptors sit in areas of the brain that regulate appetite, and tirzepatide lowers calorie intake, with effects likely mediated through appetite [4]. So all three drugs reduce appetite — the difference is that phentermine does it through stimulant pathways, while the injectables mimic gut hormones.
Day to day, the drugs also feel different. The Adipex-P label describes one 37.5 mg capsule or tablet daily, taken before breakfast or 1 to 2 hours after, with a half tablet (18.75 mg) once or twice daily possible for some patients; late-evening dosing is avoided because of insomnia risk [5]. The Drugs.com monograph lists the phentermine resin complex at a usual dosage of 15 or 30 mg once daily, and phentermine HCl 8 mg three times daily, given 30 minutes before meals [2]. Zepbound is a once-weekly injection, and both Zepbound and Wegovy are used with a reduced-calorie diet and increased physical activity [4].
One practical limit on phentermine: tolerance to its appetite-suppressing effect usually develops within a few weeks. The label says if that happens, phentermine should be discontinued and the recommended dose should not be exceeded [5].
How much weight does each one produce?
The strongest numbers available are for the injectables, from SURMOUNT-5 — a 72-week, open-label Phase 3b trial that randomized 751 adults with obesity, or overweight plus a weight-related comorbidity and no diabetes, across the US and Puerto Rico [4]. Participants were counseled on a reduced-calorie diet and increased physical activity, and took maximum tolerated doses: tirzepatide 10 or 15 mg, or semaglutide 1.7 or 2.4 mg [4].
| Result at 72 weeks | Zepbound (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Average weight loss | 20.2% | 13.7% |
| Average pounds lost | 50.3 lb (22.8 kg) | 33.1 lb (15.0 kg) |
| Reached ≥10% loss | 81.6% | 60.5% |
| Reached ≥15% loss | 64.6% | 40.1% |
| Reached ≥20% loss | 48.4% | 27.3% |
| Reached ≥25% loss | 31.6% | 16.1% |
| Waist circumference | −18.4 cm (7.2 in) | −13.0 cm (5.1 in) |
Tirzepatide’s 20.2% versus semaglutide’s 13.7% represented 47% greater relative weight loss [4].
For phentermine, the label reports no comparable weight-loss percentage. It positions phentermine only as a short-term adjunct to diet, exercise, and behavior change, and states that the limited usefulness of drugs in this class should be weighed against the possible risks [5]. Because the label carries no efficacy figure for phentermine alone, a side-by-side weight-loss number against the injectables cannot be drawn from it.
How long can you take phentermine versus a GLP-1?
Phentermine is labeled for short-term use only — “a few weeks” — as an adjunct in a weight-reduction regimen [5]. A phentermine patient information sheet states the usual duration of treatment is 12 weeks or less [6], and some states allow a doctor to prescribe only one month at a time [6].
The label also states that phentermine is indicated only as short-term monotherapy, and that combining it with any other weight-loss product — prescription, over-the-counter, or herbal — has not been established as safe or effective and is not recommended [5].
The injectables are built around longer horizons. Zepbound is FDA-approved for adults with obesity, or overweight plus at least one weight-related medical problem, to lose weight and keep it off, and it is also approved for moderate-to-severe obstructive sleep apnea in adults with obesity [4]. SURMOUNT-5 ran 72 weeks, and studies of tirzepatide in chronic kidney disease and in obesity-related morbidity and mortality are ongoing [4].
Phentermine’s label applies to adults with a starting body mass index of 30 or higher, or 27 or higher in the presence of other risk factors such as controlled hypertension, diabetes, or hyperlipidemia [5].
What does each one cost?
On average, phentermine costs far less out of pocket than the injectables. In 2025, the average amount spent out of pocket per 30-day fill was $15 for phentermine, compared with $75 for Wegovy, over $110 for Zepbound, $112 for Qsymia, $76 for Saxenda, $44 for Mounjaro, and $35 for Ozempic [8]. Without insurance, brand-name GIP/GLP-1 prescriptions run over $1,400 at retail [8], and the average insured Zepbound user can expect over $1,400 a year in copays [8].
