Tirzepatide vs semaglutide: what the trials show

Key takeaways

• In the only direct obesity head-to-head (SURMOUNT-5, 751 adults, 72 weeks), tirzepatide cut body weight 20.2 percent versus semaglutide's 13.7 percent at top dose .
• Side by side at max labeled doses: tirzepatide 15 mg ran about 20.9 percent at 72 weeks (SURMOUNT-1) and semaglutide 2.4 mg about 14.9 percent at 68 weeks (STEP-1) .
• In type 2 diabetes (SURPASS-2, 40 weeks), tirzepatide 15 mg beat semaglutide 1 mg on both blood sugar (about 2.3 versus 1.9 percent HbA1c drop) and weight (about 11.2 versus 5.7 kg) .
• Both carry an FDA boxed warning for thyroid C-cell tumors and are contraindicated with personal or family history of medullary thyroid carcinoma or MEN-2 .
• Semaglutide has the longer cardiovascular outcomes record (SELECT cut major cardiac events about 20 percent in non-diabetic obesity); tirzepatide's dedicated outcomes trial is still reading out .

Skip to:

• Which one wins, and on what
• Why the mechanisms differ (GLP-1 vs GLP-1 plus GIP)
• The FDA-approved product map
• The direct diabetes head-to-head: SURPASS-2
• The semaglutide STEP program
• The tirzepatide SURMOUNT program
• Cardiovascular outcomes: semaglutide's lead
• Tolerability, side effects, and the thyroid boxed warning
• Compounded versus labeled product
• Final read

Which one wins, and on what

Here is the side-by-side on the dimensions people actually weigh, drug, the verdict, the load-bearing number, and the source. The two obesity numbers come from separate trials; the 20.2-versus-13.7 line is the one direct comparison.

• Weight loss, direct head-to-head: tirzepatide wins. 20.2 percent versus semaglutide's 13.7 percent at top dose over 72 weeks (SURMOUNT-5) .
• Weight loss, max-dose monotherapy trials: tirzepatide 15 mg about 20.9 percent (SURMOUNT-1) versus semaglutide 2.4 mg about 14.9 percent (STEP-1) .
• Blood sugar in type 2 diabetes: tirzepatide wins. About 2.3 versus 1.9 percent HbA1c reduction (SURPASS-2) .
• Cardiovascular outcomes: semaglutide leads. SELECT cut major adverse cardiac events about 20 percent; tirzepatide's outcomes trial is still pending .
• Boxed warning and tolerability: tie. Same thyroid C-cell boxed warning, same class-typical GI side effects, discontinuation rates in a similar single-digit-percent range .

Why the mechanisms differ (GLP-1 vs GLP-1 plus GIP)

Semaglutide is a GLP-1 receptor agonist. Structural tweaks (an Aib substitution at position 2 and a fatty-acid sidechain) resist the enzyme that normally degrades the hormone and bind it to albumin, which is what gives it a roughly weekly half-life . Tirzepatide is a 39-amino-acid engineered peptide that hits two receptors: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide), with a similar sidechain for once-weekly dosing .

Both engage the GLP-1 pathway and produce the appetite suppression and slower gastric emptying that drive the weight effect. Tirzepatide adds GIP activity, which the mechanistic literature links to extra effects on fat tissue and insulin secretion. Whether GIP agonism is doing the heavy lifting is still argued, but the consistent efficacy edge points to a real pharmacodynamic difference, not a fluke of trial design.

The FDA-approved product map

• Ozempic (semaglutide injection): approved 2017 for type 2 diabetes; weekly; 0.25 mg start, 0.5 / 1 / 2 mg maintenance.
• Wegovy (semaglutide injection): approved 2021 for chronic weight management; weekly; titrated to 2.4 mg, with a 7.2 mg dose later approved for additional reduction in eligible adults.
• Rybelsus (semaglutide tablets): approved 2019 for type 2 diabetes; daily oral with food restrictions; not approved for weight management.
• Mounjaro (tirzepatide injection): approved 2022 for type 2 diabetes; weekly; 2.5 / 5 / 7.5 / 10 / 12.5 / 15 mg.
• Zepbound (tirzepatide injection): approved 2023 for chronic weight management; weekly; same dose range as Mounjaro.

All of these labels are searchable on DailyMed . The brand names matter because the same molecule is sold under different names for different indications, which is a frequent source of confusion.

The direct diabetes head-to-head: SURPASS-2

SURPASS-2 was a 40-week open-label trial comparing tirzepatide (5, 10, 15 mg weekly) against semaglutide 1 mg weekly in adults with type 2 diabetes on metformin. Every tirzepatide dose beat semaglutide 1 mg on both blood sugar and weight, with tirzepatide 15 mg producing about 2.3 percent HbA1c reduction versus 1.9 percent, and about 11.2 kg weight loss versus 5.7 kg .

