Tirzepatide as an adjunct in T1D
Eli Lilly and Company
Early randomized weight evidence in T1D with obesity; T1D glucose-control use remains investigational.
What it is
A dual GIP/GLP-1 receptor agonist being tested alongside insulin for T1D glucose control. A 12-week trial randomized 24 adults with T1D and obesity; 22 completed it. Weight and insulin requirements fell, but time in range did not improve significantly. US obesity-label eligibility is separate from the unapproved T1D glycemic indication.
Editorial review: .
Most recent recorded citation date: 2026-05-12. Only explicit date metadata is included; an undated citation may be newer. This does not mean every claim was reviewed on that date.
Trial status, labels and access can change between reviews. How we review the evidence · How to read the evidence
Evidence behind this assessment
Key evidence notes. Study results, product eligibility and access answer different questions.
- Who was studied?
- Study populations and analysis groups vary. Product age limits alone do not describe who was studied.See the linked studies and their populations →
- Benefit or performance
- Glycemic benefit: In the 22 completers, the adjusted HbA1c difference was −0.4 percentage points (95% CI −0.7 to 0.0; P=0.05). Time in range and time below range did not differ significantly. These secondary analyses were not adjusted for multiple comparisons.[1]
- Important harms and treatment burden
- Safety: Among 12 participants per arm, no DKA, severe hypoglycemia or serious adverse events were reported over 12 weeks. Nonserious events affected 9 on tirzepatide versus 1 on placebo. Anxiety was associated with one discontinuation; this small, selected cohort cannot establish uncommon or long-term risks.[1]
- Approval and country access
- The August 2026 US labels do not authorize T1D glucose control. Zepbound obesity treatment can fall within its label for an eligible adult who also has T1D. This record does not establish access or funding in other jurisdictions.Approval, trial recruitment, local supply and funding are separate. Check the cited label or access source.
- Follow-up and remaining uncertainty
- Read the full discussion and original sources for follow-up duration and study limitations.
Research status alone does not establish approval, clinical benefit or local availability. This record cites company or conference reports. Those sources may describe interim findings; their source type is shown in the source list.
Editorial score: calculation and evidence
A weighted editorial judgment on a 0–100 scale, not a probability of success or a measured treatment effect. Higher criterion scores mean more favorable assessments.
Score limitations. Scores are editorial judgments about adjunctive T1D use, not measured treatment effects. The randomized evidence is a small 12-week obesity trial; secondary glucose outcomes and rare-event safety remain uncertain. The access score does not describe eligibility for separately licensed obesity treatment.
Default calculation: 42 × 25 + 45 × 25 + 84 × 15 + 58 × 20 + 32 × 15 = 5075; divide by total weight 100. Unrounded weighted result: 50.75.
In the 22 completers, the adjusted HbA1c difference was −0.4 percentage points (95% CI −0.7 to 0.0; P=0.05). Time in range and time below range did not differ significantly. These secondary analyses were not adjusted for multiple comparisons.[1]
Among 12 participants per arm, no DKA, severe hypoglycemia or serious adverse events were reported over 12 weeks. Nonserious events affected 9 on tirzepatide versus 1 on placebo. Anxiety was associated with one discontinuation; this small, selected cohort cannot establish uncommon or long-term risks.[1]
The trial reported an 8.8% placebo-adjusted weight reduction at 12 weeks. Total daily insulin requirements were approximately 35% lower relative to placebo in a secondary analysis of 22 completers. This trial did not establish cardiovascular or kidney outcome benefits in T1D.[1]
SURPASS-T1D-1 is a phase-3 trial listed active but not recruiting, with estimated enrollment of 905 and primary completion estimated in November 2026. The registry has no posted results; planned enrollment is not a confirmed analysis population.[4]
The US Zepbound label covers adult obesity, or overweight with a weight-related condition, rather than T1D glucose control. A person with T1D may meet that obesity indication. This access score concerns the investigational T1D glycemic use; it does not measure individual obesity-treatment eligibility or insurance coverage.[7]
These ratings concern add-on treatment alongside insulin in T1D. Maturity reflects the T1D evidence, and safety receives substantial editorial weight because risks differ by drug and population. For example, SGLT2 inhibitors can cause ketoacidosis even without markedly high glucose; this is not exclusive to T1D. Access reflects the stated indication and region, including whether a separate obesity indication applies.