GoodRx lists these average cash prices and coupon lows for common phentermine products (prices vary by pharmacy, state, and date, and coupon use on a controlled substance is restricted in some states) [7]:
| Phentermine product | Average cash price | GoodRx low price |
|---|---|---|
| 37.5 mg, 30 tablets | $37.81 | $14.40 |
| 37.5 mg, 30 capsules | $38.04 | $16.50 |
| 30 mg, 30 capsules | $38.33 | $14.70 |
| 15 mg, 30 capsules | $40.69 | $16.50 |
| Lomaira 8 mg, 90 tablets | $77.02 | $29.24 |
A manufacturer savings card exists for brand-name Lomaira [7].
Does insurance cover phentermine more easily than GLP-1s?
Yes, by a wide margin. As of January 2026, 63.0% of people with commercial insurance had unrestricted coverage of phentermine, 21.0% had restricted coverage, and 16.0% had none [8]. For the injectables prescribed for weight loss, unrestricted coverage was 3.9% for Zepbound (with 56.4% having no coverage) and 9.4% for Wegovy (21.2% no coverage) [8].
By plan type, 90% of commercial-plan enrollees had coverage of phentermine 37.5 mg tablets, versus 27.5% on ACA marketplace plans, 3% on Medicare, and 51.2% on Medicaid, according to Managed Markets Insight & Technology data as of May 12, 2025 [7]. Prior authorization was required for 16.3% of commercial enrollees and 36% of Medicaid enrollees, while step therapy was rare (0% to 2.1%) [7]. Medicare is barred from covering medications solely for weight loss [8].
Use patterns reflect this. Zepbound fills have grown over 600% since the beginning of 2024 and Wegovy’s fill rate has doubled since the start of 2024, while phentermine fills declined slightly in the last year [8].
What are the side effects of each?
Phentermine’s common side effects are dry mouth, restlessness, difficulty sleeping, increased heart rate, and increased blood pressure [6]. The label reports adverse events across the cardiovascular, central nervous, gastrointestinal, allergic, and endocrine systems [5]. It may impair the ability to drive or operate machinery, can slow thinking and motor skills, and may affect vision [5][6]. Alcohol should not be drunk while taking phentermine because of the risk of an adverse reaction [5][6].
Two rare but serious risks appear on the phentermine label. Primary pulmonary hypertension (PPH), a frequently fatal lung disease, has been reported in patients taking phentermine combined with fenfluramine or dexfenfluramine, and the possibility of an association with phentermine alone cannot be ruled out; the drug should be stopped if new, unexplained shortness of breath, chest pain, fainting, or lower-leg swelling occurs [5][6]. Rare cases of serious regurgitant valvular heart disease affecting the mitral, aortic, and/or tricuspid valves have also been reported, and the role of phentermine alone has not been established [5][6]. The label also urges caution even in patients with mild hypertension, because of the risk of blood pressure increases [5].
For people with type 2 diabetes on diabetes medications, weight loss itself can cause hypoglycemia; blood sugar should be tested before and during treatment, and insulin or oral hypoglycemic doses may need reduction [5][6].
On the injectable side, adverse events in SURMOUNT-5 were primarily gastrointestinal and generally mild to moderate — commonly nausea, diarrhea, vomiting, constipation, and abdominal pain [4]. Discontinuation due to adverse events was 6.1% for tirzepatide versus 8.0% for semaglutide, though the trial was not powered to compare safety between the two [4]. Zepbound’s listed serious risks include thyroid tumors including thyroid cancer, pancreatitis, gallbladder problems, kidney problems from dehydration, severe allergic reactions, hypoglycemia when combined with insulin or sulfonylureas, vision changes in type 2 diabetes, mood changes or suicidal thoughts, and aspiration of food during anesthesia or deep sedation [4].
Who should not take phentermine?
The label lists these contraindications: a history of cardiovascular disease (coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension), use during or within 14 days of MAO inhibitors (risk of hypertensive crisis), hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, nursing, and hypersensitivity to sympathomimetic amines [5][6]. The Drugs.com monograph adds moderate-to-severe hypertension and advanced arteriosclerosis [2].
