Semaglutide vs Tirzepatide: Weight Loss, Evidence and Lab Results

The main difference is mechanism: semaglutide activates one gut-hormone receptor (GLP-1), while tirzepatide activates two (GLP-1 and GIP). In the one head-to-head trial in adults with obesity but without diabetes, tirzepatide produced more weight loss than semaglutide, and an earlier head-to-head trial in type 2 diabetes pointed the same direction. Both are FDA-approved drugs made by different companies: semaglutide, made by Novo Nordisk, was first approved in 2017 (Ozempic, later Wegovy and Rybelsus), and tirzepatide, made by Eli Lilly, was first approved in 2022 (Mounjaro, later Zepbound). What's sold on the grey market under either name is an unbranded, unapproved version, and that's the version our lab data covers.

At a glance

How they work differently

Semaglutide is a modified version of GLP-1, a hormone your gut releases after a meal. It binds GLP-1 receptors in the pancreas, stomach and brain, which raises insulin release when blood sugar is high, slows stomach emptying and dials down appetite (Ozempic prescribing information, DailyMed).

Tirzepatide was built from the sequence of a second gut hormone, GIP, and modified so it also activates GLP-1 receptors. That makes it a dual agonist, and the label describes it as a GIP receptor and GLP-1 receptor agonist (Zepbound prescribing information, DailyMed). Researchers think the extra GIP activity is why the results differ, but the trials measured outcomes, not the reason for them, so treat the mechanism story as a hypothesis.

Approved uses differ a little too. Wegovy's current label covers weight management, cardiovascular risk reduction in adults with established cardiovascular disease plus obesity or overweight, and (under accelerated approval) a liver condition called MASH with moderate to advanced fibrosis. It now comes as an injection and a tablet (Wegovy prescribing information, DailyMed). Ozempic is labeled for type 2 diabetes, cardiovascular risk in type 2 diabetes, and kidney outcomes in type 2 diabetes with chronic kidney disease. Zepbound is labeled for weight management and for moderate-to-severe obstructive sleep apnea in adults with obesity, and Mounjaro for type 2 diabetes and cardiovascular risk in high-risk adults with type 2 diabetes.

What the evidence shows

Two randomized trials have put the drugs directly against each other. Both were sponsored by Eli Lilly, and both were open-label, so participants and investigators knew which drug was being used.

In obesity without diabetes: SURMOUNT-5

SURMOUNT-5 randomly assigned 751 adults with obesity but without type 2 diabetes to tirzepatide or semaglutide, each at the maximum tolerated dose, for 72 weeks. Average weight change at the end was -20.2% with tirzepatide and -13.7% with semaglutide, and waist circumference fell by an average of 18.4 cm versus 13.0 cm. Tirzepatide recipients were also more likely to reach weight reductions of at least 10%, 15%, 20% and 25%. Gastrointestinal events were the most common side effects in both groups, mostly mild to moderate, and mostly during the dose-escalation phase (Aronne et al., NEJM 2025).

Both drugs worked. Semaglutide's average result in this trial is a large weight loss by any earlier standard; the finding is about the gap between the two, not about semaglutide failing.

That is the cleanest answer available to "which one produces more weight loss?", and the answer is tirzepatide. Two caveats belong next to it. The trial was open-label, and "maximum tolerated dose" means people ended up on different doses, so the results describe each drug as people actually tolerated it.

In type 2 diabetes: SURPASS-2

SURPASS-2 randomized 1,879 adults with type 2 diabetes to one of three tirzepatide doses or to semaglutide over 40 weeks. Tirzepatide at every dose lowered A1c (a marker of average blood sugar) more than semaglutide and produced greater weight loss, and the authors called it noninferior and superior. The comparator was the semaglutide dose used for diabetes, which is lower than the dose in the weight-management trials (Frías et al., NEJM 2021).

