GLP-1, or glucagon-like peptide-1, was not discovered in a single eureka moment but pieced together over decades by researchers working on the gut’s role in blood sugar control. The pivotal breakthrough came in the early 1980s, when molecular biologists cloning the gene for glucagon unexpectedly found it also encoded two additional hormone sequences, one of which turned out to be GLP-1. From that genetic accident of discovery, an entire class of blockbuster drugs eventually emerged. Understanding who made the key contributions and why GLP-1 matters requires tracing a story that runs from century-old physiology experiments through lizard venom research to some of the most prescribed medications on the planet.
The Incretin Idea That Preceded GLP-1 by Decades
Long before anyone knew GLP-1 existed, scientists noticed something odd: swallowing glucose triggered far more insulin release than injecting the same amount directly into the bloodstream. This observation, dating back to the early twentieth century, suggested that the gut itself was sending signals to the pancreas to ramp up insulin production. Researchers called this the “incretin effect,” from “intestine secretion insulin.” The concept has more than a century of history, full of periods of enthusiasm and skepticism, and only in recent decades has it been tied firmly to specific hormones.
1Europe PMC / Frontiers in Endocrinology. The Origin and Understanding of the Incretin ConceptBy the 1970s, one incretin hormone had been identified: GIP (glucose-dependent insulinotropic polypeptide). But GIP alone could not fully explain the incretin effect. Something else was at work. That something turned out to be GLP-1, though discovering it required tools that would not exist until the molecular biology revolution of the 1980s.
Cloning the Gene and Finding a Hidden Hormone
In 1982 and 1983, two groups working independently made the critical find. Graeme Bell and colleagues at the University of Chicago, along with Joel Habener’s laboratory at Massachusetts General Hospital, cloned and sequenced the mammalian proglucagon gene. They were primarily interested in glucagon, a hormone made in the pancreas that raises blood sugar. But when they read the full gene sequence, they found it encoded not just glucagon but also two additional glucagon-like peptides, which they named GLP-1 and GLP-2.
2PubMed. The discovery of glucagon-like peptide 1The gene was the same in the pancreas and the intestine, but the way cells processed the resulting protein differed. Pancreatic cells chopped up proglucagon to produce mainly glucagon. Intestinal cells, by contrast, processed it to release GLP-1 and GLP-2 instead. This tissue-specific processing meant that the same gene could serve opposite metabolic purposes depending on where it was expressed. It was a textbook example of how biology squeezes multiple functions out of a single genetic blueprint.
Identifying GLP-1’s sequence was only the first step. Researchers still needed to figure out which form of the peptide was biologically active and what it actually did. Through the mid-to-late 1980s, groups led by scientists including Jens Juul Holst in Copenhagen and Daniel Drucker in Toronto demonstrated that a truncated version of GLP-1, released from intestinal L-cells after meals, powerfully stimulated insulin secretion in a glucose-dependent manner. That glucose dependence turned out to be crucial: GLP-1 boosted insulin only when blood sugar was already elevated, making it far less likely to cause dangerous drops in blood sugar compared with other insulin-stimulating agents.
3PubMed Central. GLP-1 receptor activated insulin secretion from pancreatic β-cells: mechanism and glucose dependenceHow GLP-1 Works in the Body
GLP-1 is released by specialized enteroendocrine L-cells scattered throughout the intestinal lining. These cells detect the arrival of food, particularly fats and carbohydrates, through nutrient-sensing receptors on their surface, and respond by secreting GLP-1 into the bloodstream.
4PubMed Central. The L-Cell in Nutritional Sensing and the Regulation of AppetiteOnce released, GLP-1 acts on multiple organs at once. At the pancreas, it binds to GLP-1 receptors on beta cells, triggering a signaling cascade that amplifies insulin release when glucose is present.
5PubMed Central. Mechanisms of action of glucagon-like peptide 1 in the pancreas It also stimulates the gene responsible for making insulin, so the cell does not just release stored insulin but gears up to produce more.
