Two randomized trials have compared tirzepatide vs semaglutide directly, and both reported larger average reductions with tirzepatide. In SURMOUNT-5 — 751 adults with obesity but without type 2 diabetes, followed for 72 weeks — participants averaged 20.2% body-weight loss on tirzepatide versus 13.7% on semaglutide. In SURPASS-2 — 1,879 adults with type 2 diabetes taking metformin, followed for 40 weeks — every tirzepatide dose lowered blood sugar and body weight more than semaglutide 1.0 mg. Averages are not the whole decision, though. The two medications differ in mechanism, evidence history, and cardiovascular data, and the right one for a given patient is a clinical call. Below is the comparison, number by number.
Tirzepatide: one molecule, two receptors
Tirzepatide is a once-weekly injectable peptide that activates two incretin receptors at once — the GLP-1 receptor and the GIP receptor. It is a 39-amino-acid molecule engineered on the native GIP backbone, and its plasma half-life of about five days in humans is what makes weekly dosing possible.
The GLP-1 side of its mechanism works the way semaglutide does: reduced appetite signaling in the hypothalamus and brainstem, slower gastric emptying, and insulin release that rises only when blood glucose is elevated. The GIP side adds effects on fat-tissue metabolism and — in preclinical studies — on reward-driven eating, while having little effect of its own on gastric emptying. That second receptor is the design idea behind the molecule: more metabolic signaling without a matching increase in the gastrointestinal side effects that limit GLP-1 dosing.
Dosing starts at 2.5 mg weekly and steps up roughly every four weeks through 5, 7.5, 10, and 12.5 mg to a 15 mg maximum — about five months to the top dose. In SURMOUNT-1, the pivotal placebo-controlled trial in 2,539 adults with obesity or overweight and no diabetes, participants averaged 15.0% (5 mg), 19.5% (10 mg), and 20.9% (15 mg) body-weight loss over 72 weeks versus 3.1% with placebo. The full trial program — including results in adults who also have type 2 diabetes, where average losses are smaller — is covered in our tirzepatide clinical trials review.
Semaglutide: the single-receptor comparator
Semaglutide is a once-weekly injectable GLP-1 receptor agonist — one incretin receptor, not two. The molecule mimics GLP-1, a gut hormone released after eating, and its half-life of roughly 165–168 hours in humans (native GLP-1 lasts one to two minutes) supports weekly dosing. It served as the comparator arm in both trials that tested tirzepatide head to head.
Its evidence base spans the STEP trial program, long-term extensions, and a dedicated cardiovascular outcomes trial. In STEP 1, 1,961 adults with overweight or obesity and no diabetes averaged 14.9% body-weight loss on semaglutide 2.4 mg over 68 weeks versus 2.4% with placebo, and 32.0% of treated participants lost more than 20% of their body weight. In adults with type 2 diabetes (STEP 2), the same dose averaged 9.64% over 68 weeks. Semaglutide is also the only one of the two with a placebo-controlled cardiovascular outcomes trial: in SELECT, adults with established cardiovascular disease and overweight or obesity — but no diabetes — had roughly 20% fewer major cardiovascular events on semaglutide 2.4 mg than on placebo. Full trial detail, including what happened after participants stopped, lives in our semaglutide weight-loss results article.
Tirzepatide vs semaglutide in head-to-head trials
Two randomized trials have tested the medications against each other, in different populations: SURPASS-2 in type 2 diabetes and SURMOUNT-5 in obesity without diabetes. Tirzepatide arms averaged larger reductions in both, with the gap widest at the highest doses.
| Trial | Population | Length | Weekly doses compared | Average results per arm |
|---|---|---|---|---|
| SURPASS-2 (NEJM, 2021) | 1,879 adults with type 2 diabetes on metformin | 40 weeks | Tirzepatide 5, 10, or 15 mg vs semaglutide 1.0 mg | HbA1c: −2.01 / −2.24 / −2.30 points vs −1.86 points. Weight: −7.6 / −9.3 / −11.2 kg vs −5.7 kg |
| SURMOUNT-5 (NEJM, 2025) | 751 adults with obesity, without type 2 diabetes | 72 weeks | Maximum-tolerated tirzepatide (10 or 15 mg) vs maximum-tolerated semaglutide (1.7 or 2.4 mg) | Weight: −20.2% (−22.8 kg) vs −13.7% (−15.0 kg) |
Three qualifiers matter. First, SURPASS-2's comparator was semaglutide 1.0 mg — the highest dose approved for type 2 diabetes at the time, not the 2.4 mg dose later used for weight management — so it is a diabetes-treatment comparison, not an obesity-dose one. Second, both trials were open-label: participants and clinicians knew which medication was being taken. Third, SURMOUNT-5 compared maximum-tolerated dosing strategies rather than fixed doses. Within those limits the results were consistent. In SURPASS-2, tirzepatide's HbA1c advantage was statistically significant at every dose (p=0.02 at 5 mg; p<0.001 at 10 and 15 mg), and every weight difference reached p<0.001. In SURMOUNT-5, the weight difference was significant (p<0.001), waist circumference fell 18.4 cm versus 13.0 cm, and 19.7% of tirzepatide participants versus 6.9% of semaglutide participants lost at least 30% of their body weight.
