Which is more effective for weight reduction?

Tirzepatide, clearly, in the only large head-to-head trial: 20.2% against 13.7% over 72 weeks.

Is tirzepatide worse tolerated because it is stronger?

Not on this measure. Gastrointestinal discontinuation was lower on tirzepatide at 2.7% than semaglutide at 5.6%.

Why would anyone use semaglutide then?

It has far more cardiovascular outcome evidence, including SELECT's 20% MACE reduction across 17,604 participants, and broader approved and pending indications.

GLP-1 & Incretin Science

Semaglutide vs Tirzepatide: What the Direct Comparison Showed

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-233 cited sources

SURMOUNT-5 compared the two directly over 72 weeks in 751 adults with obesity and without diabetes. Tirzepatide produced 20.2% mean weight reduction against semaglutide's 13.7%, with greater waist reduction and fewer gastrointestinal discontinuations. Semaglutide retains the larger cardiovascular outcome evidence base.

Key facts

Trial: SURMOUNT-5, 751 adults, 72 weeks
Tirzepatide result: −20.2% (22.8 kg)
Semaglutide result: −13.7% (15.0 kg)
Waist circumference: −18.4 cm vs −13.0 cm
GI discontinuation: 2.7% vs 5.6%
Doses compared: Max tolerated: 10/15 mg vs 1.7/2.4 mg
Significance: p<0.001 on both endpoints

Why this trial settles more than cross-trial comparison could

Most comparisons in this field are indirect — one drug's trial against another drug's trial, with different populations, durations and estimands. SURMOUNT-5 randomised the same population to both drugs at the same time for the same duration, which removes essentially every confound that makes indirect comparison unreliable. It is the strongest evidence available on this question.

The headline result

Over 72 weeks, tirzepatide produced 20.2% mean weight reduction, averaging 22.8 kg, against semaglutide's 13.7%, averaging 15.0 kg. Both arms used the maximum tolerated dose — 10 or 15 mg for tirzepatide, 1.7 or 2.4 mg for semaglutide. Tirzepatide participants were also more likely to reach every threshold from 10% through 25%.

Body composition and cardiometabolic measures

Waist circumference fell 18.4 cm on tirzepatide against 13.0 cm on semaglutide. Tirzepatide also produced significantly greater improvement in systolic and diastolic blood pressure, HbA1c, fasting insulin, triglycerides and HDL cholesterol at 72 weeks.

The tolerability finding is the counterintuitive one

Larger effect usually costs tolerability across this class. Here it did not. Gastrointestinal adverse events causing discontinuation occurred in 2.7% on tirzepatide against 5.6% on semaglutide — the more effective drug was also the better tolerated one on that measure. This is a genuine exception to the pattern that holds almost everywhere else in incretin pharmacology.

What semaglutide still has

Outcome data. SELECT enrolled 17,604 participants and demonstrated a 20% reduction in major adverse cardiovascular events, and semaglutide has been under review for further indications including heart failure with preserved ejection fraction and peripheral artery disease. Weight reduction is a surrogate; hard cardiovascular outcomes are not, and that evidence base takes years to build.

A detail worth noticing

Weight reduction was approximately 6% lower in men than in women across both treatment arms. Sex differences of that size are rarely foregrounded in coverage of these trials, and they matter for interpreting any single reported average.

Quick reference

Endpoint (72 wks)	Tirzepatide	Semaglutide
Mean weight change	−20.2%	−13.7%
Absolute weight	−22.8 kg	−15.0 kg
Waist circumference	−18.4 cm	−13.0 cm
GI discontinuation	2.7%	5.6%
Receptors	GIP + GLP-1	GLP-1
CV outcome trial	—	SELECT, 20% MACE reduction

Extended research context

The GLP-1 & Incretin Science deep dive

Deep dive: the two routes to a bigger effect

Every compound trying to beat GLP-1 alone has taken one of two routes. The first adds more receptors from the same hormone family — GIP in tirzepatide, GIP and glucagon in retatrutide. The second adds a non-incretin satiety hormone, which in practice means amylin: CagriSema combines cagrilintide with semaglutide, and amycretin engages both receptors from one molecule. Both routes work, because they recruit signalling pathways that do not fully overlap. Neither has escaped the constraint that binds all of them, which is that gastrointestinal tolerability worsens as effect size grows.

Deep dive: why a percentage is not a result

The most-quoted numbers in this field are the least comparable. REDEFINE 1 reported 22.7% and 20.4% for the same compound in the same trial — the first among participants who adhered to treatment, the second across everyone randomised. TRIUMPH-1 reported 28.3% in an uncomplicated obesity population while TRIUMPH-3 reported up to 22.6% in adults with established cardiovascular disease, using the same compound. Before any two figures can be compared they have to match on estimand, population, duration, comparator and whether the number is placebo-adjusted. Most published comparisons match on none of them.

Deep dive: what happens after the trial stops

Every headline figure describes weight while treatment continues. The STEP-1 extension found that a year after semaglutide was stopped, participants had given back roughly two-thirds of what they lost, moving from 17.3% mean reduction to a net 5.6% — though average weight remained below baseline and nearly half stayed at least 5% down. Meta-analysis puts regain at around 0.8 kg per month. This is why maintenance studies such as TRIUMPH-6 matter more to the field's future than another two points of peak reduction.

Research applications

▸Comparing incretin and amylin compounds on a like-for-like basis
▸Interpreting estimands, thresholds and placebo-adjusted figures in trial reports
▸Tracking the obesity pipeline across sponsors and jurisdictions
▸Understanding receptor pharmacology behind GLP-1, GIP, glucagon and amylin
▸Distinguishing licensed medicines from investigational compounds

Handling checklist

✓Identify which estimand a quoted percentage comes from before citing it
✓Check the trial population and baseline BMI against the comparison you are making
✓Confirm the duration and whether the reduction curve had plateaued
✓Read discontinuation rates alongside efficacy figures
✓Verify every NCT identifier against ClinicalTrials.gov rather than secondary reporting

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Comparing headline percentages across different trials

Fix: Population, duration, estimand and comparator all differ; the numbers are not interchangeable.

✗ Quoting the larger of two figures from the same trial

Fix: Name the estimand. Efficacy and treatment-policy answer different questions.

✗ Treating peak reduction as a durable outcome

Fix: Substantial regain follows cessation across the class; peak figures describe a maintained state.

✗ Assuming an oral route means a weaker mechanism

Fix: Route and receptor count are independent. Orforglipron is weaker because it hits one receptor, not because it is a tablet.

✗ Reading investigational compounds as available treatments

Fix: Most of this pipeline holds no authorisation anywhere; mazdutide is approved only in China.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

Which is more effective for weight reduction?: Tirzepatide, clearly, in the only large head-to-head trial: 20.2% against 13.7% over 72 weeks.
Is tirzepatide worse tolerated because it is stronger?: Not on this measure. Gastrointestinal discontinuation was lower on tirzepatide at 2.7% than semaglutide at 5.6%.
Why would anyone use semaglutide then?: It has far more cardiovascular outcome evidence, including SELECT's 20% MACE reduction across 17,604 participants, and broader approved and pending indications.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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