In the first RCT of tirzepatide in type 1 diabetes, participants lost an average of 10.3 kg (8.8%) over 12 weeks while also reducing insulin needs by 35% with no serious adverse events.
100% achieved ≥5% weight lossEvery single participant on tirzepatide lost at least 5% of their body weight in just 12 weeks, with nearly half losing 10% or more — while also reducing their insulin needs by over a third.
What the researchers found
Over 12 weeks, tirzepatide produced a mean weight loss of 10.3 kg versus 0.7 kg with placebo (treatment difference: -8.7 kg, p<0.0001), representing 8.8% body weight reduction. Every participant (100%) on tirzepatide achieved at least 5% weight loss, and 45% achieved at least 10% loss, compared to 9% and 0% on placebo.
Tirzepatide also improved HbA1c by -0.4% versus placebo (p=0.05) and reduced total daily insulin dose by 24.2 units/day (35.1% reduction vs placebo, p=0.0002). No significant adverse events occurred in either group. Twenty-two of 24 participants completed the study.
Why it matters
Obesity affects a growing proportion of people with type 1 diabetes and increases cardiovascular risk, but no weight loss medications have been specifically studied or approved for this population. This first-ever RCT of tirzepatide in type 1 diabetes shows it can produce substantial weight loss and reduce insulin needs without safety concerns — potentially opening a new treatment avenue for millions of patients.
How the study worked
This was a 12-week, phase 2, double-blind, placebo-controlled trial in adults with type 1 diabetes and BMI >30 kg/m². Participants were randomized to weekly subcutaneous tirzepatide (2.5 mg for 4 weeks, then 5.0 mg for 8 weeks) or placebo. The primary endpoint was change in body weight at 12 weeks. Secondary endpoints included HbA1c, insulin dose, and safety.
What this study cannot tell us
This was a small phase 2 trial with only 24 participants and 12-week duration. The tirzepatide dose was capped at 5 mg (lower than the 10-15 mg used in type 2 diabetes obesity studies), so the full potential may not have been captured. Longer-term safety, durability of weight loss, and effects at higher doses need evaluation. The study excluded participants with BMI under 30, so applicability to overweight (non-obese) type 1 diabetes patients is unknown.
How to read the evidence
This is a phase 2, double-blind, placebo-controlled randomized trial — a strong study design. However, the small sample size (n=24) and short duration (12 weeks) mean these are preliminary results that require confirmation in larger, longer phase 3 trials.
When this study was published
Published in 2026, this is the first randomized controlled trial of tirzepatide in type 1 diabetes, making it a landmark study that could shift clinical practice if confirmed in larger trials.
The bigger picture
Tirzepatide and other GLP-1-based peptide drugs have transformed type 2 diabetes and obesity treatment, but type 1 diabetes has been overlooked. This trial bridges that gap, showing the dual GIP/GLP-1 agonist is effective and safe even when patients depend entirely on exogenous insulin. With obesity rates rising among type 1 diabetes patients, this could lead to the first approved weight management therapy specifically studied in this population.
Questions still open
- Would higher tirzepatide doses (10-15 mg) produce even greater weight loss in type 1 diabetes, and would they remain safe?
- Is the weight maintained after stopping tirzepatide, or does it return as seen in type 2 diabetes studies?
- Could the 35% insulin dose reduction translate to reduced hypoglycemia risk in longer-term treatment?
Common questions
Why haven't weight loss drugs been studied in type 1 diabetes before?
Could tirzepatide replace insulin in type 1 diabetes?
Read the original research
Tirzepatide in Adults With Type 1 Diabetes: A Phase 2 Randomized Placebo-Controlled Clinical Trial.
Diabetes care, 49(1), 161-170
Citation
Snaith, Jennifer R; Frampton, Ruth; Samocha-Bonet, Dorit; Greenfield, Jerry R. (2026). Tirzepatide in Adults With Type 1 Diabetes: A Phase 2 Randomized Placebo-Controlled Clinical Trial.. Diabetes care, 49(1), 161-170. https://doi.org/10.2337/dc25-2379