Semaglutide vs Tirzepatide vs Retatrutide: What the Data Actually Shows

Semaglutide is a pure GLP-1 receptor agonist - it slows gastric emptying, suppresses appetite via hypothalamic GLP-1R signaling, and drives incretin-mediated insulin release. Its C18 fatty diacid chain provides ~7-day plasma half-life through albumin binding (MW 4,113.58 Da). The SUSTAIN and STEP trials confirmed ~15–17% body weight reduction at maximum doses.

Tirzepatide adds co-agonism at the GIP receptor, which has a fundamentally different action profile: GIP signaling in adipose tissue increases lipid clearance and reduces inflammatory cytokine expression, while central GIP receptor activation augments the satiety signal beyond what GLP-1 alone achieves. The SURMOUNT-1 trial showed 20.9% mean body weight reduction at 15 mg - roughly 5 percentage points above semaglutide comparator arms.

Retatrutide (LY3437943) adds glucagon receptor agonism to the GIP + GLP-1 dual axis of tirzepatide. Glucagon receptor activation drives hepatic fat oxidation independently of the GLP-1 pathway - directly increasing resting energy expenditure in a way that neither semaglutide nor tirzepatide achieves. Phase 2 NEJM data (Jastreboff et al. 2023) showed 24.2% mean body weight reduction at 12 mg over 48 weeks - the highest recorded in any weekly injectable peptide trial.

Critically, the glucagon component also drives the most pronounced gastrointestinal side effect profile of the three compounds. Nausea and vomiting rates in the 12 mg arm reached 45% during uptitration - approximately double those seen with tirzepatide at its maximum dose. This is a direct consequence of combined GLP-1 + glucagon gastric motility suppression.

All three compounds are lyophilized peptides requiring BAC water reconstitution, but their molecular architectures create different handling requirements. Semaglutide's C18 fatty diacid chain makes it the most temperature-sensitive of the three - UV exposure accelerates asparagine deamidation at residue 34. Tirzepatide's dual-agonist backbone is more thermally stable, tolerating brief room temperature exposure better than semaglutide.

Retatrutide is the newest and has the least published stability data outside of Lilly's proprietary formulations. Based on its molecular weight (~4,680 Da) and structural similarity to tirzepatide, conservative handling (2–8°C, light-protected, use within 28 days) is the appropriate default until more stability data is published.