Phentermine vs Ozempic, Wegovy and Zepbound: Is Phentermine a GLP-1?

Phentermine is not a GLP‑1, and it does not do what Ozempic, Wegovy or Zepbound do. It is a stimulant-type appetite suppressant, a Schedule IV controlled substance, and its label approves it for “a few weeks”.^[1] Semaglutide and tirzepatide are weekly injections approved for long-term use, and their main trials produced roughly 15% to 21% weight loss over about a year and a half.^[9][10] No randomized trial has put phentermine against either one. In the one large health system that tracked all of them side by side, people on phentermine lost a median 2.5% of their weight; people on semaglutide lost 7%.^[11]

Is phentermine a GLP-1?

No. The phentermine label calls it a sympathomimetic amine and says it acts much like amphetamine, the prototype of the older appetite-suppressant drugs.^[1] It was first approved in 1959.^[2] The label admits the mechanism is unsettled. It has not been established, it says, that phentermine works mainly by suppressing appetite rather than through other effects on the brain or metabolism.^[1]

Semaglutide, the drug in Ozempic and Wegovy, is a glucagon-like peptide‑1 (GLP‑1) receptor agonist, acting on the receptor for a hormone made in the gut.^[3] Tirzepatide, in Zepbound and Mounjaro, acts on the GLP‑1 receptor and on a second hormone receptor, GIP.^[4] Neither is a stimulant, and neither label carries a controlled-substance schedule.^[3][4] Which drugs belong to the class is sorted on which drugs are GLP-1s.

One brand detail matters for the “phentermine vs Ozempic” question. Ozempic’s approval covers type 2 diabetes only; every indication on its label concerns adults with diabetes.^[5] Wegovy is the same molecule approved for weight, and Zepbound is tirzepatide’s weight-loss brand. The full list of approved drugs is on medications.

Phentermine vs Ozempic, Wegovy and Zepbound at a glance

What the phentermine trials measured

Phentermine’s trials are short, small and old by current standards. Its own label sums up the evidence base in unusual language: the extra loss on the drug, beyond placebo, came to less than a pound a week,^[2] and because the studies lasted only weeks, the total effect over diet alone “must be considered clinically limited”.^[1] The randomized trials found on PubMed:

Over a few months, phentermine works: in the two Korean trials most people lost at least 5% of their weight, against a minority on placebo.^[7][8] EQUATE, the largest and longest of the three, was built to test Qsymia, the phentermine–topiramate capsule, and its phentermine-only arms show the drug alone doing less well. Fewer than half of those on phentermine reached 5%, and the combination beat each of its parts.^[6] How Qsymia compares with the GLP‑1s is on Wegovy vs Qsymia and Zepbound vs Qsymia.

What the semaglutide and tirzepatide trials measured

STEP 1 randomized 1,961 adults without diabetes to semaglutide 2.4 mg, Wegovy’s dose, or placebo for 68 weeks. Counting everyone regardless of whether they stopped the drug, weight fell 14.9% on semaglutide and 2.4% on placebo, and 86.4% lost at least 5%, against 31.5% on placebo.^[9]

SURMOUNT‑1 gave 2,539 adults one of three weekly tirzepatide doses, 5, 10 or 15 mg, or a placebo, and followed them for 72 weeks. Under its treatment-regimen estimand, which also counts people who stopped, weight fell 15.0%, 19.5% and 20.9% by dose, against 3.1% on placebo; 85% to 91% lost at least 5%.^[10]

Set beside phentermine’s trials, the gap is in length as much as size. The GLP‑1 figures were measured after a year and a half of treatment; phentermine’s were measured after three to seven months, which is already longer than its label contemplates.

The closest thing to a head-to-head comparison

Phentermine has never been randomized against semaglutide or tirzepatide. The nearest substitute is a review of weight-loss drugs prescribed across 37 Veterans Affairs medical centers between March 2020 and March 2022, covering 1,959 veterans, with every drug measured the same way in the same health system.^[11]

Semaglutide produced nearly three times the median loss, and twice the share of people reaching 5%. Phentermine had the lowest rate of side effects of any drug in the review, and it was the only one for which stomach and bowel problems were not the most common complaint.^[11] The limits are real: nobody was randomized, people chose or were steered to drugs for reasons the review could not fully account for, phentermine was off the VA’s national formulary, and the semaglutide group mixed Ozempic and Wegovy doses.^[11] So the figures describe practice and cannot show cause.