6Molecular and Cellular Endocrinology. Glucagon-like peptide-1(7–37)/(7–36)amide is a new incretinIn the stomach, GLP-1 slows gastric emptying through signals carried by the vagus nerve, keeping food in the stomach longer and blunting the post-meal spike in blood sugar.
7PubMed. Glucagon-like peptide-1 inhibits gastric emptying via vagal afferent-mediated central mechanisms In the brain, GLP-1 receptors in areas that control appetite contribute to feelings of fullness and reduced food intake. Research has identified the dorsomedial hypothalamus as a key region where GLP-1 receptor agonists act to promote satiation.
8PubMed Central. Glucagon-Like Peptide-1 and Hypothalamic Regulation of Satiation: Cognitive and Neural Insights from Human and Animal StudiesThere is one major catch with natural GLP-1: it is destroyed almost immediately. An enzyme called DPP-IV, found on the surface of blood vessel cells, chops up GLP-1 within minutes of its release.
9PubMed. Mechanisms underlying the rapid degradation and elimination of the incretin hormones GLP-1 and GIP That rapid breakdown is why natural GLP-1 cannot simply be injected as a drug. Making GLP-1 medically useful required finding ways to keep it active in the body for hours or days instead of minutes.
From Gila Monster Venom to the First Approved Drug
The solution came from an unlikely source. In the 1990s, John Eng, an endocrinologist at the Veterans Affairs Medical Center in the Bronx, was systematically cataloging peptides in the venom of the Gila monster, a venomous lizard native to the American Southwest. He isolated a 39-amino-acid peptide he called exendin-4. It shared roughly half its structure with human GLP-1 and activated the same receptor, but it resisted breakdown by DPP-IV, lasting far longer in the bloodstream.
10PubMed. The development of Byetta (exenatide) from the venom of the Gila monster as an anti-diabetic agentThat stability made exendin-4 a viable drug candidate. A synthetic version, exenatide, entered clinical trials and came to market in 2005 under the brand name Byetta, becoming the first GLP-1 receptor agonist approved for type 2 diabetes. It required twice-daily injections, which limited convenience, but it proved the concept: a drug mimicking GLP-1 could lower blood sugar effectively while carrying a low risk of hypoglycemia.
Engineering Longer-Lasting and More Potent Analogs
Exenatide opened the door, but pharmaceutical chemists quickly set about designing molecules that would last longer and work better. The key innovation for the next generation of drugs, liraglutide and semaglutide, was attaching a fatty acid chain to the GLP-1 molecule. This chain binds reversibly to albumin, a protein abundant in blood, which acts as a slow-release carrier. By optimizing the fatty acid structure and the chemical linker connecting it to the peptide, researchers extended the drug’s effective life from hours to days or even a week.
11PubMed Central. The Discovery and Development of Liraglutide and SemaglutideLiraglutide, marketed as Victoza for diabetes and Saxenda for weight management, requires daily injection. Semaglutide, available as Ozempic for diabetes and Wegovy for obesity, works as a once-weekly injection. An oral form of semaglutide (Rybelsus) also exists, though it requires careful dosing on an empty stomach because peptides are fragile in the digestive tract.
More recently, dual-agonist drugs have entered the picture. Tirzepatide, sold as Mounjaro and Zepbound, activates both the GLP-1 receptor and the GIP receptor simultaneously. It is engineered to favor the GIP receptor, which allows more aggressive dosing without the nausea and vomiting that often limit how much GLP-1 activity a patient can tolerate.
12The Journal of Clinical Investigation. Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist By engaging both incretin pathways, tirzepatide has shown particularly large effects on both blood sugar and body weight. The GIP component also has a distinct role in fat metabolism: GIP directly promotes fat storage in healthy fat cells, while GLP-1 indirectly promotes fat breakdown, and together these actions help redistribute fat away from organs where it does the most damage.
