GLP-1, GIP, and Triple Agonists: How the Weight-Loss Peptide Class Actually Works

Semaglutide, tirzepatide, and retatrutide are often discussed interchangeably, but they're mechanistically distinct, and the differences explain the diverging trial results.

Semaglutide is a single-receptor GLP-1 agonist. GLP-1 is an incretin hormone released after eating that slows gastric emptying, increases insulin secretion in response to food, and acts on appetite centers in the brain. Semaglutide's long half-life (about a week) comes from a fatty-acid side chain that binds albumin and resists enzymatic breakdown.

Tirzepatide adds a second receptor target: GIP, another incretin hormone. GIP alone has a more complex relationship with body weight, but combined with GLP-1 agonism in a single molecule, tirzepatide's Phase 3 program (SURMOUNT) showed larger average weight loss than GLP-1-only therapy in head-to-head-adjacent trial populations.

Retatrutide goes a step further, adding glucagon receptor agonism to the GIP/GLP-1 combination. Glucagon is typically associated with raising blood glucose, but at the receptor level it also increases energy expenditure. Retatrutide's Phase 2 data showed the largest average weight loss recorded for an obesity therapy to date, though it remains in Phase 3 trials and isn't yet approved.

The pattern across the class: each additional receptor target has, so far, correlated with larger average effect sizes in trials, at the cost of a shorter track record. Semaglutide has the most post-market safety data; retatrutide has the least.

None of this changes the practical basics that apply to any GLP-1-class therapy: these are potent endocrine-acting drugs with gastrointestinal side effects that scale with dose, and semaglutide and tirzepatide are FDA-approved prescription medications, meaning legitimate access runs through a physician, not a research-chemical vendor.

← See the Semaglutide directory entry