GLP-1 vs phentermine
You are deciding between two options.
Reference and calculator, not medical advice. Dosing figures below are transcribed from FDA-approved prescribing information and link back to it. Last reviewed August 2026.
Both are prescription medications for weight, and the difference that matters most is printed in the indication rather than anywhere else: phentermine is approved for short-term use, and the GLP-1 weight products are labelled for long-term weight management.
What Phentermine actually is
An older prescription appetite suppressant, a sympathomimetic amine.
- FDA-approved medication
- Yes
- Approved for
- Short-term — the label says "a few weeks" — adjunct to diet and exercise in exogenous obesity at BMI ≥ 30, or ≥ 27 with risk factors.
- Difference that matters
- The labelled duration is the difference that matters. Phentermine is approved for short-term use; the GLP-1 weight products are labelled for long-term weight management.
This page describes what each thing is and what it is approved for. It does not compare how well they work, because that is a question for a prescriber with your history in front of them.
The phentermine label says "short-term (a few weeks)" adjunct in a regimen of weight reduction. The Wegovy label says reduce excess body weight and maintain weight reduction long term. Those are different treatment models, not different strengths of the same idea — one is a limited course, the other is ongoing therapy.
Side-by-side
Structural facts from each product’s prescribing information.
| Product | Active ingredient | Indication | Frequency | Start | Max labelled | Steps to max |
|---|---|---|---|---|---|---|
| Wegovy Novo Nordisk | Semaglutide | Weight management | Weekly | 0.25 mg | 2.4 mg | 5 |
| Wegovy tablets Novo Nordisk | Semaglutide (oral) | Weight management | Daily | 1.5 mg | 25 mg | 4 |
| Zepbound Eli Lilly | Tirzepatide | Weight management | Weekly | 2.5 mg | 15 mg | 6 |
The BMI thresholds in the phentermine label — 30, or 27 with risk factors — are close to those used for the GLP-1 weight products, so eligibility is not usually where they diverge. Duration, mechanism and cost are.
Common questions
- Is phentermine cheaper?
- It is an older generic and generally much less expensive, which is a real consideration. It is also labelled for a few weeks rather than ongoing use, so comparing monthly costs compares two different treatment durations.
- Can they be combined?
- A prescriber question. Neither label instructs on combining them, and this site does not advise on off-label combinations.
- Which works better?
- Not something this site answers. The labels support describing what each is approved for and for how long; anything beyond that requires someone assessing you.
Sources and last review
Every dosing figure on this page is transcribed from an FDA-approved label and links back to it. Conversions are arithmetic you can repeat by hand. Nothing here is reviewed by a clinician, and this site does not employ one — which is exactly why what I do publish stays traceable to its source.
- Wegovy (Semaglutide) prescribing information — Novo Nordisk, label revised 30 June 2026, checked 7 August 2026
- Zepbound (Tirzepatide) prescribing information — Eli Lilly, label revised 6 May 2026, checked 7 August 2026
Transcribed from the labels above and checked by me — Gavin, who builds this site, takes a GLP-1, and is not a clinician. Last reviewed August 2026. Found an error? [email protected] — I treat corrections as bugs. How this site is built and funded.
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