Tirzepatide: A Literature Course on What Published Studies Report

What This Course Covers

Tirzepatide is a synthetic peptide designed to activate two incretin receptors with a single molecule: the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. It has been examined in a large phase 3 programme in type 2 diabetes, in trials in obesity, and in a phase 2 trial in liver disease. This page walks through that literature module by module: what each study population was, what was administered, how long the study ran, and what the authors reported. This page is for educational purposes only and is not medical advice; consult a licensed physician for any question about diagnosis, treatment or medication.

Nothing here is a protocol, a regimen or a suggestion. Doses appear only where a cited trial described them, and they are presented as study design details rather than instructions.

Module 1: Receptor Pharmacology

The molecular starting point for this literature is how tirzepatide interacts with its two target receptors. A 2020 pharmacology study characterised tirzepatide as an imbalanced and biased dual GIP and GLP-1 receptor agonist, reporting that it bound the GIP receptor with affinity comparable to native GIP while binding the GLP-1 receptor with weaker affinity than native GLP-1 (PMID 32730231). That same study reported biased agonism at the GLP-1 receptor, with signalling that favoured cAMP generation over β-arrestin recruitment (PMID 32730231).

Why this matters for reading the clinical literature: two agonists that both "activate GLP-1 receptors" may not behave identically at the receptor level, and the authors of the pharmacology work framed tirzepatide's profile as distinct from a balanced co-agonist (PMID 32730231). A 2022 systematic update collected the preclinical and early clinical work on the molecule and summarised its development as a dual incretin receptor agonist (PMID 36498958).

Product and regulatory context

A 2023 drug monograph described tirzepatide, marketed as Mounjaro, as a once-weekly subcutaneous injection authorised for glycaemic control in adults with type 2 diabetes alongside diet and exercise (PMID 36751934). A 2024 review in Drugs summarised the accumulated evidence for tirzepatide in type 2 diabetes across the SURPASS trial programme (PMID 38388874). Regulatory status differs by country and by indication, and approved product labelling is the authoritative source for any licensed use.

Module 2: Glycaemic Trials in Type 2 Diabetes

SURPASS-1: monotherapy versus placebo

SURPASS-1 was a double-blind, randomised phase 3 trial in adults with type 2 diabetes inadequately controlled with diet and exercise, in which participants received once-weekly subcutaneous tirzepatide at 5 mg, 10 mg or 15 mg, or placebo, for 40 weeks (PMID 34186022). The study reported greater reductions in HbA1c and in body weight in each tirzepatide group than in the placebo group at 40 weeks (PMID 34186022). Researchers reported that the most common adverse events were gastrointestinal and were generally mild to moderate, and that no severe hypoglycaemia was reported in the trial (PMID 34186022).

SURPASS-5: added to titrated insulin glargine

SURPASS-5 randomised adults with type 2 diabetes whose glycaemia was inadequately controlled on titrated insulin glargine to receive once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo for 40 weeks (PMID 35133415). The study reported significantly greater reductions in HbA1c with each tirzepatide dose than with placebo when added to insulin glargine over 40 weeks (PMID 35133415). As in the monotherapy trial, researchers reported that gastrointestinal events were the most frequently observed adverse events in that study (PMID 35133415).

Reading the two trials together

These trials answer different questions. One examined tirzepatide without background glucose-lowering therapy (PMID 34186022); the other examined it on top of basal insulin, where background hypoglycaemia risk is inherently different (PMID 35133415). Effect sizes from separate trials with different populations are not directly interchangeable.

Module 3: Body-Weight Trials

SURMOUNT-4: continued treatment versus withdrawal

SURMOUNT-4 used an open-label lead-in period of 36 weeks in adults with obesity, after which participants were randomised either to continue tirzepatide at their maximum tolerated dose of 10 mg or 15 mg weekly or to switch to placebo for a further 52 weeks (PMID 38078870). The study reported that participants who continued tirzepatide had further weight reduction during the randomised phase, whereas those switched to placebo regained a substantial proportion of the weight lost during the lead-in (PMID 38078870). Researchers framed this as a maintenance question: what happened to weight reduction when treatment stopped (PMID 38078870).

