How each drug works
Semaglutide is a GLP-1 receptor agonist (glucagon-like peptide-1). It mimics a gut hormone that regulates appetite, slows gastric emptying, and improves insulin response. FDA-approved in 2017 for type 2 diabetes (Ozempic®) and in 2021 for weight loss (Wegovy®).
Tirzepatide is a dual GLP-1 and GIP receptor agonist (glucose-dependent insulinotropic polypeptide). By activating both receptors simultaneously, it produces a more potent hormonal signal than semaglutide — resulting in greater appetite reduction and better glycemic control. FDA-approved in 2022 for type 2 diabetes (Mounjaro®) and in 2023 for weight loss (Zepbound®).
Efficacy in clinical trials
The pivotal trial for semaglutide was STEP 1 (NEJM 2021): 1,961 adults with obesity received semaglutide 2.4 mg weekly or placebo over 68 weeks. The semaglutide group lost 14.9% of body weight on average, vs. 2.4% on placebo. A third of participants lost more than 20%.
The pivotal trial for tirzepatide was SURMOUNT-1 (NEJM 2022): 2,539 adults with obesity received tirzepatide (5, 10, or 15 mg) or placebo over 72 weeks. The 15 mg group lost 20.9% on average; the 10 mg group, 19.5%; the 5 mg group, 15.0%. More than 50% of participants on higher doses lost more than 20%.
SURMOUNT-5 (2025) was the first direct head-to-head trial between tirzepatide and semaglutide for weight loss. Tirzepatide produced significantly greater weight loss (~47% more weight lost on average). This confirmed what indirect trials had suggested.
Side effects compared
Both drugs share the GLP-1 family side-effect profile:
- Gastrointestinal (nausea, constipation, diarrhea): common in both, usually mild, usually resolving after the first few weeks with titration.
- Fatigue: 10–15% in both.
- Pancreatitis: rare but possible in both. Risk ~0.2%/year.
- Gallstones: slightly increased risk in both, especially with rapid weight loss (>1.5 kg/week).
- MTC/MEN2: absolute contraindication for both (FDA black box warning).
In head-to-head trials, tirzepatide shows slightly less severe nausea, possibly because the GIP component may have centrally-mediated anti-nausea effects. But this varies a lot between patients — some tolerate semaglutide better, others tirzepatide.
Real cost
Retail prices without insurance are similar for brand versions:
- Wegovy® (semaglutide): $1,349/mo MSRP
- Zepbound® (tirzepatide): $1,059/mo MSRP
- Mounjaro® (tirzepatide off-label): $1,069/mo
- Ozempic® (semaglutide off-label): $935/mo
For compounded versions, the difference is more noticeable because tirzepatide's active ingredient is more expensive for compounding pharmacies to source:
- Compounded semaglutide (REMEVi): from $199/mo all-inclusive
- Compounded tirzepatide (REMEVi): from $269/mo all-inclusive
The annual difference is ~$1,560. For many patients, that's decisive. For others, the additional weight loss with tirzepatide justifies the cost.
How to decide
This is a conversation you should have with a licensed provider who knows your full medical history. That said, the following factors usually guide the decision:
Consider tirzepatide if:
- Your BMI is high (>35) and you're looking for significant weight loss.
- You have type 2 diabetes with elevated A1c.
- You've already tried semaglutide and results stalled.
- The additional monthly cost is feasible for you.
Consider semaglutide if:
- It's your first GLP-1 and you want to start with the medication with the longest track record.
- Budget is a meaningful factor.
- Your BMI is moderate (27–34) and a 15% loss gets you meaningfully closer to your goal.
- You have limited access or supply issues with tirzepatide.