13PubMed Central. Mechanisms of action and therapeutic applications of GLP-1 and dual GIP/GLP-1 receptor agonistsThe Push Toward Oral Pills
One major limitation of current GLP-1 drugs is that most require injection. Even the existing oral semaglutide has strict dosing requirements and lower bioavailability than the injectable version. Researchers are now developing entirely new molecules, small enough to survive the gut and absorb efficiently, that are not peptides at all. Orforglipron is the most advanced of these: a small-molecule compound that activates the GLP-1 receptor and can be taken as a straightforward daily pill without the fasting and timing constraints of oral semaglutide.
14PubMed Central. Orforglipron: A Comprehensive Review of an Oral Small-Molecule GLP-1 Receptor Agonist for Obesity and Type 2 DiabetesChemists have also been modifying orforglipron’s structure to improve how well it is absorbed. By tweaking the part of the molecule exposed to water, researchers have produced variants with dramatically better gut permeability and oral bioavailability in animal testing, without sacrificing the drug’s ability to activate the GLP-1 receptor at very low concentrations.
15ACS Medicinal Chemistry Letters. Discovery of Small-Molecule GLP‑1 Receptor Agonists with Improved Oral Pharmacokinetics Based on Orforglipron If these compounds succeed in human trials, they could make GLP-1 therapy far more accessible to people who are reluctant to inject or who lack access to cold-chain storage.
Cardiovascular and Kidney Benefits
What elevated GLP-1 drugs from useful diabetes medications to transformative therapies was the discovery that they do more than control blood sugar. Large randomized trials have consistently shown that GLP-1 receptor agonists reduce major cardiovascular events, including heart attacks, strokes, cardiovascular death, and heart failure, across diverse populations that include people with obesity but without diabetes.
16PubMed. Cardiovascular Risk Reduction With GLP-1 RA DrugsA meta-analysis pooling results from major trials found that GLP-1 receptor agonists reduced the combined rate of major cardiovascular events by about 12%, with similar reductions in cardiovascular death and all-cause mortality. Stroke risk dropped by roughly 16%. The drugs also reduced hospital admissions for heart failure by about 9% and a broad composite kidney outcome by about 17%, primarily through reduced protein leakage in the urine.
17The Lancet Diabetes & Endocrinology. Cardiovascular, mortality, and kidney outcomes with GLP-1 receptor agonists in patients with type 2 diabetes: a systematic review and meta-analysisThe cardiovascular benefits appear to go beyond what you would expect from lower blood sugar and weight loss alone. GLP-1 receptor agonists stimulate blood vessels to produce nitric oxide (which relaxes arteries), reduce oxidative stress, and have anti-inflammatory and anti-atherosclerotic effects.
18PubMed Central. The benefits of GLP1 receptors in cardiovascular diseasesOn the kidney side, the protective effects have attracted intense interest, especially because GLP-1 receptors are only modestly expressed in the kidney itself. The renoprotective mechanism appears to work partly through reducing inflammation: GLP-1 receptor agonists block activation of immune cells within kidney tissue, reduce infiltration by inflammatory cells, and lower levels of inflammatory signaling molecules. They may also reduce oxidative stress, fibrosis, and cell death in the kidney.
19Nature Reviews Nephrology. Incretin drugs in diabetic kidney disease: biological mechanisms and clinical evidenceLiver Disease and Fatty Liver
Another area of growing clinical relevance is metabolic dysfunction-associated steatohepatitis, or MASH (formerly called NASH), a form of fatty liver disease that can progress to cirrhosis. GLP-1 receptor agonists, especially semaglutide and liraglutide, reduce liver fat, hepatic inflammation, and liver enzyme levels in trials.
20International Journal of Innovative Technologies in Social Science. GLP-1 RECEPTOR AGONISTS IN METABOLIC DYSFUNCTION-ASSOCIATED STEATOTIC LIVER DISEASE: THERAPEUTIC POTENTIAL AND CLINICAL CHALLENGES A recent systematic review and meta-analysis of patients with moderate-to-advanced fibrosis found that GLP-1 receptor agonists were about three times as likely as placebo to produce resolution of MASH without worsening fibrosis, and also significantly improved fibrosis itself.