One caveat carried this comparison for years: SURPASS-2 used semaglutide 1 mg, the diabetes dose, not the 2.4 mg obesity dose. So it did not settle the obesity question on its own. SURMOUNT-5 closed that gap in 2025 by running tirzepatide 15 mg against semaglutide 2.4 mg directly in obesity, and tirzepatide again came out ahead, 20.2 versus 13.7 percent . The interpretation is the same in both trials: dual agonism beats single agonism on weight at the doses studied.

The semaglutide STEP program

• STEP-1: semaglutide 2.4 mg for 68 weeks, about 14.9 percent weight reduction in adults with overweight or obesity .
• STEP-2: same protocol in type 2 diabetes; smaller, about 9.6 percent, consistent with the diabetes-population pattern.
• STEP-3: combined with intensive behavioral therapy; about 16 percent.
• STEP-4: weight came back after a randomized switch to placebo, the central argument that these are maintenance drugs, not courses.
• STEP-HFpEF: semaglutide 2.4 mg in obesity-related heart failure with preserved ejection fraction; improved symptoms and exercise function .
• SELECT: semaglutide 2.4 mg in non-diabetic adults with obesity and established cardiovascular disease; cut major adverse cardiac events about 20 percent over a median 33 months .

The tirzepatide SURMOUNT program

• SURMOUNT-1: tirzepatide 15 mg for 72 weeks, about 20.9 percent weight reduction in adults with obesity and without diabetes .
• SURMOUNT-2: same trial design in type 2 diabetes; about 14.7 percent at 15 mg.
• SURMOUNT-3: intensive lifestyle lead-in then tirzepatide; added effect on top of the lifestyle loss.
• SURMOUNT-4: maintenance trial; weight regain followed the switch to placebo, the same pattern as STEP-4.
• SURMOUNT-OSA: tirzepatide in obesity with moderate-to-severe obstructive sleep apnea; cut the apnea-hypopnea index about 50 percent versus placebo at 52 weeks .

Cardiovascular outcomes: semaglutide's lead

This is where semaglutide is genuinely ahead, and it is the one place the weight number is not the whole story. Semaglutide has hard cardiovascular outcome trials: SUSTAIN-6 (2016) established benefit in type 2 diabetes , and SELECT (2023) extended it to non-diabetic adults with obesity and established cardiovascular disease, cutting major adverse cardiac events about 20 percent .

Tirzepatide's dedicated cardiovascular outcomes trial, SURPASS-CVOT, was designed against an active comparator (dulaglutide) rather than placebo, because by the time it launched, incretin therapy was already standard of care . Until it reads out, tirzepatide's cardiovascular case rests on surrogate markers and the weight loss itself, not on a completed hard-outcome trial. That is a real difference for someone choosing on cardiovascular risk rather than weight.

Tolerability, side effects, and the thyroid boxed warning

The GI side effects (nausea, diarrhea, vomiting, constipation, abdominal pain) are class-typical for both and worst during titration. SURPASS-2 reported broadly similar tolerability, with adverse-event discontinuation in roughly the same single-digit-percent range across arms . Both labels also warn on gallstones, pancreatitis, and acute kidney injury from dehydration when GI symptoms are severe .

Both also flag hypoglycemia risk when combined with insulin or a sulfonylurea, which usually means the prescriber lowers those doses. And both carry the FDA boxed warning for thyroid C-cell tumors. That warning rests on rodent data, not confirmed human cases, and long-term human surveillance has not confirmed the signal at clinical doses, but it stays in the label and is the reason a personal or family history of medullary thyroid carcinoma or MEN-2 is screened before prescribing .

Compounded versus labeled product

Every trial number above comes from FDA-approved labeled product manufactured under chemistry and manufacturing controls. Compounded semaglutide and tirzepatide do not have equivalent oversight, and the FDA has flagged dosing-error patterns and adverse events with compounded GLP-1 products . Semaglutide salt forms (sodium, acetate) are not the same active ingredient as the approved base.

The shortages that opened the compounding lane for both drugs were resolved in 2024 and 2025, and the agency has since acted against compounding outside that framework. Cross-trial efficacy numbers do not transfer to a compounded preparation of unverified identity. The full picture is in compounded GLP-1s vs labeled product.

Final read

Both drugs transformed obesity and type 2 diabetes treatment in under a decade, and both clearly work. Tirzepatide has the larger weight signal and now the direct trial to back it (20.2 versus 13.7 percent). Semaglutide has the longer cardiovascular outcomes record (SELECT) and the broader real-world track record. The choice is fit, not a winner: cardiovascular history, diabetes status, sleep apnea, tolerability, prescriber experience, and what insurance will actually cover .

For research and educational purposes only. Not medical advice.

pepSmart has not commissioned independent clinical review of this article. Choosing or switching between GLP-1 drugs, especially alongside insulin, with thyroid or cardiovascular disease, or in pregnancy, belongs with a qualified clinician who can review your individual history.

For how this article was sourced and reviewed, see Editorial process and contributor disclosure and Sourcing posture.

Spot an error? Email corrections via /about.

Sources: 15 entries, all primary canon (PubMed, ClinicalTrials.gov, DailyMed, FDA), last reviewed 2026-05-28.

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