Phentermine vs GLP-1s: Cost, Results and Who Each One Suits

Phentermine is the oldest weight-loss drug still on the American market and, alongside semaglutide and tirzepatide, one of the three most prescribed [1]. It costs a fraction of what the injectables cost. For a lot of people that makes it the obvious first question: why not just take the cheap one?

Here is the honest comparison, including the parts the cheap-option framing tends to skip.

What is phentermine and how does it work?

Phentermine is a sympathomimetic amine, structurally related to amphetamine, first approved in 1959 [2]. It increases norepinephrine release in the hypothalamus, which reduces appetite. It is sold as Adipex-P, Lomaira and various generics, in tablets and capsules, and it is a Schedule IV controlled substance [2].

GLP-1 receptor agonists work completely differently. Semaglutide and tirzepatide mimic gut hormones released after eating. They slow gastric emptying, increase insulin secretion when blood sugar is high, and act on appetite centers in the brain. Tirzepatide adds a second receptor, GIP.

That difference matters for how the two feel. Phentermine is a stimulant; many people describe it as making them less interested in food but also more wired. GLP-1s are not stimulants; people describe food noise going quiet and getting full much faster.

How much weight does each one produce?

This is the core of the comparison.

Drug	Trial	Duration	Mean weight loss
Phentermine alone	Pooled short-term data	~12 weeks	About 3–5%
Phentermine/topiramate ER (Qsymia) 15/92 mg	EQUIP	56 weeks	10.9%
Semaglutide 2.4 mg (Wegovy)	STEP 1 (n=1,961) [16]	68 weeks	14.9% (vs 2.4% placebo)
Semaglutide 2.4 mg (Wegovy)	SURMOUNT-5	72 weeks	13.7% (15.0 kg)
Tirzepatide (Zepbound)	SURMOUNT-5	72 weeks	20.2% (22.8 kg)
Orforglipron 36 mg (Foundayo)	ATTAIN-1	72 weeks	12.4% (efficacy estimand)

A 2019 retrospective cohort of 3,411 patients across phentermine, phentermine-topiramate, naltrexone-bupropion and lorcaserin found average weight loss of 3.45% of body weight at 12 weeks across the group [3].

In SURMOUNT-5, the only large open-label head-to-head between the two leading GLP-1 class drugs, tirzepatide produced 20.2% versus semaglutide’s 13.7% at 72 weeks, and 48.4% of tirzepatide patients reached 20% or greater weight loss versus 27.3% on semaglutide [4].

So the gap is real and large. Roughly three to five times more weight loss with the modern incretins, and over a much longer period of demonstrated effect.

What about the timeframe difference?

This is the point most comparisons handle badly.

Phentermine’s trials are short because its label is short. The FDA label for Adipex-P describes it as indicated “only as short-term (a few weeks) monotherapy for the management of exogenous obesity” as an adjunct to exercise, behavioral modification and caloric restriction [2][5]. The American Association of Clinical Endocrinology’s patient information sheet states the usual duration of treatment is 12 weeks or less [6].

GLP-1 drugs are approved for chronic weight management, meaning indefinite use, with trial data out to 68 to 72 weeks and beyond in extension studies.

That difference is not a technicality. Obesity behaves like a chronic condition, and weight tends to return when treatment stops, whichever drug it is. A drug labeled for a few weeks and a drug labeled for chronic management are answering different questions.

Many clinicians do prescribe phentermine for longer than the label, and some obesity medicine practices treat it as a long-term agent. That is an off-label decision your prescriber makes with you, and it should be an explicit conversation rather than an assumption.

What does each one cost in 2026?

This is where phentermine wins decisively.

Phentermine. Average retail is about $37.69 for 30 tablets of 37.5 mg, $38.05 for 30 capsules of 30 mg and $40.55 for 30 capsules of 15 mg. With a GoodRx coupon it can be as low as about $14.40 at some pharmacies, though because phentermine is a controlled substance, some states do not permit coupons on it [7]. GoodRx claims data put the average out-of-pocket spend at about $15 per fill for phentermine in 2025, versus over $110 per fill for Zepbound [8].

Semaglutide (Wegovy). NovoCare Pharmacy self-pay is $349 per month for standard doses of 0.25 mg through 2.4 mg, and $399 for Wegovy HD 7.2 mg. Patients new to the savings offer pay $199 per month for the two starter doses for the first two monthly fills through December 31, 2026 [9]. List price is roughly $1,349 per month.

Tirzepatide (Zepbound). LillyDirect self-pay is $299 per month for the 2.5 mg starting dose, $399 for 5 mg and $449 for higher doses [10]. One condition worth reading before you budget: the $449 tier applies only when you refill within 45 days. Lilly’s stated regular self-pay prices without that refill cadence are higher — $499 for 7.5 mg and $699 for 10, 12.5 and 15 mg. A missed refill window, which is common when a prescription lapses or a dose changes, can raise the monthly cost substantially.