Phentermine is not recommended for children 16 and under, and caution is advised in older adults and in people with reduced kidney function, since the drug is substantially excreted by the kidneys [5]. The label and monograph also advise avoiding SSRIs (fluoxetine, sertraline, fluvoxamine, paroxetine), other anorectic agents such as phendimetrazine, and over-the-counter or herbal weight-loss products [2][5].
Who should not take Zepbound (tirzepatide)?
Zepbound should not be used by anyone with a personal or family history of medullary thyroid carcinoma, with Multiple Endocrine Neoplasia syndrome type 2, or who has had a serious allergic reaction to tirzepatide [4]. It should not be combined with other tirzepatide-containing products or any GLP-1 receptor agonist, and its safety and effectiveness in children are not known [4].
Was phentermine part of the fen-phen problem?
The history is on the label. PPH and serious regurgitant valvular heart disease were reported in patients taking phentermine combined with fenfluramine or dexfenfluramine — the “fen” in fen-phen — though the label states that the possible role of phentermine alone has not been established and cannot be ruled out [5]. Fenfluramine and dexfenfluramine are no longer commercially available in the US [2].
The monograph recommends a medical history and cardiovascular exam for anyone who received fenfluramine or dexfenfluramine, alone or with phentermine, with an echocardiogram if there are signs or symptoms of valvular disease [2]. Phentermine itself remains on the market, and its label retains the monotherapy-only wording [5].
What about Qsymia as a middle option?
Qsymia is a combination of phentermine and topiramate, and it is listed among phentermine alternatives that insurers may cover [7]. It sits between the two approaches on cost: the average out-of-pocket spend in 2025 was $112 per 30-day fill [8]. Commercial coverage as of January 2026 was 15.9% unrestricted, 65.7% restricted, and 18.4% none [8].
Which one tends to fit which situation?
Phentermine’s profile favors people whose main constraint is cost or coverage — $15 average out-of-pocket spend per 30-day fill and 63.0% unrestricted commercial coverage [7][8] — who prefer a pill, and who are planning a short, defined course, since the usual duration is 12 weeks or less [6].
The GLP-1 and GIP/GLP-1 injectables favor people seeking a large reduction: 13.7% to 20.2% of body weight at 72 weeks in the head-to-head trial [4]. Zepbound also carries a separate approval for moderate-to-severe obstructive sleep apnea with obesity [4], and tirzepatide and semaglutide are marketed for type 2 diabetes as Mounjaro and Ozempic [4].
Health history can decide the question. Cardiovascular disease, uncontrolled hypertension, hyperthyroidism, or glaucoma rule out phentermine per its label [5]. A personal or family history of medullary thyroid carcinoma or MEN 2 rules out tirzepatide [4].
For anyone weighing a move from phentermine to a GLP-1, the phentermine label does not support combining it with any other weight-loss drug [5], and both drug types are prescribed alongside a reduced-calorie diet and increased physical activity [4][5].
What should I check or ask before starting either one?
Before assuming a price, check the plan formulary and summary of benefits — these are usually available online or through the number on the insurance card [7]. If a drug is not covered, options include a letter of medical necessity, a formulary exception, an appeal, a discount card, or asking about an alternative agent the plan does cover [7].
Anyone taking Zepbound should tell all healthcare providers about it before surgery or any procedure involving anesthesia or sedation, because of the aspiration risk [4]. And because phentermine is a controlled substance, coupon availability varies by state [7].
Sources
- JAMA Network Open. Longitudinal Analysis of Obesity Drug Use and Public Awareness.
- Drugs.com. Phentermine Monograph for Professionals. November 10, 2025.
- Cureus. FDA-Approved Pharmacotherapy for Weight Loss Over the Last Decade.
- Eli Lilly and Company. Zepbound (tirzepatide) showed superior weight loss over Wegovy (semaglutide) in complete SURMOUNT-5 results published in The New England Journal of Medicine. May 11, 2025.
- US Food and Drug Administration. Adipex-P (phentermine hydrochloride) capsules label. 2012.
