Tirzepatide (Mounjaro) is the first dual GIP/GLP-1 receptor agonist approved for type 2 diabetes. The SURPASS program — six trials including a dedicated cardiovascular outcomes trial — established tirzepatide as the most potent glycemic agent in the GLP-1 class, reducing HbA1c by up to 2.4% and body weight by up to 10.7 kg. This guide covers the complete SURPASS data, mechanism, dosing, and head-to-head comparison vs Semaglutide.
The GIP receptor component is the key differentiator. GIP agonism enhances insulin secretion in a glucose-dependent manner, improves peripheral insulin sensitivity, and suppresses glucagon — effects that are additive to GLP-1 receptor agonism. The result is a more complete glycemic correction than semaglutide or liraglutide can achieve with GLP-1 alone.
GIP agonism enhances glucose-dependent insulin secretion from beta cells and suppresses glucagon from alpha cells. GLP-1 agonism adds appetite suppression, gastric emptying delay, and additional insulin secretion. The combined effect produces greater glycemic control than either mechanism alone.
SURPASS-2 showed tirzepatide 15 mg reduced HbA1c by 2.4% vs 1.9% for semaglutide 1 mg — a statistically significant difference. At all three doses (5, 10, 15 mg), tirzepatide outperformed semaglutide on HbA1c reduction in the direct head-to-head trial.
The GIP receptor component improves peripheral insulin sensitivity in muscle and adipose tissue, reducing insulin resistance independent of weight loss. This insulin-sensitizing effect is additive to the insulin secretion enhancement, producing a more complete glycemic correction than GLP-1 monotherapy.
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The SURPASS program comprised six Phase 3 trials and one cardiovascular outcomes trial (SURPASS-CVOT) enrolling over 20,000 patients. Together they established tirzepatide's efficacy across the full spectrum of T2D management — monotherapy, combination with oral agents, add-on to insulin, and high cardiovascular risk populations.
| Trial | Comparator | N | Duration | HbA1c 5mg | HbA1c 10mg | HbA1c 15mg |
|---|---|---|---|---|---|---|
| SURPASS-1 | Placebo | 478 | 40 wks | −1.6% | −1.9% | −2.0% |
| SURPASS-2 | Semaglutide 1 mg | 1,879 | 40 wks | −1.9% | −2.1% | −2.4% |
| SURPASS-3 | Insulin degludec | 1,444 | 52 wks | −1.8% | −2.0% | −2.1% |
| SURPASS-4 | Insulin glargine | 2,002 | 52 wks | −1.9% | −2.1% | −2.3% |
| SURPASS-5 | Placebo + insulin | 475 | 40 wks | −1.9% | −2.0% | −2.1% |
| SURPASS-CVOT | Placebo | 13,884 | ~4 yrs | N/A | N/A | −2.0% |
| Metric | Tirzepatide | Semaglutide |
|---|---|---|
| Mechanism | GIP + GLP-1 dual agonist | GLP-1 agonist |
| HbA1c reduction (max) | −2.4% | −1.9% (1 mg) |
| Weight loss | −8.5 to −10.7 kg | −4.5 kg |
| CV outcomes data | Yes (SURPASS-CVOT, −17%) | Yes (SUSTAIN-6, −26%) |
| Hypoglycemia risk | Low (monotherapy) | Low (monotherapy) |
| FDA approval for T2D | Yes (Mounjaro, 2022) | Yes (Ozempic, 2017) |
| Dosing frequency | Weekly injection | Weekly injection |
| Timepoint | Tests & Assessments |
|---|---|
| Baseline | HbA1c, FPG, eGFR, LFTs, lipid panel, weight, BP |
| Week 4 | FPG (assess early response), GI tolerance check |
| Week 12 | HbA1c, FPG, weight, BP, dose escalation decision |
| Week 24 | HbA1c, FPG, weight, lipid panel, eGFR |
| Week 40–52 | Full panel: HbA1c, FPG, eGFR, LFTs, lipid panel, weight, BP, eye exam |
| Every 6 mos | HbA1c, FPG, weight, BP, eGFR (maintenance) |
Yes. Tirzepatide (Mounjaro) received FDA approval for type 2 diabetes in May 2022. It is the first dual GLP-1/GIP receptor agonist approved for diabetes management. SURPASS-2 showed it outperforms semaglutide on HbA1c reduction at equivalent doses.
SURPASS trials show tirzepatide reduces HbA1c by an average of 2.0–2.4% at the 15 mg dose — the largest reduction of any approved GLP-1 class agent. At 5 mg and 10 mg doses, reductions average 1.6% and 1.9% respectively. This compares to approximately 1.5% for semaglutide 1 mg (SURPASS-2 direct comparison).
SURPASS-2 directly compared tirzepatide to semaglutide 1 mg in type 2 diabetes. Tirzepatide at all doses (5, 10, 15 mg) produced greater HbA1c reduction and weight loss than semaglutide 1 mg. The dual GLP-1/GIP mechanism provides additive insulin sensitization that semaglutide's GLP-1-only mechanism does not.
FDA-approved Mounjaro dosing starts at 2.5 mg/week for 4 weeks, then 5 mg/week. The dose can be increased by 2.5 mg every 4 weeks as tolerated, up to a maximum of 15 mg/week. Research protocols vary from this clinical dosing schedule.
Tirzepatide has a low risk of hypoglycemia when used as monotherapy because it stimulates insulin secretion in a glucose-dependent manner. However, when combined with insulin or sulfonylureas, hypoglycemia risk increases substantially. SURPASS trials reported hypoglycemia rates of 1–2% with tirzepatide monotherapy vs 10–20% with insulin combinations.
SURPASS-CVOT (n=13,884) showed tirzepatide reduced major adverse cardiovascular events (MACE) by 17% vs placebo in patients with T2D and established cardiovascular disease or high CV risk. This is comparable to semaglutide's SELECT trial results. The CV benefit is thought to be driven by both direct GLP-1 receptor effects on the heart and indirect benefits from weight loss and glycemic improvement.
For many patients with T2D who are not insulin-dependent, tirzepatide can achieve glycemic targets without insulin. SURPASS-3 compared tirzepatide to insulin degludec and showed tirzepatide achieved greater HbA1c reduction with significantly less hypoglycemia and weight loss vs weight gain with insulin. However, patients with T1D or advanced T2D with beta cell failure still require insulin.
Fasting glucose improvements are typically seen within the first 1–2 weeks at the 2.5 mg starting dose. HbA1c reductions are measurable by week 12 and continue improving through week 40–52 as the dose is titrated. Maximum glycemic benefit is typically achieved at the 10–15 mg maintenance dose after 20–24 weeks of titration.
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