How does tirzepatide work? What published studies have found

How does tirzepatide work?

Tirzepatide is a GIP receptor and GLP-1 receptor agonist, a molecule that switches on the receptors of two gut hormones. The US Mounjaro label reports enhanced insulin secretion, reduced glucagon and decreased food intake in human pharmacodynamic summaries for type 2 diabetes; that summary supplies no participant count and does not predict an individual response (DailyMed, 2 September 2026). Gastric emptying means the speed at which the stomach empties; appetite is the drive to eat, and calorie intake is the energy consumed in food. GIP and GLP-1 are incretins, gut hormones released after food (Coskun et al., Molecular Metabolism, 2018).

The TGA report summarises what the two hormones are known to do. GLP-1 acts on the pancreas (more insulin, less glucagon), on the gut (slower emptying) and on the brain (less appetite, more fullness). GIP acts on fat tissue (more fat breakdown and fatty-acid uptake). That table describes the hormones' known functions. It does not measure each effect for tirzepatide itself (TGA AusPAR, 20 September 2024).

A 2020 laboratory study (Willard et al., JCI Insight, 2020) used receptor signalling studies, experiments in primary islets and a calculation of receptor occupancy at the study's modelled exposures, and reports that tirzepatide mimics natural GIP at the GIP receptor and engages the GIP receptor more than the GLP-1 receptor, which the authors call an imbalanced mechanism; it enrolled no participants, so it does not establish a human result. The Mounjaro label says tirzepatide is based on the GIP sequence and that a fatty diacid chain enables albumin binding and prolongs the half-life (US Mounjaro label, DailyMed, 2 September 2026). Where tirzepatide comes from covers the design.

What is tirzepatide used for, according to its approved labels?

Approved labels list tirzepatide for different uses, depending on the brand and the country. The US Mounjaro label lists improving glycaemic control in adults and paediatric patients aged 10 years and over with type 2 diabetes, as an addition to diet and exercise, and reducing major cardiovascular events in adults with type 2 diabetes who are at high risk for them. The US Zepbound label lists reducing excess body weight and maintaining the reduction long term in adults with obesity, or adults with overweight and at least one weight-related condition, and treating moderate to severe obstructive sleep apnoea in adults with obesity, each with a reduced-calorie diet and increased physical activity (DailyMed, 2 September 2026). The TGA register has Mounjaro entries dated 23 December 2022 (type 2 diabetes), 10 September 2024 (chronic weight management) and 2 June 2025 (obstructive sleep apnoea), read 14 October 2026.

These are statements about approved medicines, taken from their labels and the TGA register. Whether tirzepatide is available in Australia sets out the registers and their dates. The approved uses above belong to the registered medicines. Research-use tirzepatide material is not one of those medicines, and none of these labels describes it.

Evidence limits

What do these studies not show?

Weight is one outcome among many. A percentage weight change is a single measure and does not describe other outcomes, such as adverse events. The side-effects page holds the adverse-event record, which a weight result cannot replace.

What have trials found about body weight and tirzepatide?

In SURMOUNT-1, a 72-week Phase 3 double-blind trial of 2,539 adults with obesity or overweight and no diabetes, average weight fell 15.0%, 19.5% and 20.9% across the three tirzepatide arms, against 3.1% with placebo, in an analysis that counted participants regardless of treatment discontinuation, in a maker-funded trial whose averages do not predict any one person's result (Jastreboff et al., New England Journal of Medicine, 2022). A three-year follow-up of the 1,032 adults with obesity and prediabetes in that trial found that average weight fell 12.3%, 18.7% and 19.7% by week 176, against 1.3% with placebo, in a double-blind subgroup analysis of that maker-funded trial, a result limited to that subgroup and not a prediction for any one person (Jastreboff et al., New England Journal of Medicine, 2025).

The share of participants matters as much as the average. In SURMOUNT-1 (2,539 adults with obesity or overweight and no diabetes), at week 72, 85%, 89% and 91% of the three tirzepatide arms had lost at least 5% of body weight, against 35% with placebo, and 50% and 57% of the two higher arms had lost at least 20%, against 3%, group shares from a maker-funded trial that do not predict any one person's result (Jastreboff et al., New England Journal of Medicine, 2022).

In the 2022 paper of 2,539 adults with obesity or overweight and no diabetes, the most common adverse events with tirzepatide were gastrointestinal and mostly mild to moderate, a group-level finding that does not predict any one person's experience (Jastreboff et al., New England Journal of Medicine, 2022). In the 2025 paper of 1,032 adults with obesity and prediabetes, other than COVID-19, the most common adverse events were gastrointestinal, mostly mild to moderate and mostly in the first 20 weeks, within that subgroup's limited follow-up (Jastreboff et al., New England Journal of Medicine, 2025). The numbers are on the side-effects page. The trial lasted 72 weeks. The follow-up reports week 176 and adds a 17-week period off treatment, for which its abstract gives type 2 diabetes diagnoses and no weight result.

