Tirzepatide (Mounjaro, Zepbound) and semaglutide (Ozempic, Wegovy) are the two most-prescribed weekly injectables for type 2 diabetes and weight management. They share the same once-weekly subcutaneous dosing, similar side-effect profile, and overlapping FDA-approved indications — but they're different molecules with different receptor targets, different half-lives, and meaningfully different weight-loss outcomes in the trials. This article compares them head-to-head with numbers from the FDA Ozempic label, the FDA Mounjaro label, and the SURPASS, STEP, and SURMOUNT trial programs.
Semaglutide binds the GLP-1 receptor only. GLP-1 (glucagon-like peptide-1) is an incretin hormone released after meals — it stimulates glucose-dependent insulin secretion, slows gastric emptying, and acts centrally to reduce appetite.
Tirzepatide does both. It binds GLP-1 and also GIP (glucose-dependent insulinotropic polypeptide), the other major incretin. The dual mechanism appears to drive larger weight loss, although the precise contribution of GIP is still being characterized in mechanistic studies. The molecule's structure, dosing, and PK profile are documented in the FDA Mounjaro label and the StatPearls tirzepatide monograph.
Both drugs use albumin-binding fatty-acid acylation to extend their half-lives well beyond what native GLP-1 (≈2 minutes) would allow. The dual-target tirzepatide molecule clears slightly faster than semaglutide:
| Parameter | Semaglutide (Ozempic, Wegovy) | Tirzepatide (Mounjaro, Zepbound) |
|---|---|---|
| Half-life (t½) | ~168 h (7 d) | ~120 h (5 d) |
| Tmax (SC) | ~48 h | ~36 h |
| Bioavailability (SC) | 89% | 80% |
| Time to steady state | ~4–5 weeks | ~4 weeks |
| Clearance after stop (5 t½) | ~35 days | ~25 days |
The practical implication of the half-life difference: a missed dose on tirzepatide drops you slightly faster than a missed dose on semaglutide. Both are still buffered by ~50% trough levels at steady state, so a single skipped week is rarely noticeable. For the full math on semaglutide clearance see How long does Ozempic stay in your system.
The flagship weight-loss trials for each drug used different populations and durations, so the cleanest comparison is the SURMOUNT-5 head-to-head and the diabetes-population SURPASS-2.
| Trial | Drug · Dose | Population | Duration | Mean weight loss |
|---|---|---|---|---|
| STEP-1 | Semaglutide 2.4 mg | Obesity, no T2D | 68 wk | 14.9% |
| SURMOUNT-1 | Tirzepatide 5 / 10 / 15 mg | Obesity, no T2D | 72 wk | 16.0% / 21.4% / 22.5% |
| SURMOUNT-5 (H2H) | Tirzepatide max-tolerated | Obesity, no T2D | 72 wk | 20.2% |
| SURMOUNT-5 (H2H) | Semaglutide max-tolerated | Obesity, no T2D | 72 wk | 13.7% |
| SURPASS-2 | Tirzepatide 15 mg vs Sema 1 mg | T2D | 40 wk | 11.2 kg vs 5.7 kg |
The signal is consistent: at maximum approved doses, tirzepatide produces roughly 5–8 percentage points more mean weight loss than semaglutide over comparable durations. The SURMOUNT-5 trial (Aronne et al., NEJM 2025) is the cleanest direct comparison; SURPASS-2 (Frías et al., NEJM 2021) made the same point in a diabetes population. STEP-1 (Wilding et al., NEJM 2021) and SURMOUNT-1 (Jastreboff et al., NEJM 2022) are the foundational obesity trials for each drug.
For glucose control, SURPASS-2 reported A1c reductions of 2.01% (tirzepatide 5 mg), 2.24% (10 mg), and 2.30% (15 mg) versus 1.86% on semaglutide 1 mg over 40 weeks. Both drugs are clinically meaningful glucose-lowering agents; tirzepatide's higher doses extend the curve further. For the average T2D patient, both substantially exceed the typical 0.5–1% A1c reduction from oral agents.
The dominant side effects on both drugs are gastrointestinal and dose-dependent. SURPASS-2 nausea rates: 17–22% on tirzepatide vs 18% on semaglutide. Vomiting: 6–10% vs 8%. Diarrhea: 12–17% vs 12%. Most resolve over the first 4–8 weeks of titration as the gut adapts.
Less common but labeled effects on both drugs:
The much-discussed claim that tirzepatide's GIP activity adds new side effects has not been borne out. Trial-pooled rates of GI events are similar; the SURMOUNT trials did not surface a new tirzepatide-specific adverse event class.
Both drugs use 4-week titration steps to manage GI tolerability:
| Week | Ozempic (T2D) | Wegovy (Obesity) | Mounjaro (T2D) | Zepbound (Obesity) |
|---|---|---|---|---|
| 1–4 | 0.25 mg | 0.25 mg | 2.5 mg | 2.5 mg |
| 5–8 | 0.5 mg | 0.5 mg | 5 mg | 5 mg |
| 9–12 | 1 mg | 1 mg | 7.5 mg | 7.5 mg |
| 13–16 | 2 mg | 1.7 mg | 10 mg | 10 mg |
| 17+ | 2 mg max | 2.4 mg | 12.5 → 15 mg | 12.5 → 15 mg |
Doses listed are per the current FDA labels. Many clinicians slow titration further if GI side effects are not tolerated.
Standard practice when switching is to restart at the lowest dose of the new drug — 2.5 mg for tirzepatide, 0.25 mg for semaglutide — and follow the standard 4-week titration. There is no FDA-approved equivalency table for cross-titration. Because semaglutide's half-life is 7 days, expect 4–5 weeks of pharmacologic overlap when switching off it; tirzepatide overlap is shorter at 3–4 weeks.
Some clinicians shorten the restart in patients already at high tolerated doses of the prior drug, but the conservative approach is the labeled one. If you switch, log both drugs in your tracker so you can see the overlap and titration arc visually.
U.S. list prices in 2025–2026:
Insurance coverage is the dominant variable. Diabetes labels (Ozempic, Mounjaro) are widely covered when criteria are met. Weight-management labels (Wegovy, Zepbound) are excluded from many commercial and most Medicare Part D plans, though that is shifting. Both manufacturers run direct cash-pay programs that price single-dose vials below the official list.
That decision belongs to you and your prescriber. The objective comparison: tirzepatide produces larger mean weight loss and slightly larger A1c reductions in the trials; semaglutide has a longer half-life, longer real-world track record, and a larger cardiovascular outcomes dataset (SUSTAIN-6, SELECT). Side-effect rates are similar. Cost is similar at list, but real out-of-pocket cost depends on your plan and which cash-pay program you qualify for. Tolerance varies by individual; some people tolerate one well after stopping the other for nausea.
Dose Track models the full PK curve for both semaglutide and tirzepatide using the half-life and Tmax values from the FDA labels above. Log each weekly injection and the app overlays peak, trough, accumulation across weeks, and the decay tail after stopping — so a switch between drugs becomes a single visualization, not a guessing game. See GLP-1 tracking features, the full supported medications list, or download on the App Store.