Against older weight-loss drugs, both medications sit well above the field. A 132-trial network meta-analysis covering 48,209 adults with overweight or obesity (Shi et al., The Lancet) found average losses, versus lifestyle intervention alone, of 11.40% for semaglutide and 7.98% for phentermine–topiramate, with the GLP-1 class as a whole at 5.79%. Its literature search closed in March 2021 — before tirzepatide's obesity trials — so it ranks semaglutide among the older agents but cannot place tirzepatide.
Side effects: more overlap than difference
The two medications share one side-effect profile, led by gastrointestinal symptoms that cluster during dose escalation and usually ease at maintenance. In SURPASS-2, the rates sat close together: nausea affected 17–22% of tirzepatide-treated participants across the three doses versus 18% on semaglutide 1.0 mg; diarrhea 13–16% versus 12%; vomiting 6–10% versus 8%. Most events in both arms were mild to moderate, and gastrointestinal symptoms were likewise the most common adverse events on both sides of SURMOUNT-5. Across the tirzepatide phase-3 programs, 4.3–7.1% of tirzepatide-treated participants stopped treatment because of adverse events, versus 2.1–2.6% on placebo. Serious adverse events in SURPASS-2 were reported in 5–7% of participants across the tirzepatide doses versus 3% with semaglutide 1.0 mg.
The class cautions are shared too. Severe hypoglycemia was rare in SURPASS-2 — blood glucose below 54 mg/dL occurred in 0.2–1.7% of tirzepatide-treated participants and 0.4% of semaglutide-treated participants, all also taking metformin — because both medications raise insulin only when glucose is elevated. Labels of the approved branded products warn about possible pancreatitis and gallbladder disease for both molecules, and both are avoided in people with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome, a caution that originates in rodent C-cell tumor findings. A provider screens for all of this before prescribing either one.
What compounded formulations mean here
Tirzepatide is the molecule in Mounjaro and Zepbound; semaglutide is the molecule in Ozempic, Wegovy, and Rybelsus. Promise dispenses each as a compounded medication, which is different from an FDA-approved product: the formulations offered here are not FDA-approved. Compounded medications are prepared by a licensed U.S. compounding pharmacy to fill an individual prescription rather than mass-manufactured; our guide to how compounded medications are made walks through the process. One related fact from the regulatory record: the oral semaglutide tablet (Rybelsus) is approved for type 2 diabetes only — no oral form of either molecule is approved for weight management. A licensed provider may still prescribe a compounded formulation — that decision is between you and your doctor.
How a provider weighs the choice
No single number decides between these medications; a provider weighs the goal, the history, and the data together. The considerations usually run in this order:
- Treatment goal. For combined blood-sugar and weight goals in type 2 diabetes, SURPASS-2 is the relevant comparison; for weight management without diabetes, SURMOUNT-5 is.
- Magnitude versus track record. Tirzepatide's arms averaged larger reductions in both head-to-head trials; semaglutide has been prescribed in branded form since 2017 and has the longer accumulated follow-up.
- Cardiovascular history. Semaglutide showed a placebo-controlled reduction of roughly 20% in major cardiovascular events (SELECT, adults with established cardiovascular disease and overweight or obesity, without diabetes). Tirzepatide's cardiovascular outcomes trial, SURPASS-CVOT, compared it against dulaglutide — an active medication, not placebo — in more than 13,000 adults with type 2 diabetes and established cardiovascular disease: major events occurred in 12.2% versus 13.1% over a median of about four years, meeting noninferiority while superiority was not met. Noninferiority against an active comparator is not evidence of added cardiovascular benefit.
- Tolerability and practicalities. Prior response to either medication, side-effect history, the contraindication screen above, and availability all move the decision.
At Promise, a licensed provider reviews every request; not everyone qualifies, and the provider may decline to prescribe either medication if your health history argues against it.