Guidelines point the same way. The 2026 American College of Physicians review included phentermine among the drugs it assessed and found semaglutide and tirzepatide had “the most favorable results across outcomes”.^[12] The guideline that followed ranks semaglutide and tirzepatide first-line, phentermine–topiramate second, liraglutide third and naltrexone–bupropion fourth. Phentermine on its own does not appear on the list.^[13] The broader ranking of 19 drugs from the BMJ is covered in what 262 trials say together, and the case for the cheaper combination in the old cheap drug keeps winning.

Side effects: a stimulant's risks against a gut hormone's

The two drug families fail people in different ways. On a GLP‑1 the trouble is mostly digestive and concentrated during dose increases: in STEP 1, 4.5% stopped semaglutide because of stomach or bowel effects against 0.8% on placebo,^[9] and in SURMOUNT‑1 between 4.3% and 7.1% stopped tirzepatide for any side effect, against 2.6%.^[10] Phentermine’s trouble is stimulant-type. In both Korean trials, dry mouth and insomnia were the leading side effects, and neither trial found a significant difference in blood pressure between drug and placebo in the volunteers it enrolled.^[7][8] That does not cancel the label’s cardiovascular contraindications, which rule phentermine out for anyone with a history of heart disease, stroke or uncontrolled blood pressure.^[1] The brand-by-brand pulse figures for GLP‑1s are collected on heart rate and palpitations on a GLP-1.

On misuse, the label tells prescribers to weigh the possibility of abuse because phentermine is related to amphetamine.^[1] The measured risk in weight-loss patients looks low. In a study of 269 patients treated with phentermine for weight, including 117 who had taken it for between one and 21 years, none met criteria for abuse or psychological dependence, and stopping abruptly produced no amphetamine-type withdrawal beyond more hunger.^[14] It was one study, without a comparison group of people never given the drug.

Weeks or years

The deepest difference is how long each drug is meant to be taken. Phentermine’s label expects the appetite effect to wear off, saying tolerance usually sets in within a few weeks; at that point the instruction is to stop the drug, not to take more of it.^[2] Wegovy and Zepbound are approved to “maintain weight reduction long term”.^[3][4]

Longer use does happen, and the best data on it are observational. Among 13,972 adults whose records were followed after a first phentermine prescription, continuous use beyond a year went with 7.4% more weight lost at the two-year mark than use of three months or less, and heart events or deaths were no more common.^[15] That is a comparison between phentermine users, not with a GLP‑1, and people who keep taking a drug often differ from those who stop. Weight returns when either kind of drug ends; what is known about regain after semaglutide is in stopping semaglutide and weight regain.

Cost

Phentermine is a generic first approved in 1959,^[2] with no manufacturer cash program, so its price depends on the pharmacy and any discount card used. The brand GLP‑1s have published cash prices. Novo Nordisk sells Wegovy at $349 a month for doses up to 2.4 mg,^[16] and Ozempic at $349 a month, or $499 at 2 mg.^[18] Lilly sells Zepbound pens at $299 for the 2.5 mg starting dose, $449 at 7.5 mg and above when refilled on time, and up to $699 when not.^[17]

Insurance changes the arithmetic in both directions. Many plans exclude weight-loss drugs entirely, and a commercial plan that does cover Wegovy or Zepbound can bring the manufacturer price down to as little as $25 a month with the savings card.^[16][17] Ozempic is covered for type 2 diabetes, not for weight.^[5]

Which one a prescriber is likely to offer

• For lasting weight loss, the guideline answer is a GLP‑1. The ACP ranks semaglutide and tirzepatide first and does not list phentermine alone.^[13]
• Phentermine is ruled out for many people. Heart disease, uncontrolled blood pressure, hyperthyroidism, glaucoma, a history of drug abuse, recent use of an MAO inhibitor, pregnancy and breastfeeding are all contraindications.^[1]
• Some people cannot use a GLP‑1. Anyone who has had medullary thyroid cancer, has a relative who did, or has the MEN 2 syndrome is barred from Wegovy and Zepbound.^[3][4]
• Cost and coverage often decide it. Phentermine is a generic; the GLP‑1s are brand drugs that many plans exclude for weight loss, and their cash prices run to hundreds of dollars a month.^[16][17]

A few telehealth sellers carry both kinds of drug, so one prescriber can weigh them against each other. Hone Health lists phentermine and naltrexone beside compounded liraglutide, all behind a membership. ShedRx sells a daily compounded oral combination of metformin, naltrexone and topiramate next to its compounded semaglutide and tirzepatide, and Tyde Wellness lists low-dose naltrexone alongside the same two GLP‑1s. The combination and low-dose naltrexone are not what the trials on this page tested, so ask what evidence stands behind whichever one you are offered.