Oral incretins. The Wegovy pill runs $149 per month for the 1.5 mg starting dose, $199 for 4 mg and $299 for the 9 mg and 25 mg doses self-pay. Foundayo (orforglipron) starts at $149 per month.

Middle ground. Generic phentermine/topiramate ER, which produced around 10% weight loss in EQUIP and CONQUER, runs roughly $59 to $63 per month with a coupon. That is worth knowing if phentermine alone is not enough and $299 is out of reach.

With commercial insurance that covers weight-loss medication, GLP-1 copays can drop to $25 or even $0 per fill, and eligible Medicare members may access a $50 monthly copay through the GLP-1 Bridge program running July 2026 through 2027. Coverage is the single biggest variable in what you actually pay.

What does each one do to you?

Phentermine side effects. Increased heart rate and blood pressure, insomnia, dry mouth, constipation, nervousness, restlessness and headache. It is a stimulant, and many people feel that. The label warns against drinking alcohol and against driving or operating machinery until you know how it affects you, because it can slow thinking and motor skills and affect vision [6]. Because it can lower blood sugar in people with type 2 diabetes who take diabetes medicines, blood sugar monitoring is advised [6].

It is habit-forming and federally controlled.

Phentermine contraindications. Cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, agitated states, a history of drug abuse, MAOI use within 14 days, pregnancy and nursing, and known hypersensitivity to sympathomimetic amines [5].

GLP-1 side effects. Predominantly gastrointestinal: nausea, vomiting, diarrhea, constipation, abdominal pain and reflux. Less common but serious concerns include pancreatitis, gallbladder disease and a boxed warning for thyroid C-cell tumors based on rodent data. See this site’s safety lane for the full profile.

The practical difference: phentermine’s problems tend to be stimulant problems, and GLP-1 problems tend to be gut problems. Which one is tolerable depends a lot on you.

Is phentermine dangerous because of fen-phen?

This comes up constantly, and the answer is more reassuring than the reputation suggests.

In the 1990s, phentermine was combined off-label with fenfluramine as “fen-phen.” On July 8, 1997, Mayo Clinic researchers publicly reported 24 cases of valvular heart disease in women treated with the combination, with lesions resembling carcinoid-induced valvular disease, a serotonin-related syndrome; the full report was published by Connolly and colleagues in the New England Journal of Medicine on August 28, 1997 [11][15]. On September 15, 1997, the manufacturers and the FDA announced the withdrawal of fenfluramine (Pondimin) and dexfenfluramine (Redux) [11].

The “roughly 30 percent” figure that follows this story around deserves a footnote, because it is routinely quoted without one. It comes from five echocardiographic prevalence surveys of exposed people reported to the FDA in 1997, in which the prevalence of valvular disease meeting the case definition ran from 30.0% to 38.3%, overall 32.8% (95% CI 27.7%–38.9%) [11]. Those surveys had no unexposed control groups, and later controlled studies found much lower rates, with heart-valve complications under 3% among people who took the combination for three months or less. So 30% is what the 1997 uncontrolled surveys showed, not the settled attributable risk.

On the litigation, the widely repeated “$13 billion” figure traces to a 2003 Forbes report of what Wyeth had by then paid in claims related to Pondimin and Redux; the court-approved national class settlement with American Home Products was $4.75 billion when approved in 2000 and was later valued at roughly $7.65 billion. Treat $13 billion as reported company payouts across all diet-drug litigation, not as a single judgment.

The withdrawn drug was fenfluramine, a serotonergic agent. Phentermine was not withdrawn and has stayed on the market for the nearly three decades since. The episode is why the label warns explicitly that co-administration of phentermine with other weight-loss drugs is not recommended [5], and it is part of why the FDA later required cardiovascular outcome trials for new obesity drugs.

Who does each one suit?

Phentermine tends to make sense when:

• Cost or lack of coverage is the binding constraint. At roughly $15 a fill it is accessible to almost anyone.
• You need a short, defined push rather than indefinite therapy.
• You cannot tolerate GLP-1 gastrointestinal effects, or have a contraindication such as a personal or family history of medullary thyroid carcinoma or MEN2.
• You want an oral option and do not want to inject.
• A modest target, 5% or so, would meaningfully help your health.

GLP-1 drugs tend to make sense when:

• You need 10% or more weight reduction, which the 2026 Obesity Association standards say points to tirzepatide or semaglutide specifically [12].
• You also have type 2 diabetes, cardiovascular disease, sleep apnea or another condition where these drugs carry indication-specific evidence.
• You have cardiovascular contraindications to stimulants.
• Your insurance covers them, which changes the cost calculus completely.

The 2026 Standards of Care in Overweight and Obesity from the Obesity Association, a division of the American Diabetes Association, structure the decision exactly this way: match the drug to the magnitude of weight reduction needed and to the coexisting conditions [12]. Grade A recommendations include offering obesity medications as part of initial treatment for adults with or at high risk of obesity-related complications.

Is there a middle option?

Yes, and it is underused: phentermine/topiramate ER, sold as Qsymia and now available generically since May 2025.

