Lilly starts 905-person phase 3 trial of tirzepatide in type 1 diabetes

Eli Lilly has started a phase 3 trial that will test tirzepatide in adults who have type 1 diabetes along with obesity or overweight. The study, registered as SURPASS-T1D-1, plans to randomize 905 people to tirzepatide once a week or to a placebo ^[1].

Tirzepatide is the molecule sold in the U.S. as Mounjaro, for type 2 diabetes, and Zepbound, for obesity. It has no approval for type 1 diabetes.

The trial design

The main question is blood sugar control. The primary endpoint is the change in HbA1c from the start of the study to week 40 ^[1]. HbA1c is a lab measure of average blood sugar over roughly the previous two to three months. The listing describes about 49 weeks of treatment ^[1].

Primary completion, meaning the point at which the last participant finishes the main outcome measurement, is listed as November 2026 ^[1]. The trial listing does not disclose the tirzepatide doses that will be used.

The scale stands out. Earlier incretin studies in type 1 diabetes were far smaller, so the evidence behind the idea has been thin. A 905-person randomized trial is a different order of magnitude ^[1].

Safety questions from earlier incretin work

The safety readout may matter as much as the efficacy one. People with type 1 diabetes cannot make enough insulin and depend on injected insulin, so adding a second drug that also lowers blood sugar raises the risk of hypoglycemia, or low blood sugar. Ketosis and diabetic ketoacidosis are the other closely watched risks. Earlier work with the GLP-1 drug liraglutide in type 1 diabetes, run as a series of trials known as the ADJUNCT program, drew attention to both and never led to an approval for type 1 diabetes.

That history is why a large randomized, placebo-controlled design matters here. In a placebo-controlled trial, investigators can count how often low blood sugar and ketone events occur in each group, rather than guessing from small studies.

Why it matters for patients

Incretin drugs such as semaglutide (Ozempic, Wegovy, Rybelsus) and tirzepatide are approved for type 2 diabetes and for weight management, not for type 1 diabetes. That means any use of tirzepatide by someone with type 1 diabetes today is off-label, and its safety and effectiveness in that group have not been established in a large trial ^[1].

If SURPASS-T1D-1 shows a meaningful HbA1c reduction without an excess of hypoglycemia or ketoacidosis, tirzepatide could become the first incretin drug with solid phase 3 evidence in type 1 diabetes, and eventually a possible add-on to insulin. If the trial turns up safety signals, the case for using these drugs in type 1 diabetes would weaken, much as the liraglutide program did. Either outcome would inform how doctors and insurers treat off-label prescribing in this group.

Weight loss is another area of interest because every participant entered the trial with overweight or obesity, but the listing names only the HbA1c measure as the primary endpoint ^[1].

What happens next

Lilly lists primary completion for November 2026 ^[1]. That is when the last participant is expected to finish the primary outcome, not when results will be made public. Whether and when the company releases topline findings or publishes full results has not been disclosed. No interim analysis dates, enrollment milestones or site locations beyond the registry entry have been reported.

People with type 1 diabetes who want to know whether they could join the study would need to check the registry listing and any participating sites, since the listing does not break out where the 905 participants will be enrolled ^[1].