GLP-1 Receptor Agonists Explained

TL;DR

- GLP-1 receptor agonists activate the GLP-1 receptor and produce four core effects: glucose-dependent insulin release, glucagon suppression, slower gastric emptying, and greater satiety (Frontiers in Endocrinology review, 2024, doi:10.3389/fendo.2024.1431292). - In STEP 1, semaglutide (a GLP-1 only agonist) produced about 15 percent mean body weight loss, about -15.3 kg, versus about -2.6 kg with placebo over 68 weeks (Wilding et al., 2021). - Tirzepatide adds GIP activity and reached up to about 22.5 percent in SURMOUNT-1; retatrutide adds glucagon activity and reached up to about 24.2 percent at 48 weeks (Jastreboff et al., 2022; Jastreboff et al., 2023). - Semaglutide and tirzepatide carry regulatory approval for specified uses; retatrutide and survodutide are investigational and not approved (FDA prescribing labels; le Roux et al., 2024). - Trials report substantial weight regain after stopping, so the effect depends on continued use (Wilding et al., 2022; Aronne et al., 2024).

What are GLP-1 receptor agonists?

GLP-1 receptor agonists are a class of engineered molecules that bind and activate the glucagon-like peptide-1 receptor, copying a natural gut hormone that lowers blood sugar and reduces appetite (Frontiers in Endocrinology, 2024, doi:10.3389/fendo.2024.1431292).

GLP-1 is an incretin hormone the gut releases after eating. It tells the pancreas to release insulin, tells the liver to hold back glucagon, and signals the brain that you are full. GLP-1 receptor agonists are built to send that same signal, but stronger and for far longer than the natural hormone does (Frontiers in Endocrinology, 2024). The class now includes single-receptor agonists, dual agonists, and triple agonists, which differ by how many hormone receptors each drug hits.

How do GLP-1 receptor agonists work?

They activate the GLP-1 receptor and trigger four linked effects that together lower blood sugar and reduce food intake.

According to the 2024 Frontiers in Endocrinology review (doi:10.3389/fendo.2024.1431292), the main actions are:

1. Glucose-dependent insulin secretion. Insulin is released when blood sugar is high, which lowers the risk of overshooting into low blood sugar. 2. Glucagon suppression. Less glucagon means the liver makes less new glucose. 3. Slower gastric emptying. Food leaves the stomach more slowly, so you feel full longer. 4. Increased satiety. Signaling in appetite centers of the brain reduces hunger and food intake.

Dual and triple agonists layer more receptor targets on top of this. Tirzepatide also activates the GIP receptor (Jastreboff et al., 2022), and retatrutide activates the GIP and glucagon receptors as well (Jastreboff et al., 2023).

Why are GLP-1 receptor agonists engineered to resist breakdown?

Because native GLP-1 is destroyed within minutes, so these drugs are modified to survive far longer and act over days rather than seconds.

Natural GLP-1 is broken down almost immediately by an enzyme called DPP-4, which makes the raw hormone useless as a long-acting drug (Frontiers in Endocrinology, 2024). GLP-1 receptor agonists are engineered to resist that enzymatic degradation, for example by changing the molecule's structure or attaching it to carrier fragments that slow clearance. This extended duration is why several agents in the class are given once weekly rather than several times a day (Frontiers in Endocrinology, 2024).

What do the major GLP-1 trials report?

Large randomized trials report weight loss that tends to scale with the number of receptors a drug activates, and the newest triple agonist reports the largest reduction to date.

The ranges below reflect what published studies and commonly studied research protocols report. This is educational, not a prescription or a personal recommendation. The table lists trial outcomes and the doses each study used, with a source for each row.

One head-to-head comparison exists inside the class. In SURPASS-2, a trial in people with type 2 diabetes, tirzepatide produced greater reductions than semaglutide (Frias et al., 2021). The differences between semaglutide, tirzepatide, and retatrutide in the table come from separate trials with different designs, so they are not a direct head-to-head result.

Approved or investigational: which GLP-1 drugs are which?

Semaglutide and tirzepatide carry regulatory approval for specified uses, while retatrutide and survodutide remain investigational and are not approved.

Approval status matters because it tells you whether a compound has cleared full regulatory review or is still being studied. Per FDA prescribing labels, semaglutide and tirzepatide have approved uses that include long-term weight management and blood glucose control. Retatrutide's data come from clinical studies, not approved use (Jastreboff et al., 2023). Survodutide is another investigational multi-receptor agonist with early trial data (le Roux et al., 2024). Investigational means the evidence is still being gathered and the compound has not been approved for general medical use.

Why does adding GIP and glucagon activity increase weight loss?

Trials suggest that hitting more than one metabolic receptor produces larger weight reductions, with the triple agonist reporting the highest figures so far.

Reading the trials in order shows the pattern. Semaglutide, which targets GLP-1 alone, reported about 15 percent (Wilding et al., 2021). Tirzepatide, which adds GIP, reported up to about 22.5 percent (Jastreboff et al., 2022). Retatrutide, which adds glucagon on top of GLP-1 and GIP, reported up to about 24.2 percent at 48 weeks (Jastreboff et al., 2023). The glucagon receptor is thought to raise energy expenditure, which may add to the appetite-lowering effect of the other two targets. This remains a cross-trial pattern rather than proof from a single study comparing all three.

What happens when someone stops a GLP-1 receptor agonist?

Research reports that much of the lost weight returns after stopping, so these drugs manage weight while taken rather than resetting it permanently.

In a STEP 1 extension, participants regained a large share of the weight they had lost within about a year of stopping semaglutide (Wilding et al., 2022). SURMOUNT-4 reported weight regain after tirzepatide was withdrawn (Aronne et al., 2024). Part of this reflects the body defending a higher weight: after weight loss, energy expenditure tends to fall through adaptive thermogenesis (Rosenbaum and Leibel, 2010). Weight loss on therapy also tends to slow to a plateau over time (Horn et al., 2025). Together these findings explain why the class is generally studied as ongoing therapy.

How are GLP-1 receptor agonists different from other research peptides?

They act on the incretin and glucagon receptor system, which is a different pathway from growth hormone secretagogues and amylin analogs.

Growth hormone secretagogues raise growth hormone and IGF-1 through a separate receptor. Examples with published data include MK-677, which raised fasting glucose and lowered insulin sensitivity in older adults (Nass et al., 2008), CJC-1295, which sustained higher GH and IGF-1 (Teichman et al., 2006), and ipamorelin, a selective GH secretagogue (Raun et al., 1998). Cagrilintide is an amylin analog studied alongside semaglutide (Lau et al., 2021); it targets amylin, not the GLP-1 receptor, so it is not itself a GLP-1 receptor agonist. Knowing which pathway a compound targets is the fastest way to place it in the right class.

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