21JHEP Reports. Efficacy and safety of GLP-1 receptor agonists in MASH with fibrosis: A systematic review and meta-analysisThis matters because MASH currently has limited treatment options and is one of the fastest-growing reasons for liver transplant. Semaglutide became the first drug approved specifically for MASH in 2024, a milestone that brought GLP-1 therapy into hepatology, not just endocrinology and cardiology.
Side Effects and Practical Concerns
GLP-1 drugs are not free of downsides. The most common side effects are gastrointestinal: nausea, vomiting, diarrhea, and constipation. These tend to hit hardest in the first weeks of treatment, affecting roughly half to 60% of patients, and usually lessen over time. They are dose-dependent, which is why most prescribing protocols call for a gradual dose increase.
22Diabetes & Metabolism Journal. Exploring the Side Effects of GLP-1 Receptor Agonist: To Ensure Its Optimal PositioningRarer but more serious concerns include gallbladder disorders and acute pancreatitis. The delayed gastric emptying that helps control blood sugar also creates practical complications: patients undergoing surgery face increased aspiration risk under anesthesia, and bowel preparation for colonoscopies can be less effective.
23PubMed Central. Glucagon-like Receptor-1 agonists for obesity: Weight loss outcomes, tolerability, side effects, and risksLean mass loss is another ongoing concern. About 40% of the weight people lose on GLP-1 drugs comes from lean body mass, including muscle, not just fat. That has raised questions about whether the drugs could impair muscle function over time, particularly in older adults. Current imaging evidence suggests the muscle changes may be adaptive rather than harmful. Muscle quality seems to improve through better insulin sensitivity and reduced fat infiltration into muscle tissue, even as total muscle mass decreases.
22Diabetes & Metabolism Journal. Exploring the Side Effects of GLP-1 Receptor Agonist: To Ensure Its Optimal Positioning Still, the long-term implications remain unclear, and resistance exercise during treatment is widely recommended.
GLP-1, Addiction, and the Brain’s Reward System
Some of the most intriguing research on GLP-1 has nothing to do with blood sugar or body weight. GLP-1 receptors are found in reward-related areas of the brain, and activating them appears to dampen the dopamine surges that reinforce addictive behaviors. In animal studies, GLP-1 and its agonists reduce intake of palatable food and also reduce consumption of alcohol, cocaine, amphetamine, and nicotine.
24PubMed Central. Can GLP-1 Be a Target for Reward System Related Disorders? A Qualitative Synthesis and Systematic Review Analysis of Studies on Palatable Food, Drugs of Abuse, and AlcoholThe alcohol data is particularly developed. Preclinical work shows that GLP-1 receptor activation blunts alcohol’s ability to stimulate the mesolimbic dopamine system, the circuit most associated with the rewarding effects of drugs. Animals given GLP-1 receptor agonists drink less and work less hard to obtain alcohol in behavioral tasks.
25PubMed. GLP-1 signaling and alcohol-mediated behaviors; preclinical and clinical evidence Anecdotal reports from patients on semaglutide and liraglutide have echoed these findings, with many describing reduced cravings for alcohol and other substances. Clinical trials are now underway to test whether this translates into a formal treatment for alcohol use disorder and other addictions.
Neuroprotection and Neurodegenerative Disease
GLP-1 receptors are expressed throughout the brain, not just in appetite-regulating regions, and researchers have been investigating whether GLP-1 receptor agonists could protect neurons against degeneration. In preclinical models of Alzheimer’s and Parkinson’s disease, GLP-1 receptor activation triggers a broad set of protective responses in neurons and the support cells surrounding them. These include reduced inflammation, lower oxidative stress, improved energy metabolism, protection against toxic protein buildup (including amyloid-beta and alpha-synuclein), and even increased production of growth factors that help neurons survive.
