In May 2025, the New England Journal of Medicine dropped SURMOUNT-5, the first randomised head-to-head of tirzepatide (Zepbound) and semaglutide (Wegovy) in adults with obesity and no T2D (NEJM, 2025). The verdict: tirz produced 47% greater relative weight loss. What does that look like in practice, and who actually feels the gap?
Key Takeaways
- —Tirz hit −20.2% mean body weight at 72 weeks vs −13.7% for sema (SURMOUNT-5, NEJM 2025).
- —31.6% of tirz users dropped ≥25% of body weight vs 16.1% on sema, roughly twice as many.
- —Side-effect profiles look similar on paper. Both lean GI-heavy, both see comparable dropouts.
- —Tirz is a dual GIP/GLP-1 agonist; sema is GLP-1 only. The extra GIP arm is the leading explanation for the efficacy gap.
- —Real-world 12-month data from a tele-doc weight programme tracks the trial signal directionally (PMC, 2026).
SURMOUNT-5 enrolled 751 adults with BMI ≥30, or ≥27 with at least one weight-related comorbidity, and no T2D. Participants were randomised 1:1 to maximum tolerated doses of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) over 72 weeks (Applied Clinical Trials, 2025).
The primary endpoint, least-squares mean percent change in body weight, came out like this:
| Metric | Tirzepatide | Semaglutide | Difference |
|---|---|---|---|
| Mean weight loss | −20.2% | −13.7% | −6.5 pp |
| ≥25% weight loss | 31.6% | 16.1% | ~2× more likely |
| ≥15% weight loss | 64.6% | 40.1% | −24.5 pp |
| Waist circumference | −18.4 cm | −13.0 cm | −5.4 cm |
| HbA1c change (pre-diabetic subset) | −0.46% | −0.33% | −0.13 pp |
The p-value was <0.001 across every primary and secondary endpoint. That's not a marginal result. It's a clean, consistent separation across multiple outcome measures.
The mechanism difference is the headline. Sema is a GLP-1 receptor agonist, full stop. It mimics glucagon-like peptide 1, slowing gastric emptying, dialling up satiety, and quieting food noise. It has cardiovascular benefit data behind it and is approved as Ozempic for diabetes and Wegovy for obesity.
Tirz adds a second arm: GIP (glucose-dependent insulinotropic polypeptide) receptor agonism. GIP was historically labelled the "fat storage" hormone, which made it a counterintuitive obesity target. But pair it with GLP-1 agonism and GIP appears to push fat oxidation, adipose remodeling, and insulin sensitivity beyond what GLP-1 alone delivers. That's the working theory.
According to the SURMOUNT-5 investigators, the 47% greater relative weight loss with tirzepatide vs semaglutide — 20.2% vs 13.7% at 72 weeks — represents the largest efficacy gap observed in a head-to-head trial of approved anti-obesity medications to date (NEJM, 2025).
Trial populations are cherry-picked. Real-world data carries more weight clinically. A 2026 retrospective from a tele-doc remote weight management programme found tirz users dropped more weight at 3 months (−2.4% extra), 6 months (−4.3% extra), and 12 months (−6.9% extra) vs sema (PMC, 2026). The real-world gap moves in the same direction as the trial, just smaller in absolute terms, which is what you'd expect when you leave a controlled setting.
A 2024 JAMA Internal Medicine study looked at real-world prescribing data and found tirz users were 76% more likely to hit ≥5% weight loss, 154% more likely to hit ≥10%, and 224% more likely to hit ≥15% vs sema.
Both are once-weekly subq pins, but the titration ladders and ceiling doses are different beasts. Switching between them is not a 1:1 swap, and people on the subs who try to skip steps usually regret it.
| Phase | Tirzepatide (Zepbound) | Semaglutide (Wegovy) |
|---|---|---|
| Starting dose | 2.5 mg weekly × 4 weeks | 0.25 mg weekly × 4 weeks |
| Titration | +2.5 mg every 4 weeks | Doubled every 4 weeks (0.5 → 1.0 → 1.7) |
| Full efficacy doses | 5 / 10 / 15 mg | 1.7 / 2.4 mg |
| Maximum approved dose | 15 mg | 2.4 mg |
| Typical time to maintenance | 20 weeks (5 titration steps) | 16 weeks (4 titration steps) |
The slower titration on tirz is deliberate. At 15 mg you're running 6x the starting dose, and the 2.5 mg increments give the gut time to adapt. Skip steps and dropout from nausea and vomiting jumps fast. A dose hold for a week or two when GI gets ugly is normal and beats quitting.
Both drugs hit GI hardest: nausea, vomiting, diarrhoea, constipation, plus the famous sulfur burps that everyone on r/Tirzepatide and r/Semaglutide warns new users about. In SURMOUNT-5, the side effect profiles were similar between arms, which tracks with their shared GLP-1 mechanism (Aronne et al., NEJM, 2025). Discontinuation rates from adverse events were also close (~6% tirz vs ~5% sema).
Specific adverse event rates from combined trial data:
| Adverse event | Tirzepatide | Semaglutide |
|---|---|---|
| Nausea | 18–31% | 20–44% |
| Diarrhoea | 12–17% | 17–30% |
| Vomiting | 6–10% | 6–24% |
| Constipation | 6–13% | 11–24% |
| Injection site reaction | Rare, mild | Rare, mild |
| Gallbladder disease (serious) | ~0.6% | ~1.6% (SELECT, NEJM, 2023) |
| Pancreatitis (serious) | <0.5% | <0.5% |
Both carry boxed warnings for medullary thyroid carcinoma based on rodent carcinogenicity data. The human relevance is unsettled, but personal or family history of MTC or MEN2 syndrome is an absolute no-go for both.