Editor’s take
The weight outcome supports further study in adults with T1D and obesity. The trial does not establish improved time in range, long-term safety or benefits for people with T1D without obesity. The insulin-requirement result is a group-level trial finding, not a rule for changing an individual insulin regimen.
The full picture
Tirzepatide activates GIP and GLP-1 receptors. This record concerns its investigation alongside insulin for T1D glucose control. The August 2026 US Mounjaro label covers T2D glucose control from age 10 and cardiovascular-risk reduction in adults with T2D at high risk. The US Zepbound label covers weight management in adults with obesity, or overweight with a weight-related condition, and moderate-to-severe obstructive sleep apnea in adults with obesity. Neither label supplies a T1D glycemic indication. An adult with T1D who meets the obesity-label criteria may nevertheless receive it for that separate indication.67
What the randomized trial found
TIRTLE1 was a single-site, double-blind trial in adults aged 18–60 with T1D for at least two years and BMI at least 30 kg/m². It randomized 24 participants, 12 per arm, to tirzepatide or placebo for 12 weeks. 22 completed it, 11 per arm. The primary weight analysis included all 24 using imputation for missing data; secondary analyses used the 22 completers without imputation.12
Weight fell by 10.3 kg in the tirzepatide arm and 0.7 kg with placebo. The baseline-adjusted between-group difference was −8.7 kg (95% CI −12.0 to −5.5), described by the authors as an 8.8% placebo-adjusted reduction. The adjusted estimate is not a simple subtraction of the two arm changes. Total daily insulin requirements were approximately 35% lower relative to placebo in a secondary analysis: the baseline-adjusted ratio of geometric means was 0.65 (95% CI 0.53–0.79). These are group-level estimates, not individual dose-adjustment instructions.1
Glucose outcomes were less certain. The adjusted HbA1c difference was −0.4 percentage points (95% CI −0.7 to 0.0; P=0.05). The time-in-range difference, using 70–180 mg/dL, was +8.4 percentage points (95% CI −4.0 to 20.9; P=0.17), not statistically significant. Time below range also did not differ significantly. The study was powered for weight, and its secondary-outcome confidence intervals were not adjusted for multiple comparisons; these analyses do not establish definitive glucose benefits.1
Safety and the limits of a small trial
No DKA, severe hypoglycemia or serious adverse events were reported. Nonserious events affected 9 of 12 participants receiving tirzepatide versus 1 of 12 on placebo, with 14 versus 1 events; gastrointestinal symptoms predominated. The supplement also lists one depression event and one anxiety event as possibly related, and one discontinuation. The main paper reports preexisting symptoms/history and associates the anxiety-related withdrawal with difficulty meeting weekly facility-visit demands. These observations neither establish causation nor justify omitting the events from the safety account.12
The trial excluded people with recent DKA or severe hypoglycemia and several other higher-risk conditions, including gastroparesis and proliferative retinopathy or macular edema. Treatment was delivered during facility visits, with study-team support for insulin adjustments and discussion of ketone and hypoglycemia risks. Twelve weeks in this selected population cannot establish uncommon-event or long-term safety. Garvan sponsored the study; the paper reports foundation, diabetes-society and philanthropic project funding, with no funder role in trial design, interpretation or presentation.1
The current US labels contain a boxed warning about thyroid C-cell tumors in rodents, with relevance to humans unknown, and contraindications including personal/family medullary thyroid carcinoma or MEN2. Precautions include severe gastrointestinal reactions, pancreatitis, kidney injury from volume depletion and hypoglycemia with concomitant insulin; both labels state that use is not recommended in severe gastroparesis. These label warnings are separate from the small T1D trial's observed events.67
ADA 2026 recommendation 8.29 includes GLP-1–based obesity treatment for adults with T1D and obesity through shared, individualized decisions. Its discussion emphasizes changing insulin requirements, hypoglycemia and ketosis risks, including with automated insulin delivery. It cautions against simply extrapolating T2D titration protocols to T1D. This guidance is not an approval for T1D glucose control.8
What phase 3 is testing
At the 17 September 2026 check, both SURPASS-T1D registries were active, not recruiting. T1D-1 lists estimated enrollment of 905 and T1D-2 465; these are planned counts, not confirmed randomized or analyzed populations. Both specify adults with T1D on insulin for at least one year, BMI at least 25 and screening HbA1c 7.0–10.5%. Their primary endpoint is HbA1c change at week 40. T1D-2 also includes week-72 secondary follow-up.45
Both list estimated primary completion in November 2026. Estimated study completion is December 2026 for T1D-1 and July 2027 for T1D-2. The registry updates were posted on 12 May and 17 February 2026, respectively; neither had posted results at this check. Lilly's current T1D-1 page also states that enrollment is complete. Completion estimates do not establish when results will be released or whether a regulator will authorize a new indication.453
Coming soon
ETA · Both SURPASS-T1D studies list primary completion in November 2026. Study completion is estimated in December 2026 for T1D-1 and July 2027 for T1D-2; these are not promised results-publication dates.
