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 are informal shorthand used in the weight-loss and tracking community to group obesity medications by the number of 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 receptor agonists (like teduglutide, used for short bowel syndrome) are a completely different drug class. In this context, “GLP-2” means a dual GIP+GLP-1 agonist like tirzepatide, and “GLP-3” means a triple agonist like retatrutide.

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 format - injection logs need site and pen inventory, oral logs need tablet timing and supply count.

3. GLP-2 - Dual Agonist (Tirzepatide)

Tirzepatide (Mounjaro, Zepbound) adds a second receptor target: the glucose-dependent insulinotropic polypeptide receptor (GIPR). 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 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)

Retatrutide adds a third target: 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.

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 currently 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 context: what GLP-1 means, what Foundayo is, and the GLP-1 tracking overview.

8. GLP-1 vs GLP-2 vs GLP-3 FAQ

• What is GLP-1 in the context of weight-loss drugs?

  GLP-1 refers to medications that activate only the GLP-1 receptor. Semaglutide (Ozempic, Wegovy) is the primary example. The STEP 1 trial (NEJM 2021) showed a mean 14.9% body weight reduction at 68 weeks with 2.4 mg versus 2.4% with placebo.

• What is GLP-2 in the context of weight-loss drugs?

  GLP-2 is informal shorthand for dual agonist medications that activate both the GIP receptor and the GLP-1 receptor. Tirzepatide (Mounjaro, Zepbound) is the primary example. SURMOUNT-1 (NEJM 2022) showed a mean 20.9% weight reduction at 72 weeks on 15 mg versus 3.1% with placebo.

• What is GLP-3 in the context of weight-loss drugs?

  GLP-3 is informal shorthand for triple agonist investigational drugs that target GLP-1, GIP, and glucagon receptors simultaneously. Retatrutide is the lead candidate. A Phase 2 trial (NEJM 2023) showed 24.2% mean weight reduction at 48 weeks on 12 mg versus 2.1% with placebo. It is not FDA-approved as of May 2026.

• How does the agonist class affect a tracking record?

  The agonist class doesn't change how logging works, but it helps explain the product you're tracking. Semaglutide and tirzepatide are both weekly subcutaneous injections with similar log fields. Oral semaglutide (Rybelsus) and orforglipron shift to tablet timing and supply count. The product label - not the receptor shorthand - determines the record shape.

• Is tirzepatide a GLP-1 drug?

  Tirzepatide activates the GLP-1 receptor and the GIP receptor. It is often grouped with GLP-1 medications because it shares the receptor pathway, but its dual mechanism is distinct from semaglutide, which targets only the GLP-1 receptor. FDA labeling identifies tirzepatide as a GIP/GLP-1 receptor agonist.

9. Sources