SURMOUNT-5: head-to-head with semaglutide

A 2025 randomised trial compared tirzepatide with semaglutide for the treatment of obesity, with participants receiving the maximum tolerated dose of tirzepatide (10 mg or 15 mg weekly) or of semaglutide (1.7 mg or 2.4 mg weekly) over 72 weeks (PMID 40353578). The study reported a greater mean percentage reduction in body weight at 72 weeks with tirzepatide than with semaglutide (PMID 40353578). This was a direct randomised comparison, which is methodologically stronger than comparing results across separate trials.

Module 4: Comparative and Indirect Evidence

Much of the tirzepatide-versus-semaglutide literature is indirect, meaning it links trials through common comparators rather than randomising participants to both drugs. A 2024 systematic review and network meta-analysis of randomised controlled trials compared subcutaneous tirzepatide with semaglutide in adults with type 2 diabetes and reported greater reductions in HbA1c and body weight with tirzepatide across the doses analysed (PMID 38613667). The authors of that analysis also examined tolerability, reporting that gastrointestinal adverse events were common with both agents (PMID 38613667).

A 2025 indirect treatment comparison assessed tirzepatide 10 mg and 15 mg against semaglutide 2.4 mg for the management of obesity and overweight in people with type 2 diabetes and reported greater weight reduction with tirzepatide in that indirect analysis (PMID 40537987). Indirect comparisons depend on assumptions about how similar the underlying trial populations were, a limitation the authors of such analyses typically acknowledge (PMID 40537987).

Module 5: Liver Outcomes

A 2024 phase 2 randomised trial examined tirzepatide in adults with biopsy-confirmed metabolic dysfunction-associated steatohepatitis (MASH) and moderate or severe liver fibrosis, assigning participants to once-weekly tirzepatide 5 mg, 10 mg or 15 mg or placebo for 52 weeks (PMID 38856224). The study reported that resolution of MASH without worsening of fibrosis occurred more often in the tirzepatide groups than in the placebo group at 52 weeks (PMID 38856224). Researchers described the trial as phase 2, meaning it was designed to inform larger confirmatory studies rather than to settle the question (PMID 38856224).

Study Design Reference Table

Adverse Events: What Studies Report

Gastrointestinal events

Across the randomised trials, gastrointestinal complaints dominated the adverse-event tables. SURPASS-1 reported that the most common adverse events were gastrointestinal, mostly mild to moderate in severity, and that no severe hypoglycaemia was reported (PMID 34186022). SURPASS-5 similarly reported gastrointestinal events as the most frequent adverse events when tirzepatide was added to titrated insulin glargine (PMID 35133415). The 2024 network meta-analysis reported that gastrointestinal adverse events were common with both tirzepatide and semaglutide in the trials it pooled (PMID 38613667).

Events in the obesity trials

In the 72-week head-to-head obesity trial, researchers reported that gastrointestinal adverse events were the most common events with both tirzepatide and semaglutide (PMID 40353578). The 2025 indirect comparison also assessed safety alongside efficacy in people with obesity or overweight and type 2 diabetes (PMID 40537987).

Psychiatric reports in pharmacovigilance data

A 2024 pharmacovigilance analysis examined individual case safety reports submitted to the EudraVigilance database for semaglutide, liraglutide and tirzepatide and reported psychiatric adverse events, including anxiety, depression and suicidal ideation, among the submitted reports (PMID 38265519). The authors of that analysis noted that spontaneous reporting databases capture suspected associations and cannot establish causality or incidence (PMID 38265519). Pharmacovigilance signals are hypothesis-generating; they sit at a different evidence level from randomised trials.