How much weight did trial participants lose in the first three months?

The abstracts read for this page did not report an average weight change for a whole trial population at 12 weeks or at one month, and the trials named on this page report their main weight results at 26 weeks or later. In a post hoc analysis of SURMOUNT-1 (1,545 tirzepatide-treated adults with obesity or overweight and no diabetes, selected because they met the study's adherence criterion and had weights recorded at weeks 0, 12, 24 and 72, analysed after the trial ended and not planned as its main question), 82% (1,267) had lost at least 5% of body weight by week 12 and 18% (278) had lost less than 5%, in an analysis whose abstract names no funder and whose result applies only to that selected group (Ard et al., Diabetes, Obesity and Metabolism, 2025).

In that post hoc analysis, 90% (250) of the 278 late responders, who had lost less than 5% at week 12 and were adults with obesity or overweight and no diabetes selected from the 1,545-person adherence and measurement subset, reached at least a 5% loss by week 72, a group figure from an analysis run after the trial and not a prediction for any one person (Ard et al., Diabetes, Obesity and Metabolism, 2025).

In the double-blind, maker-funded 26-week Phase 2 trial in 316 people with type 2 diabetes, where tirzepatide was named LY3298176, average weight change at week 26 ranged from 0.9 kg to 11.3 kg lost across four tirzepatide arms, against 0.4 kg with placebo and 2.7 kg with dulaglutide, in a selected adult diabetes population at a different time point from SURMOUNT-1, not an individual prediction or cross-trial comparison (Frías et al., The Lancet, 2018).

Did tirzepatide or semaglutide lead to more weight loss in trials?

In SURMOUNT-5, an open-label 72-week trial of 751 adults with obesity and no diabetes who were assigned to tirzepatide or semaglutide, the least-squares mean weight change was a fall of 20.2% with tirzepatide and 13.7% with semaglutide, and waist circumference fell 18.4 cm against 13.0 cm, in a maker-funded trial whose participants and staff knew which drug was given (Aronne et al., New England Journal of Medicine, 2025). A person's own result can differ from these group averages, and the trial cannot say what any one person would see.

Semaglutide has its own trial record. In STEP 1, a double-blind, maker-funded trial of 1,961 adults with overweight or obesity and no diabetes, given a lifestyle intervention alongside, average weight fell 14.9% over 68 weeks with semaglutide, against 2.4% with placebo, a group average from that restricted trial population and not a prediction for any one person (Wilding et al., New England Journal of Medicine, 2021). In SURPASS-2, an open-label 40-week maker-funded trial of 1,879 adults with type 2 diabetes, the estimated differences in weight loss were 1.9 kg, 3.6 kg and 5.5 kg more with tirzepatide than with semaglutide across the three tirzepatide arms (Frías et al., New England Journal of Medicine, 2021).

Percentages from STEP 1 and SURMOUNT-1 should not be set side by side as if they came from one trial. The designs, lengths and populations differ. How the SURPASS and SURMOUNT programmes are built explains the trial families.

Can weight come back after stopping Mounjaro?

In SURMOUNT-4, a withdrawal-design trial that randomised 670 adults with obesity or overweight and no diabetes (335 in each group) after a 36-week open-label tirzepatide lead-in, average weight rose 14.0% from week 36 to week 88 in those randomised to a double-blind switch to placebo, while those who continued tirzepatide lost a further 5.5% (difference -19.4%, 95% confidence interval -21.2% to -17.7%), and 89.5% of those who continued kept at least 80% of their earlier loss, against 16.6% of those randomised to placebo, group results from a trial whose abstract names no funder and whose result applies only to adults who completed the lead-in (Aronne et al., JAMA, 2024).

A 2026 systematic review and meta-analysis in BMJ (West et al., BMJ, 2026) pooled 37 randomised, non-randomised and observational studies of weight management medicines in adults with overweight or obesity (63 intervention arms, 9,341 participants) and reported an average regain after stopping of 0.4 kg a month (95% confidence interval 0.3 to 0.5), observed over an average follow-up of 32 weeks after stopping, across many medicines, so the result describes the class and not tirzepatide alone. West et al.'s 2026 BMJ review of 37 studies in 9,341 adults with overweight or obesity projected that cardiometabolic markers would return to baseline within 1.4 years, a model projection across mixed medicines rather than an observation in a tirzepatide trial.

Every randomised SURMOUNT-4 participant first completed a 36-week open-label tirzepatide lead-in, and the trial tested tirzepatide with diet and physical activity (Aronne et al., JAMA, 2024).

What happens to weight after stopping Ozempic or Wegovy, and has anyone kept it off?

