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
Head-to-Head Efficacy Comparison
| Metric | Tirzepatide | Semaglutide | Phentermine | Metformin |
|---|---|---|---|---|
| Avg. weight loss | 20–22% | ~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
Decision Guide: Which Drug Is Right for You?
How GLP-1 Medications Compare to Older Agents
GLP-1 agonists produce substantially greater weight loss than older agents. Tirzepatide averages approximately 20–22.5% body-weight reduction in trials, semaglutide approximately 15%, phentermine approximately 5–9% (and is FDA-approved only for short-term use ≤12 weeks), and metformin approximately 3–5% (off-label for weight loss; primarily a diabetes drug). GLP-1 agonists also improve cardiometabolic outcomes that older agents do not.
The trade-off is cost: phentermine and metformin are inexpensive generics, while brand-name GLP-1 agonists cost $1,000+/month uninsured. Compounded GLP-1 options through providers like Luma Health narrow that gap significantly — $90/month for semaglutide and $165/month for tirzepatide.
Frequently Asked Questions
Ranked by average weight loss: tirzepatide (20–22%) > semaglutide (~15%) > phentermine (5–9%, short-term) > metformin (3–5%). GLP-1 medications produce significantly greater weight loss, with tirzepatide leading the group.
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 — discuss with your Luma Health provider.
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 type 2 diabetes.
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.
Both are used for prediabetes, but semaglutide produces far greater weight loss (~15% vs. 3–5%) and has more potent glucose-lowering effects. Metformin has a decades-long safety record, is inexpensive, and has Diabetes Prevention Program evidence for diabetes prevention. For patients who can afford GLP-1 therapy, semaglutide provides superior metabolic outcomes — Luma Health's $90/month compounded semaglutide makes this more accessible for cash-pay patients.
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.