Semaglutide — the GLP-1 single agonist
Semaglutide is a long-acting GLP-1 receptor agonist that mimics the body's own incretin hormone. Phase 3 trials (STEP program) showed an average ~15% body weight reduction at 68 weeks on 2.4 mg weekly. It reduces appetite, slows gastric emptying, and improves HbA1c. Half-life is ~7 days, so dosing is weekly.
Tirzepatide — dual GLP-1 / GIP
Tirzepatide adds a second receptor: GIP, the other major incretin hormone. The SURMOUNT-1 trial showed ~21% body weight reduction at 72 weeks on 15 mg weekly — significantly more than Semaglutide head-to-head. Insulin sensitivity gains are also larger. Half-life ~5 days, dosed weekly.
Retatrutide — triple GLP-1 / GIP / Glucagon
Retatrutide adds a third receptor: glucagon, which directly increases energy expenditure. Phase 2 data published in NEJM (2023) showed ~24% body weight reduction at just 48 weeks on 12 mg weekly — the largest result ever recorded for a weight-loss peptide. Phase 3 trials are ongoing. Half-life ~6 days, dosed weekly.
Cost per month — research pricing
At Atlas Labs research pricing, all three peptides are $100/vial. A typical research month uses 1 vial of Semaglutide, 1–2 vials of Tirzepatide, or 1–2 vials of Retatrutide depending on titration. Bulk discounts kick in at 3 vials (10% off) and 6 vials (15% off), meaning a 6-vial research supply works out to ~$85/vial.
Which generation to choose for research
Researchers studying baseline GLP-1 mechanisms typically choose Semaglutide for its long publication history. Those modeling next-generation incretin therapy choose Tirzepatide for its dual-agonist signal. Retatrutide is the frontier — the largest effect size to date, but the shortest published track record. All three follow the same weekly titration model.