- American Association of Clinical Endocrinology. Phentermine Patient Information.
- GoodRx. How Much Does Phentermine Cost With and Without Insurance.
- GoodRx. Fills for Weight-Loss Medications Increase Despite High Costs.
- NovoCare. Get Your GLP-1 Medications Delivered with NovoCare Pharmacy.
- CNBC. Eli Lilly cuts cash prices of Zepbound weight loss drug vials on direct-to-consumer site. December 1, 2025.
- 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.
- Medscape. Standards of Care in Overweight and Obesity: Pharmacologic Treatment. April 27, 2026.
- CardioSmart, American College of Cardiology. Study Confirms the Benefits of FDA Approved Weight Loss Pills. July 2016.
- American Society for Metabolic and Bariatric Surgery. Head-to-head Study Shows Bariatric Surgery Superior to GLP-1 Drugs for Weight Loss. June 17, 2025.
- PubMed. Valvular heart disease associated with fenfluramine-phentermine. New England Journal of Medicine. August 28, 1997.
- PubMed. Once-Weekly Semaglutide in Adults with Overweight or Obesity.
- DailyMed, National Library of Medicine. ADIPEX-P (phentermine hydrochloride) tablet label.
Frequently asked questions
Is phentermine cheaper than Wegovy or Zepbound?
Yes, by a wide margin. In 2025 the average out-of-pocket spend was $15 per 30-day fill for phentermine, versus $75 for Wegovy and over $110 for Zepbound. Without insurance, brand GIP/GLP-1 prescriptions run over $1,400 at retail, while 30 tablets of phentermine 37.5 mg average $37.81 cash and as low as $14.40 with a GoodRx discount.
Can phentermine be taken long term, or only a few weeks?
Only short term. The label describes phentermine as a short-term adjunct (a few weeks) in a weight-reduction regimen, and phentermine patient information puts the usual duration at 12 weeks or less. Tolerance to the appetite effect usually develops within a few weeks, and the label says to discontinue rather than exceed the dose.
Can phentermine and a GLP-1 be taken together?
The phentermine label does not support it. Phentermine is indicated only as short-term monotherapy, and the safety and efficacy of combining it with any other weight-loss product — prescription, over-the-counter, or herbal — have not been established, so coadministration is not recommended.
How much more weight does tirzepatide produce than semaglutide?
In the 72-week SURMOUNT-5 head-to-head trial, tirzepatide produced 20.2% average weight loss versus 13.7% for semaglutide — 47% greater relative weight loss. In pounds, that was 50.3 lb (22.8 kg) versus 33.1 lb (15.0 kg), and 48.4% versus 27.3% of participants reached at least 20% weight loss.
Does insurance cover phentermine more readily than GLP-1 drugs?
Yes. As of January 2026, 63.0% of commercially insured people had unrestricted phentermine coverage, while unrestricted coverage was 3.9% for Zepbound and 9.4% for Wegovy, with 56.4% and 21.2% having no coverage at all. Medicare is barred from covering medications solely for weight loss.
Is phentermine unsafe because of the fen-phen valve problems?
The serious valve disease and pulmonary hypertension cases were reported with phentermine combined with fenfluramine or dexfenfluramine, which are no longer sold in the US. The label states the role of phentermine alone has not been established but cannot be ruled out, and rare cases have been reported with phentermine alone.
Who cannot take phentermine?
The label contraindicates phentermine in people with a history of cardiovascular disease (including coronary artery disease, stroke, arrhythmias, congestive heart failure, uncontrolled hypertension), hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, nursing, or MAO inhibitor use within 14 days. It is not recommended under age 16.
Who cannot take tirzepatide (Zepbound)?
Zepbound should not be used by anyone with a personal or family history of medullary thyroid carcinoma, with MEN 2, or who has had a serious allergic reaction to tirzepatide. It also should not be combined with other tirzepatide products or any GLP-1 receptor agonist, and safety in children is not known.
This article summarizes FDA labeling, published research and company information current as of September 14, 2026. It is not medical advice and does not replace a conversation with your own healthcare provider. How we research and verify.