Each drug on its own

Outside head-to-head trials, comparing numbers between separate studies is unreliable: populations, durations, dose escalation and analysis methods all differ. So we won't line up percentages from the placebo-controlled trials. For the underlying evidence, the semaglutide guide covers STEP 1 (the trial behind Wegovy) and the cardiovascular and kidney indications, and the tirzepatide guide covers SURMOUNT-1 and the other approval trials.

There's one more reason to hold the head-to-head results with some care: the sponsor of both trials, Eli Lilly, makes tirzepatide. Sponsorship doesn't invalidate a randomized trial, and the results were published in a top journal, but no independent group has run a large head-to-head trial of the two that we could find, and the open-label design means neither participants nor investigators were blinded.

So the evidence is stronger, on weight loss, for tirzepatide, because it beat semaglutide in the only obesity trial that compared them. Semaglutide's advantage is a longer record: it was approved first, and its cardiovascular outcome and kidney indications rest on dedicated outcome trials.

Side effects and risks compared

The two labels look strikingly alike. Both carry a boxed warning: in rodents, the drug caused thyroid C-cell tumors, and it isn't known whether that happens in humans. Both are contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2. Both list acute gallbladder disease and pancreatitis among their warnings, and both mention pulmonary aspiration risk during anesthesia or deep sedation (Wegovy label; Zepbound label).

In the head-to-head trials, nausea, diarrhea and vomiting were the most common problems on both drugs. SURPASS-2 reported serious adverse events in 5 to 7% of tirzepatide recipients and 3% of semaglutide recipients, and low rates of clinically significant low blood sugar in both, but the trial wasn't designed to rank rare harms. Neither trial can say which drug is safer over many years.

The bigger risk difference is not between the molecules but between the approved products and the grey-market versions. An approved product comes from an inspected facility with batch release testing. A research-labeled vial has none of that, which is why what independent labs find matters.

What the lab tests show

These figures come from independent lab reports, mostly commissioned and submitted by buyers (unidentified vendors excluded). VendorVerify doesn't test anything itself. A vial counts as failed if it was empty, under 95% pure, more than 20% under label or more than 50% over.

Tirzepatide: 788 vials from 48 vendors, reported from March 2023 to September 2026. Median purity was 99.76%, with 709 of 784 vials at 99% or higher and 0 under 95%. The median measured amount was 5.5% over label. Of 761 vials with a label amount and a measured amount, 309 landed within 5% of label (519 of 764 were within 10% as of September 30, 2026). 9 were more than 20% under and 51 more than 20% over, including 3 more than 50% over, and 4 were empty. In all, 16 vials (2.0%) failed.

Semaglutide: 18 vials from 8 vendors, reported from January 2025 to May 2026. 18 of 18 were 99% pure or better (median 99.67%). The median measured amount was 6.7% over label, with 8 vials within 5% (15 were within 10% as of September 30, 2026), 0 more than 20% under and 0 more than 20% over. 0 vials failed.

Those samples are not comparable in size. Tirzepatide has 788 vials to semaglutide's 18, so its figures carry far more weight, and 18 vials from 8 vendors can shift with a single new report. The numbers don't rank one compound against the other; they showed that, as of September 30, 2026, both had mostly come back close to their labels.

Here's who has the strongest independent lab record for tirzepatide right now:

Semaglutide's sample is thin, so this table may be short or empty. Here's the ranked list for it:

Buying smarter

Check a vendor's record for the specific compound: /peptides/tirzepatide and /peptides/semaglutide show the current numbers, which move faster than this article. A vendor with a clean tirzepatide record has told you little about semaglutide. Learn to read a peptide COA and check for fake-COA patterns before trusting anything a vendor posts, because a posted certificate isn't an independent report. If you want your own answer, get your vial tested. For tirzepatide, our test-results article breaks results down further. If a report comes back bad, stop using that batch, tell anyone who bought from the same run, take the report to the vendor, and submit it to VendorVerify so the next buyer sees it.

Database figures without a date update automatically from VendorVerify's lab results; dated figures are snapshots. This isn't medical advice, and research-labeled versions of both compounds are sold for research use only.