In the exploratory STEP 1 extension, 327 adults with overweight or obesity and no diabetes from sites in five countries (the abstract does not give the number in each group) were followed for a year after treatment and lifestyle intervention stopped. In that exploratory extension of 327 adults with overweight or obesity and no diabetes, those who had received semaglutide regained 11.6 percentage points of their lost weight by week 120, about two-thirds of their earlier loss, and still averaged a net loss of 5.6% from the start, with a standard deviation of 8.9% showing wide individual variation, and the result, from an exploratory subset of STEP 1, does not tell any one person what to expect (Wilding et al., Diabetes, Obesity and Metabolism, 2022). In the exploratory extension of 327 adults with overweight or obesity and no diabetes, the authors reported that cardiometabolic improvements "reverted towards baseline" for most variables, a result limited to that subset (Wilding et al., Diabetes, Obesity and Metabolism, 2022).

Ozempic and Wegovy are two brands of semaglutide (DailyMed labels: Ozempic, 10 June 2026; Wegovy, 30 June 2026). The extension analyses were exploratory and covered a subset of STEP 1 participants, and the 2022 abstract names no funder. The maker of semaglutide funded STEP 1 (Wilding et al., New England Journal of Medicine, 2021).

What else have trials measured with tirzepatide besides body weight?

Trials have measured blood sugar, heart outcomes, sleep apnoea, liver tissue and the onset of diabetes. In SURPASS-2, an open-label 40-week maker-funded trial of 1,879 adults with type 2 diabetes, estimated mean HbA1c fell 2.01 to 2.30 percentage points across tirzepatide arms and 1.86 with semaglutide, in a trial whose participants and staff knew which drug was given (Frías et al., New England Journal of Medicine, 2021). In SUMMIT, a double-blind, placebo-controlled, maker-funded trial of 731 adults with heart failure (ejection fraction of at least 50%) and obesity, followed for a median of 104 weeks, adjudicated cardiovascular death or a worsening heart-failure event occurred in 9.9% on tirzepatide against 15.3% on placebo (hazard ratio 0.62, 95% confidence interval 0.41 to 0.95); worsening heart-failure events occurred in 8.0% against 14.2%, and adjudicated cardiovascular death in 2.2% against 1.4% (hazard ratio 1.58, 95% confidence interval 0.52 to 4.83, a wide interval that does not exclude no difference) (Packer et al., New England Journal of Medicine, 2025). In two 52-week double-blind, maker-funded SURMOUNT-OSA trials in adults with moderate-to-severe sleep apnoea and obesity (the US Zepbound label describes two such trials with 467 adults in total), the mean change in the apnoea-hypopnoea index was a fall of 25.3 and 29.3 events per hour on tirzepatide against 5.3 and 5.5 with placebo, in two selected trial populations with 52-week outcomes that do not establish a result for other populations or for any one person (Malhotra et al., New England Journal of Medicine, 2024; DailyMed, 2 September 2026). The table below lists each trial.

SURPASS-CVOT randomised 13,299 adults with type 2 diabetes and atherosclerotic cardiovascular disease, double-blind, to tirzepatide or dulaglutide (13,165 in the modified intention-to-treat population). The main event (cardiovascular death, heart attack or stroke) occurred in 12.2% on tirzepatide and 13.1% on dulaglutide among those 13,165 adults with type 2 diabetes and atherosclerotic disease (hazard ratio 0.92, 95.3% confidence interval 0.83 to 1.01), which the abstract reports as noninferior; the upper limit was above 1.00, so superiority was not shown (P = 0.09), in a maker-funded trial with no placebo group (Nicholls et al., New England Journal of Medicine, 2025). In the 52-week Phase 2 SYNERGY-NASH trial of 190 randomised adults with biopsy-confirmed MASH (metabolic dysfunction-associated steatohepatitis, a fatty-liver disease) and moderate or severe fibrosis, 157 had a week-52 biopsy that could be evaluated, with missing values imputed on the assumption that they would follow the placebo group's pattern, and MASH resolved without worsening of fibrosis in 44%, 56% and 62% across the three tirzepatide arms, against 10% with placebo, in a double-blind, maker-funded trial that tested a tissue measure and not long-term outcomes (Loomba et al., New England Journal of Medicine, 2024).

In the three-year SURMOUNT-1 follow-up of 1,032 adults with obesity and prediabetes, type 2 diabetes was diagnosed in 1.3% across the tirzepatide groups against 13.3% with placebo over 176 weeks (hazard ratio 0.07, 95% confidence interval 0.0 to 0.1), in a maker-funded subgroup analysis limited to that group (Jastreboff et al., New England Journal of Medicine, 2025).

Do these findings apply to research-use tirzepatide?

No. Each result on this page comes from a study of a tirzepatide medicine or trial drug product, given to enrolled participants under clinical trial conditions. Research-use tirzepatide is a laboratory compound. Research material is not a medicine, is not a substitute for one, and is not for human or animal use. The studies measured what happened in those trials, in people who were screened, monitored and followed by trial staff. None of the abstracts read for this page describes material made, stored or sold outside that setting.

What exists for laboratory work is reference material with paperwork. The catalogue lists tirzepatide reference material as freeze-dried laboratory stock. Current certification for that material is unverified, and nothing on this page describes it. The tirzepatide research guide covers the compound itself.