26PubMed Central. The neuroprotective effects of glucagon-like peptide 1 in Alzheimer’s and Parkinson’s disease: An in-depth reviewThe question of whether weight-loss drugs from GLP-1 receptor agonists require GLP-1 receptors in the brain has been tested directly. Research in animal models shows that the weight-loss effects of GLP-1 receptor agonists depend on receptors in the central nervous system, with a subset of neurons in the arcuate nucleus of the hypothalamus playing a central role.
27PubMed Central. Glucagon-like peptide-1 receptors in the brain: controlling food intake and body weight The fact that GLP-1 drugs must reach the brain to produce weight loss is actually encouraging for neurodegeneration research, because it confirms these molecules cross into the central nervous system. Clinical trials testing liraglutide and semaglutide in Alzheimer’s and Parkinson’s patients are currently in progress, though definitive results are still years away.
Cost, Access, and the Economic Picture
The clinical potential of GLP-1 drugs runs headlong into their price. In the United States, list prices for semaglutide and tirzepatide have exceeded $1,000 per month, and insurance coverage varies widely, particularly for obesity indications as opposed to diabetes. Health economic analyses have wrestled with how to value these drugs. The broader societal benefits, including reduced cardiovascular events, fewer hospitalizations, and lower long-term healthcare costs, are real but difficult to quantify with standard cost-effectiveness models, which tend to focus narrowly on drug price versus immediate clinical outcomes.
28Economic Affairs. Societal value and health economic benefits of GLP‐1 drugs in the United StatesWhen evaluated strictly as obesity treatments compared with lifestyle interventions alone, GLP-1 receptor agonists have not cleared conventional cost-effectiveness thresholds in pooled analyses.
29PubMed Central. GLP-1 receptor agonists for treating obesity without diabetes: A systematic review and meta-analysis of economic evaluations This does not mean the drugs lack value, but it does highlight the tension between their proven clinical benefits and what healthcare systems can afford to pay at current prices. Generic versions and biosimilars are expected to change this equation over the coming years, and some countries are already negotiating substantially lower prices than what prevails in the US market.
An Ancient Hormone with a Shifting Job Description
GLP-1’s importance is not just a human story. The proglucagon gene, which encodes GLP-1, is ancient and found across vertebrates, from fish to mammals. But its function has changed dramatically over evolutionary time. In fish, GLP-1 acts more like glucagon, raising blood sugar. Somewhere along the evolutionary lineage leading to mammals, GLP-1 flipped its role entirely and became an incretin that lowers blood sugar by stimulating insulin.
30BioScience. Evolution of Hormone Function: Proglucagon-derived Peptides and Their ReceptorsPhylogenetic analysis suggests this switch may have occurred because GLP-1 lost its ability to act on insulin-secreting cells in fish and was instead recruited as a ligand for a duplicated glucagon receptor, giving it a glucagon-like function. In mammals, the receptor relationships reshuffled, and GLP-1 acquired its modern insulin-boosting role. The evolutionary rates of change for GLP-1 and its sibling peptides have been uneven, with bursts of rapid evolution in certain lineages suggesting new functions were being acquired.
31PubMed. Molecular evolution of proglucagon Across mammals, the proglucagon gene and its three receptors show enough variation to hint at biological roles beyond what has been characterized so far.
32PubMed Central. Variation in the Evolution and Sequences of Proglucagon and the Receptors for Proglucagon-Derived Peptides in MammalsThere is something poetic in the fact that a hormone whose function reversed itself over hundreds of millions of years of evolution is now being re-engineered by chemists to do things neither fish nor early mammals could have anticipated: shrinking livers scarred by fat, protecting aging brains, and quieting the reward circuits that drive addiction. The scientists who first spotted two unexpected peptide sequences hiding in the glucagon gene could not have imagined where those sequences would lead.