Semaglutide vs Tirzepatide vs Phentermine vs Metformin for Weight Loss
Four-way comparison of the most prescribed weight loss medications in 2026. Compare efficacy, side effects, cost, duration of use, and who each drug is best for.
More on Weight Loss Medication Comparisons
Overview: Four Very Different Weight Loss Approaches
Semaglutide, tirzepatide, phentermine, and metformin are all prescribed for weight management in 2026 — but they work through entirely different mechanisms, produce dramatically different results, and carry different safety profiles. This guide puts them side-by-side so you can understand where each fits in modern obesity medicine.
The short summary: GLP-1 medications (semaglutide and tirzepatide) are in a different efficacy class entirely. Phentermine is an older stimulant approved only for short-term use. Metformin produces modest benefits and is primarily a diabetes medication with weight effects as a side benefit. Each has a role in the right patient at the right time.
Individual Drug Profiles
Tirzepatide
Semaglutide
Phentermine
Metformin
Head-to-Head Efficacy Comparison
| Metric | Tirzepatide | Semaglutide | Phentermine | Metformin |
|---|---|---|---|---|
| Avg Weight Loss | 20–22% | 12–15% | 5–9%* | 3–5% |
| FDA Approval (Weight) | Yes (Zepbound) | Yes (Wegovy) | Yes (short-term) | No (off-label) |
| CV Outcomes Data | SURPASS-CVOT ✓ | SELECT trial ✓ | Limited data | UKPDS ✓ |
| Injection Required | Yes (weekly) | Yes (weekly) | No (oral) | No (oral) |
| Treatment Duration | Long-term | Long-term | ≤12 weeks | Long-term |
| Generic Available | No (2031+) | No (2031+) | Yes | Yes |
| Compounded Option | Yes | Yes | N/A | N/A |
*Phentermine weight loss is short-term only; most weight is regained after discontinuation.
Side Effect Comparison
| Side Effect | Tirzepatide | Semaglutide | Phentermine | Metformin |
|---|---|---|---|---|
| Nausea/Vomiting | Common (titration) | Common (titration) | Rare | Common (initially) |
| Heart Rate/BP | Small increase possible | Small increase possible | Often elevated | No effect |
| Sleep Disruption | Possible initially | Possible initially | Yes (insomnia common) | Rare |
| Anxiety/Jitteriness | Rare | Rare | Common | Rare |
| Vitamin B12 Deficiency | Rare | Rare | No | Yes (long-term) |
| Pancreatitis Risk | Rare | Rare | No | No |
| Dependency Risk | None | None | Yes (Schedule IV) | None |
Cost Comparison 2026
See our detailed guides: semaglutide cost breakdown and cheapest GLP-1 options in 2026.
Decision Guide: Which Drug Is Right for You?
Frequently Asked Questions
Which weight loss medication is most effective: semaglutide, tirzepatide, phentermine, or metformin?
Ranked by average weight loss: tirzepatide (20–22%) > semaglutide (12–15%) > phentermine (5–9%, short-term) > metformin (3–5%). GLP-1 medications produce significantly greater weight loss, with tirzepatide leading the group.
Can you take phentermine and semaglutide together?
Combination therapy is sometimes used off-label, but this must be done with close medical supervision. Both drugs affect appetite through different mechanisms (phentermine via CNS sympathomimetic activity, semaglutide via GLP-1 receptor). Cardiovascular monitoring is important. Most providers would not routinely combine these without specific clinical justification.
Is metformin good for weight loss?
Metformin produces modest weight loss (3–5% of body weight) as a side benefit of its primary action (reducing hepatic glucose production). It is not FDA-approved as a weight loss medication, but its favorable safety profile, low cost, and additional metabolic benefits make it a reasonable adjunct in patients with insulin resistance, prediabetes, or T2D.
Is phentermine safe long-term?
Phentermine is FDA-approved for short-term use only (typically up to 12 weeks). It acts as a sympathomimetic stimulant and carries risks including elevated heart rate and blood pressure, potential for dependence, and rebound weight gain upon discontinuation. It is not recommended for patients with cardiovascular disease.
Is semaglutide or metformin better for prediabetes?
Both are used for prediabetes, but semaglutide produces far greater weight loss (12–15% vs 3–5%) and has more potent glucose-lowering effects. Metformin has a decades-long safety record, is inexpensive, and has DPP evidence for diabetes prevention. For patients who can afford GLP-1 therapy, semaglutide provides superior metabolic outcomes.
Sources & References
- Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity." NEJM. 2022;387(3):205–216.
- Wilding JPH, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." NEJM. 2021;384(11):989–1002.
- Halpern B, et al. "Phentermine for Weight Loss in Adults." Curr Obes Rep. 2018;7(4):410–414.
- Seifarth C, et al. "Effectiveness of Metformin on Weight Loss in Non-Diabetic Individuals." Exp Clin Endocrinol Diabetes. 2013;121(1):27–31.
- Lincoff AM, et al. "Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT)." NEJM. 2023;389(24):2221–2232.