Context from reviews

A 2024 review in Drugs summarised the tolerability profile of tirzepatide in type 2 diabetes alongside its efficacy data (PMID 38388874), and a 2023 monograph summarised the licensed product's once-weekly subcutaneous administration in adults with type 2 diabetes (PMID 36751934).

Module 6: Reading the Evidence Critically

1. Check the population. Trials in drug-naive type 2 diabetes (PMID 34186022) and trials in obesity without diabetes (PMID 40353578) answer different questions.
2. Check the comparator. Placebo-controlled results are not the same as head-to-head results, and the 2025 obesity trial randomised participants directly to the two agents (PMID 40353578).
3. Check whether the comparison was indirect. Network meta-analyses and indirect treatment comparisons link trials statistically rather than by randomisation (PMID 38613667, PMID 40537987).
4. Check the duration and what happened after it. The maintenance trial specifically examined what occurred when treatment was withdrawn after a lead-in period (PMID 38078870).
5. Check the phase. The liver trial was phase 2 and used histological endpoints over 52 weeks (PMID 38856224).

Glossary

• Dual agonist: a molecule that activates two receptor types; tirzepatide was characterised as an imbalanced and biased dual GIP and GLP-1 receptor agonist (PMID 32730231).
• HbA1c: a blood marker reflecting average glucose over roughly three months, used as the primary endpoint in the SURPASS trials (PMID 34186022).
• Lead-in period: an open-label phase before randomisation, used in the maintenance trial in obesity (PMID 38078870).
• MASH: metabolic dysfunction-associated steatohepatitis, the condition studied in the 52-week phase 2 liver trial (PMID 38856224).
• Pharmacovigilance: post-marketing safety surveillance based on submitted case reports, as used in the EudraVigilance psychiatric analysis (PMID 38265519).

What the Cited Literature Does Not Settle

The studies summarised here reported endpoints such as HbA1c, body weight and liver histology over defined study periods; none of the cited work reported outcomes beyond its own duration. Long-term questions, individual suitability, interactions with other therapies, and use outside licensed indications are matters for clinicians and regulators rather than for a literature summary. Comparative claims drawn from network meta-analysis and indirect comparison carry the methodological caveats their authors described (PMID 38613667, PMID 40537987). This page is for educational purposes only and is not medical advice; consult a licensed physician about any medical decision.

References

• Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist (JCI Insight, 2020)
• Efficacy and safety of a novel dual GIP and GLP-1 receptor agonist tirzepatide in patients with type 2 diabetes (SURPASS-1): a double-blind, randomised, phase 3 trial (Lancet, 2021)
• Effect of Subcutaneous Tirzepatide vs Placebo Added to Titrated Insulin Glargine on Glycemic Control in Patients With Type 2 Diabetes: The SURPASS-5 Randomized Clinical Trial (JAMA, 2022)
• Tirzepatide: A Systematic Update (International Journal of Molecular Sciences, 2022)
• New Drug: Tirzepatide (Mounjaro) (The Senior Care Pharmacist, 2023)
• Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial (JAMA, 2024)
• Psychiatric adverse events associated with semaglutide, liraglutide and tirzepatide: a pharmacovigilance analysis of individual case safety reports submitted to the EudraVigilance database (International Journal of Clinical Pharmacy, 2024)
• Tirzepatide: A Review in Type 2 Diabetes (Drugs, 2024)
• Subcutaneously administered tirzepatide vs semaglutide for adults with type 2 diabetes: a systematic review and network meta-analysis of randomised controlled trials (Diabetologia, 2024)
• Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis (New England Journal of Medicine, 2024)
• Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (New England Journal of Medicine, 2025)
• Indirect comparative efficacy and safety of tirzepatide 10 and 15 mg versus semaglutide 2.4 mg for the management of obesity and overweight in patients with type 2 diabetes (Diabetes, Obesity & Metabolism, 2025)