Adding low-dose topiramate to phentermine roughly doubles the effect. EQUIP found 10.9% weight loss on the 15/92 mg dose versus 1.6% on placebo at 56 weeks, with 67% of patients losing at least 5% and 47% losing at least 10%. CONQUER found 9.8% on the same dose and 7.8% on 7.5/46 mg.

A 2016 network meta-analysis of 29,018 participants found phentermine-topiramate the most effective of the pre-incretin approved drugs: patients were about nine times more likely than placebo recipients to lose 5% of body weight [13].

The trade-off is that topiramate brings its own profile: paresthesia, cognitive slowing, kidney stones, metabolic acidosis, and a risk of oral clefts in pregnancy that requires reliable contraception. But at roughly $60 a month generic, or $74.99 through the manufacturer’s cash program, it sits in an interesting place between $15 phentermine and $299-plus injectables.

What happens when you stop?

Both drugs are treatments, not cures. Weight tends to return after either one is stopped, which is why the label duration difference matters so much in practice.

A real-world study presented at the 2025 ASMBS meeting found that GLP-1 patients prescribed the drugs for at least six months lost 4.7% of total body weight at two years, and those on continuous therapy for a full year lost 7%, well below trial figures. The lead author noted that as many as 70% of patients may discontinue treatment within one year [14]. Whatever you start, sustaining it is the harder problem than choosing it.

The short version

Phentermine is cheap, oral, fast-acting, decades-old and labeled for a few weeks. It produces about 3% to 5% weight loss over about 12 weeks.

GLP-1s are expensive, mostly injectable, labeled for chronic use, and produce 13% to 20% over 68 to 72 weeks.

If money were no object, the evidence points clearly at the incretins. Money is usually an object, which is exactly why phentermine is still among the three most prescribed weight-loss drugs in America two-thirds of a century after approval.

This article summarizes what FDA-approved labeling and Instructions for Use, published trials and professional guidelines report. It is not medical advice, and it does not recommend a drug or a dose for you. Follow your own prescription and the directions your prescriber and pharmacist give you, and talk to a healthcare provider about which option fits your health history before starting, stopping or changing any medication.

Sources

1. Berning P et al., JAMA Network Open 2025 — Longitudinal analysis of obesity drug use: https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2829700
2. Drugs.com — Phentermine monograph: https://www.drugs.com/monograph/phentermine.html
3. FDA-approved pharmacotherapy for weight loss over the last decade (review): https://www.cureus.com/articles/103005-fda-approved-pharmacotherapy-for-weight-loss-over-the-last-decade
4. Eli Lilly — SURMOUNT-5 complete results published in NEJM: https://investor.lilly.com/news-releases/news-release-details/zepbound-tirzepatide-showed-superior-weight-loss-over-wegovy
5. FDA label, Adipex-P (phentermine hydrochloride): https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/085128s065lbl.pdf
6. AACE phentermine patient information: https://pro.aace.com/files/obesity/toolkit/phentermine_patient_info.pdf
7. GoodRx — How much does phentermine cost with and without insurance: https://www.goodrx.com/phentermine/phentermine-cost-for-weight-loss
8. GoodRx Research — Weight-loss medication fill tracker: https://www.goodrx.com/healthcare-access/research/weight-loss-medication-fill-tracker
9. NovoCare Pharmacy: https://www.novocare.com/pharmacy.html
10. CNBC — Eli Lilly cuts cash prices of Zepbound vials: https://www.cnbc.com/2025/12/01/eli-lilly-prices-zepbound-weight-loss-drug-vials.html
11. CDC MMWR — Cardiac valvulopathy associated with exposure to fenfluramine or dexfenfluramine: https://www.cdc.gov/mmwr/preview/mmwrhtml/00049815.htm
12. Obesity Association / ADA — Standards of Care in Overweight and Obesity, pharmacologic treatment 2026: https://reference.medscape.com/cc2/p10/standards-care-overweight-obesity-ada-guidelines-2026a1000cjs
13. CardioSmart / American College of Cardiology — Study confirms benefits of FDA-approved weight loss pills (Khera 2016): https://www.cardiosmart.org/news/2016/7/study-confirms-the-benefits-of-fda-approved-weight-loss-pills
14. ASMBS — Head-to-head study shows bariatric surgery superior to GLP-1 drugs for weight loss: https://asmbs.org/news_releases/head-to-head-study-shows-bariatric-surgery-superior-to-glp-1-drugs-for-weight-loss
15. Connolly HM, Crary JL, McGoon MD, et al. — Valvular heart disease associated with fenfluramine-phentermine, N Engl J Med 1997;337(9):581-588: https://pubmed.ncbi.nlm.nih.gov/9271479
16. Wilding JPH et al., N Engl J Med 2021;384(11):989-1002 — Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1, NCT03548935): https://pubmed.ncbi.nlm.nih.gov/33567185
17. DailyMed — Adipex-P label: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=cf6e2aa5-e8e1-4478-93b9-47632a924482