GLP-TRZWhat the studies show

Q: What kind of evidence exists for tirzepatide?

Large human trials. Large randomised, placebo-controlled phase 3 trials in thousands of people, published in top journals and reviewed by regulators. That is strong evidence over 40–72 weeks; what happens over decades is not yet known.

A dual GIP/GLP-1 agonist, approved as Mounjaro and Zepbound. In the 72-week SURMOUNT-1 trial, the 15 mg group lost an average of 20.9% of body weight.

5 min read · Large human trials · Updated November 2026

In short

Name: tirzepatide (sold by Progression Labs as GLP-TRZ)
Evidence: Large human trials
Bottom line: Large randomised, placebo-controlled phase 3 trials in thousands of people, published in top journals and reviewed by regulators. That is strong evidence over 40–72 weeks; what happens over decades is not yet known.

Before you read this

This guide explains what published studies found. That is all it is.

It is not medical advice and it is not instructions for using anything. Where a dose appears, we are reporting what a named study gave the people or animals in it — a fact about the study, not an instruction to you.

The compounds here are not approved by the FDA for use in people and are sold for laboratory research only. A study tells you what happened to the subjects in it; it does not tell you what would happen to you. Talk to a doctor first.

What it is

Tirzepatide is a 39-amino-acid peptide built on the sequence of GIP, one of the two gut hormones released after a meal, and altered so that it also activates the GLP-1 receptor. A fatty-acid chain attached to the peptide binds it to albumin in the blood, which is why it lasts long enough to be given once a week. It is an Eli Lilly compound, and unlike most of what this site covers it is an approved drug: the U.S. FDA approved it as Mounjaro for type 2 diabetes in May 2022 and as Zepbound for chronic weight management in November 2023 ¹. The two products are the same molecule under different labels, and both carry the same list of side effects and warnings.

The studies below call this molecule tirzepatide. Progression Labs sells it as GLP-TRZ; the guide uses the store's name and the papers use theirs.

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The SURMOUNT-1 obesity trial

The trial behind the weight-loss approval was published in the New England Journal of Medicine in 2022. It enrolled 2,539 adults with obesity, or overweight plus at least one weight-related condition, none of whom had diabetes, and randomised them to placebo or to tirzepatide at 5, 10 or 15 mg once a week for 72 weeks ².

At 72 weeks the average change in body weight was −15.0% at 5 mg, −19.5% at 10 mg and −20.9% at 15 mg, against −3.1% on placebo ². Nine in ten participants on the two higher doses lost at least 5% of their body weight, and more than half of those on 15 mg lost 20% or more; on placebo, about one in three reached 5% ².

A later trial, SURMOUNT-5, put tirzepatide head to head with semaglutide 2.4 mg in 751 adults with obesity and no diabetes. Over 72 weeks the tirzepatide group lost an average of 20.2% of body weight and the semaglutide group 13.7% ³.

Type 2 diabetes: the SURPASS trials

Tirzepatide was approved for diabetes first, on the strength of the SURPASS programme. The most-cited trial, SURPASS-2, compared it with semaglutide 1 mg in 1,879 people with type 2 diabetes already on metformin, over 40 weeks. HbA1c fell by 2.01, 2.24 and 2.30 percentage points at 5, 10 and 15 mg, against 1.86 points with semaglutide, and weight fell by 7.6, 9.3 and 11.2 kg against 5.7 kg ⁴. Every tirzepatide dose beat the comparator on both measures.

What happens when it stops

SURMOUNT-4 answered the question most people ask next. After 36 weeks on tirzepatide, during which participants lost an average of 20.9% of body weight, 670 of them were randomised to keep going or to switch to placebo for another 52 weeks. Those who continued lost a further 5.5%; those switched to placebo regained 14% of their body weight over the year ⁵. The drug's effect, in other words, lasted as long as the drug.

Side effects and what is unknown

The common side effects in every trial were the ones shared across this class: nausea, diarrhoea, constipation and vomiting, mostly mild to moderate and clustered around dose increases ². In SURMOUNT-1, between 4.3% and 7.1% of participants on tirzepatide stopped because of side effects, against 2.6% on placebo ². The U.S. label carries a boxed warning about thyroid C-cell tumours, which were seen in rats and have not been shown in people ¹.

What the trials cannot yet say is what happens over many years, or in people who were not eligible for them: pregnancy, a history of pancreatitis and a family history of certain thyroid cancers were all exclusions. The weight-loss figures above also come from trials in which the drug was paired with regular lifestyle counselling, and the people in them were weighed, examined and asked about side effects every few weeks.

What these studies are, and aren't

Large human trials. Large randomised, placebo-controlled phase 3 trials in thousands of people, published in top journals and reviewed by regulators. That is strong evidence over 40–72 weeks; what happens over decades is not yet known.