GLP-1 vs GLP-2 vs GLP-3

1. What Do GLP-1, GLP-2, and GLP-3 Mean Here?

These labels aren't official FDA drug categories. They're informal shorthand that has spread through the weight-loss and peptide-tracking community to describe generations of obesity medications by how many hormone receptors they target. Each added receptor is thought to contribute an additional metabolic effect, and the trial data so far supports incrementally larger weight reductions as the receptor count increases.

The numbering doesn't match the biology exactly - GLP-2 the receptor agonist (teduglutide, used for short bowel syndrome) is a completely different drug class. In this community context, "GLP-2" means a dual GIP+GLP-1 agonist like tirzepatide, and "GLP-3" means a triple agonist like retatrutide. This page uses the community framing to explain what each generation tracks differently.

2. GLP-1 - Single Agonist (Semaglutide)

GLP-1 receptor agonists activate the glucagon-like peptide-1 receptor (GLP-1R), which is expressed in the pancreas, brain, and gut. Activation stimulates insulin release, suppresses glucagon, slows gastric emptying, and signals satiety to the brain (NCBI Bookshelf). Semaglutide is the most widely used member of this class.

The STEP 1 trial - a Phase 3 randomized controlled trial in adults with obesity and no diabetes - reported a mean 14.9% body weight reduction with semaglutide 2.4 mg at 68 weeks versus 2.4% with placebo (Wilding et al., NEJM 2021). About 86% of participants on the active drug lost at least 5% of body weight.

Semaglutide exists as a weekly subcutaneous injection (Ozempic for T2D, Wegovy for weight management) and as a daily oral tablet (Rybelsus, for T2D only). Orforglipron (Foundayo) is a separate oral non-peptide GLP-1 agonist approved in 2026. The receptor class is the same across these products; the route and label differ. For a tracking record, the route determines the record shape - injection logs need site and pen inventory, oral logs need tablet timing and supply count.

3. GLP-2 - Dual Agonist (Tirzepatide)

Adding a second receptor target - the glucose-dependent insulinotropic polypeptide receptor (GIPR) - is the defining feature of tirzepatide (Mounjaro, Zepbound). GIP is a gut hormone that stimulates insulin release and, when its receptor is activated alongside GLP-1R, appears to enhance weight reduction beyond what GLP-1 activation alone achieves. FDA labeling identifies tirzepatide as a GIP/GLP-1 receptor agonist, not a GLP-1 agonist.

The SURMOUNT-1 trial - a Phase 3 RCT in adults with obesity and no diabetes - found a mean 20.9% weight reduction with tirzepatide 15 mg at 72 weeks versus 3.1% with placebo (Jastreboff et al., NEJM 2022). About 91% of participants on the highest dose lost at least 5% of body weight, and 57% lost at least 20%.

The tracking record for tirzepatide looks nearly identical to semaglutide in format: weekly subcutaneous injection, pen inventory, dose titration, injection-site rotation, side-effect notes, and progress metrics. The product name in the log is what matters - the tracker records what the user enters and doesn't infer mechanism from it.

4. GLP-3 - Triple Agonist (Retatrutide)

The third receptor added is the glucagon receptor (GCGR). Glucagon raises blood glucose and energy expenditure. Combining glucagon receptor activation with GIP and GLP-1 is hypothesized to produce additive effects on energy balance - more thermogenesis from the glucagon arm, more satiety and insulin response from the GLP-1 arm, and additional metabolic benefits from the GIP arm. Retatrutide is the lead investigational compound in this class.

A Phase 2 dose-ranging trial reported a mean 24.2% body weight reduction with retatrutide 12 mg at 48 weeks in adults with obesity and no diabetes, versus 2.1% with placebo (Jastreboff et al., NEJM 2023). Nearly all participants on the highest dose lost at least 5%, and about 83% lost at least 15% - outcomes not previously seen at this stage in obesity drug development. Phase 3 trials are ongoing as of May 2026.

Retatrutide has no FDA approval, no dispensing label, and no commercial product name. There is no product to log. If it reaches approval, the record structure would resemble a standard weekly subcutaneous injection log.

5. Weight Reduction by Agonist Class

The chart below shows mean body weight reduction from the highest-dose arm versus placebo in pivotal trials for each class. These are not head-to-head comparisons - different populations, durations, and designs. They indicate what each class has demonstrated in controlled conditions, not what any individual should expect.

Sources: Wilding et al. NEJM 2021 (STEP 1) for semaglutide; Jastreboff et al. NEJM 2022 (SURMOUNT-1) for tirzepatide; Jastreboff et al. NEJM 2023 (Phase 2) for retatrutide. Trials differ in design, duration, and population - not directly comparable. Retatrutide is not FDA-approved.

6. Side by Side

The table below compares the three classes by receptor target, approval status, primary example, and record context. This is a terminology and tracking reference, not a prescribing guide.

7. What This Means for a Tracking Record

The GLP-1/2/3 shorthand helps explain which generation of medication someone is on, but it doesn't drive how a tracker works. Peptide Tracker records what the user enters: product name, route, dose units, schedule, injection site, inventory, and notes. A semaglutide record and a tirzepatide record look nearly identical in the app - both are weekly injectables with the same log structure. What differentiates them is the product name, not the receptor-class label.

For adjacent tracking context: what GLP-1 means, the Foundayo oral GLP-1 tracking guide, and the GLP-1 tracking overview.

8. Sources