The takeaway: neither has a clean tolerability win at comparable effective doses. Anecdotally, people who don't tolerate one GLP-1 usually struggle with the other too. The mechanism, not the molecule, is doing most of the GI damage. From what people report, the first week is rough on either drug, then most settle in by week three or four.
Not everyone is shooting for 20% weight loss. For someone with a modest target (5-10%), sema's established cardiovascular outcome data (SELECT, 2023) and longer paper trail make it the more defensible pick. For people with obesity-related comorbidities who need substantial weight reduction, where 6.5 percentage points translates to real differences in joint load, sleep apnoea severity, or metabolic disease progression, tirz's efficacy edge is the call.
Both drugs face hard access walls. US list prices clear $1,000/month for either without insurance, and only a slice of commercial plans cover them for obesity (T2D coverage is much broader).
| Access pathway | Tirzepatide | Semaglutide |
|---|---|---|
| Brand list price (monthly) | ~$1,060 (Zepbound) | ~$1,349 (Wegovy) |
| Manufacturer cash program | $499/mo (LillyDirect, 2.5/5 mg) | ~$499/mo (NovoCare, select doses) |
| Compounded (503A, where legal) | $229–349/mo | Mostly ended Feb 2026 |
| Typical insurance copay (when covered) | $25–150/mo | $25–150/mo |
The compounded landscape flipped hard in early 2026. The FDA called the semaglutide shortage over in February 2026, which kicked off enforcement against most compounded sema. People relying on compounded sema have mostly had to move to brand Wegovy/Ozempic or NovoCare cash. Tirz's shortage resolution did the same in late 2024, though limited compounding exceptions still exist for people with documented clinical needs that branded products can't meet (specific dose requirements, allergies to inactive ingredients).
For most people paying out-of-pocket in 2026, tirz via LillyDirect at ~$499/mo is the cheapest legal route to GLP-1 therapy, cheaper than compounded sema at its peak, and with Lilly's QC behind it.
Beyond weight loss, both drugs have outcome-trial data that shifts who they're for.
Semaglutide (more mature evidence):
Tirzepatide (evidence is building):
Bottom line for the prescriber: if there's established cardiovascular disease in the chart, sema has the receipts today. If the goal is raw weight loss magnitude, tirz wins on efficacy and the outcome data is coming. The gap will close as tirz's CVOTs read out.
Both drugs work while you take them. The STEP-1 extension showed people who stopped sema regained two-thirds of the weight they lost within a year (Wilding et al., Diabetes Obes Metab, 2022). Tirz follows the same pattern in the SURMOUNT-4 withdrawal arm. This isn't a flaw in the drugs. Obesity is a chronic, relapsing condition being treated symptomatically, not cured.
The practical read for users and prescribers: GLP-1 therapy for obesity should be planned as long-term. "Drug holidays" and indefinite stalls off the drug almost always mean the weight comes back.
Where most endocrinologists and obesity medicine specialists are landing right now:
Before picking between these two FDA-approved compounds, or walking into a tele-doc consult about either, the Next Pep comparison tool puts tirz and sema side-by-side across mechanism, pharmacokinetics, and dosing data in one objective view. The tirzepatide research profile breaks down the dual GIP/GLP-1 mechanism in detail with full SURMOUNT-5 data, and the peptide library gives you both compounds' full molecular and clinical summaries. Neither source has a financial stake in which one you pick.
Per SURMOUNT-5 (NEJM, 2025), yes. Tirz produced 20.2% mean weight loss vs 13.7% for sema over 72 weeks in adults with obesity and no T2D. That's 47% greater relative weight loss, the largest gap shown in a head-to-head between approved anti-obesity drugs.
Sema is a GLP-1 receptor agonist. Tirz is a dual GIP/GLP-1 receptor agonist. Adding GIP agonism appears to push metabolic outcomes beyond what GLP-1 alone does, which is the leading explanation for the efficacy gap.
Both run GI-heavy: nausea, vomiting, diarrhoea, constipation, sulfur burps. In head-to-head, the overall side-effect profiles were similar and discontinuation rates were close. Most users report the first week or two being the worst, then a settling-in period through titration.
Sema has the deeper bench from the SELECT trial (2023), which showed a 20% reduction in major adverse cardiovascular events in high-risk patients. Tirz's CVOT (SURPASS-CVOT) is still running. The 2024 SUMMIT trial showed tirz cut heart failure events by 38% in HFpEF patients with obesity.
No. Tirz needs its own titration starting at 2.5 mg weekly regardless of prior GLP-1 exposure. Skipping tirz's titration steps just spikes nausea and vomiting with no efficacy upside. A 4-week start at 2.5 mg is the shortest reasonable on-ramp.
Both drugs lose effect once you stop. STEP-1 extension data on sema shows people regain about two-thirds of lost weight within a year of stopping. Tirz's SURMOUNT-4 withdrawal data tracks the same pattern. Plan GLP-1 therapy for obesity as long-term, not a short cycle.
Mostly no. The FDA called both shortages over (tirz in late 2024, sema in February 2026), and the 503A compounding carve-out has ended for most users. Limited clinical-exception compounding remains for documented individualised needs. For cash-pay in 2026, LillyDirect runs tirz at around $499/month and NovoCare puts sema in the same range.
This article is for research and informational purposes only. Both tirzepatide and semaglutide are prescription medications. Consult a licensed healthcare professional for personalised medical advice.
Research Disclaimer. All content on Next Pep is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment. Consult a licensed healthcare professional before considering any peptide protocol.
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