- →SURPASS-T1D-1 primary HbA1c endpoint at week 40 · primary completion estimated November 2026; publication date not established
- →SURPASS-T1D-2 longer-term follow-up, including week-72 secondary outcomes · study completion estimated July 2027; publication date not established
Sources
- [1]Tirzepatide in Adults With Type 1 Diabetes: A Phase 2 Randomized Placebo-Controlled Clinical Trial · Peer-reviewed study · 2025-11-20 — Methods, Table 2 and Safety and Tolerability: 24 randomized, 22 completers; weight analysis included imputation for all 24, other outcomes used 22. Confidence intervals were not adjusted for multiple comparisons. January 2026 journal issue.
Snaith et al. Tirzepatide in Adults With Type 1 Diabetes: A Phase 2 Randomized Placebo-Controlled Clinical Trial. Methods, Table 2, Safety and Tolerability, and Article Information.
- [2]TIRTLE1 supplementary appendix: participant flow and safety · Peer-reviewed study — Figure S1 (page 6) and Table S2 (page 9): one active-arm discontinuation for increased anxiety; possibly related depression and anxiety events; 9/12 versus 1/12 participants with nonserious events.
TIRTLE1 supplementary appendix, Figure S1 (page 6) and Table S2 (page 9).
- [3]Lilly SURPASS-T1D-1 study listing · Manufacturer — Sponsor page checked 17 September 2026 lists completed enrollment and an enrollment goal of 905; this is not the number analyzed.
Lilly SURPASS-T1D-1 study listing, checked 17 September 2026.
- [4]SURPASS-T1D-1 — NCT06914895 · Trial registry · 2026-05-12 — Registry last update posted 12 May 2026. Active, not recruiting; 905 estimated; primary HbA1c outcome at week 40; primary completion November 2026 and study completion December 2026 are estimates. No results posted at the 17 September check.
ClinicalTrials.gov NCT06914895, status, design, eligibility and outcomes; checked 17 September 2026.
- [5]SURPASS-T1D-2 — NCT06962280 · Trial registry · 2026-02-17 — Active, not recruiting; 465 estimated. Primary HbA1c outcome at week 40; secondary follow-up includes week 72. Primary completion November 2026 and study completion July 2027 are estimates. No results posted at the 17 September check.
ClinicalTrials.gov NCT06962280, status, design, eligibility and outcomes; checked 17 September 2026.
- [6]Mounjaro US prescribing information, revised August 2026 · Manufacturer — Section 1: T2D glycemic indication from age 10 and cardiovascular-risk indication in adults with T2D at high risk; neither is a T1D glycemic indication. Boxed warning and sections 4–5 describe contraindications and precautions.
Mounjaro US prescribing information, revised August 2026, section 1, boxed warning and sections 4–5.
- [7]Zepbound US prescribing information, revised August 2026 · Manufacturer — Section 1 distinguishes adult obesity/overweight weight management and obstructive sleep apnea indications from T1D glycemic treatment. Boxed warning and sections 4–5 describe contraindications and precautions; the highlights record removal of the suicidal-behavior/ideation warning in February 2026.
Zepbound US prescribing information, revised August 2026, section 1, boxed warning and sections 4–5.
- [8]ADA 2026 Standards: obesity management including adults with T1D · Peer-reviewed study — Recommendation 8.29 and Treatment of Obesity in Type 1 Diabetes: individualized adult obesity care, shared decisions and attention to hypoglycemia, ketosis and changing insulin requirements. This is clinical guidance, not regulatory approval.
ADA Standards of Care 2026, section 8, recommendation 8.29 and Treatment of Obesity in Type 1 Diabetes.