How can one drug affect appetite, blood sugar, and weight all at once?
Think of GLP-1 as a master key that fits locks (receptors) in several different rooms of the body. The natural hormone GLP-1 is released from your gut after you eat, and it travels through the bloodstream to reach the brain, pancreas, and stomach. GLP-1 drugs (like semaglutide, liraglutide, and tirzepatide) are engineered versions that last much longer and are more potent, but they work through the same receptors. Because those receptors are spread across multiple organs, activating them produces a coordinated set of effects: you feel fuller and less hungry (brain), your pancreas releases more insulin only when blood sugar is high (pancreas), and food moves more slowly through your stomach (stomach), which also helps you feel full and blunts blood sugar spikes [2][7].
The brain effect is central to appetite and weight loss. A 2021 meta-analysis of 7 randomized controlled trials (4,824 participants) found that semaglutide reduced body weight by an average of 10.6% more than placebo—that's about 9.4 kg (20.7 lbs) [6]. The same review noted that the weight loss is driven by appetite suppression and delayed stomach emptying, not by speeding up metabolism [6]. A 2026 computational model of semaglutide's effects on the gut-brain axis confirmed that the drug reduces activity in 'hunger' neurons (AgRP) and increases activity in 'fullness' neurons (POMC) and reward-related dopamine neurons, which together reduce the drive to eat [3].
The blood sugar effect is partly independent of weight loss, but the two are deeply connected. GLP-1 directly stimulates insulin secretion from the pancreas when glucose is high—this is why it rarely causes dangerously low blood sugar [7]. In a 4-week trial of a new dual GLP-1/GIP drug (HS-20094) in 54 people with type 2 diabetes, blood sugar (HbA1c) dropped by 0.6–0.8% and fasting glucose fell by 2–3 mmol/L, while body weight decreased by 1.3–4.4% depending on the dose [1]. A separate 2.5-year study of 256 patients on various GLP-1 drugs found that the long-term improvement in blood sugar was almost entirely explained by the amount of weight lost: patients who lost the most weight (average 12.2%) saw their HbA1c drop by nearly 1%, while those who gained weight had almost no blood sugar improvement [5]. This tells us that while the direct insulin-boosting effect is real, the lasting blood sugar benefit comes largely from shedding pounds.
What kind of results do people actually see—and is it the same for everyone?
The short answer is that results vary widely, but the best-case outcomes are impressive. In a phase 2 trial of the dual GLP-1/GIP agonist tirzepatide (not yet approved for weight loss in Australia at the time of the review), people lost 15–20 kg (33–44 lbs) over 72 weeks, with more than half of those on the highest doses losing 20% or more of their body weight [2]. For semaglutide at the 2.4 mg weekly dose approved for obesity, the meta-analysis found that people were 4.3 times more likely to lose at least 10% of their body weight compared to placebo, and 6.7 times more likely to lose at least 15% [6]. In a 4-week study of a new biased dual agonist (CT-388), people lost 4.7–8.0% of their body weight in just one month, compared to 0.5% with placebo [4].
But not everyone responds the same way. The 2.5-year study of 256 patients found that about one-third of people actually gained weight while on GLP-1 drugs, and that group saw no meaningful improvement in blood sugar [5]. This is a crucial real-world finding: the drugs are powerful on average, but individual responses vary. The meta-analysis also noted that gastrointestinal side effects like nausea (2.7 times more common than placebo) and diarrhea (2 times more common) are common, especially when starting or increasing the dose [6]. These side effects are usually mild and temporary, but they can lead some people to stop treatment.
The bottom line: GLP-1 drugs are not a magic bullet—they work best as part of a comprehensive plan that includes diet and exercise. The 2023 review of obesity medications emphasizes that lifestyle changes remain the foundation, and that weight loss triggers biological changes that make it hard to keep weight off, which is why long-term medication use is often needed [2]. The drugs address the biology of appetite and blood sugar regulation, but they don't override the need for healthy habits.
About These Sources
This answer is built on 7 peer-reviewed studies — published from 2021 to 2026, 4 from 2024 or later, 7 in Q1 journals — selected as the most relevant from 8 studies that passed quality screening, drawn from 55 papers retrieved from a database of over 500 million.
Sources used in this answer
733-P: Efficacy and Safety of HS-20094 in Patients with Type 2 Diabetes—A Randomized, Double-Blind, Placebo-Controlled, Phase 2 Trial
In a 4-week randomized controlled trial of 54 people with type 2 diabetes, the dual GLP-1/GIP agonist HS-20094 reduced HbA1c by 0.6–0.8% and body weight by 1.3–4.4% across doses, with decreased appetite as a common side effect.
Current and emerging medications for the management of obesity in adults
This 2023 narrative review of obesity medications explains that GLP-1 receptor agonists reduce food intake by acting on appetite and reward centers in the brain, stimulate insulin secretion only when blood sugar is high, and slow stomach emptying—all via widely distributed GLP-1 receptors.
SemaGBA: A System Dynamics Model of the Semaglutide-Responsive Gut-Brain Axis A Model of How the Brain and Semaglutide Regulate Appetite and Weight.
A 2026 computational model of semaglutide's effects on the gut-brain axis predicted that the drug reduces hunger neuron (AgRP) activity and increases fullness neuron (POMC) and dopamine activity, leading to reduced energy intake and weight loss of 15.1% in obesity (matching clinical trial data of 14.9–17.1%).
Effects of CT-388, a once-weekly signaling-biased dual GLP-1/GIP receptor agonist, on weight loss and glycemic control in preclinical models and participants with obesity
In a phase 1 randomized trial of 4 weeks in people with overweight or obesity, the biased dual GLP-1/GIP agonist CT-388 produced 4.7–8.0% weight loss vs 0.5% with placebo, and improved blood sugar during fasting and after a glucose challenge.
1800-PUB: The Long-Term Effect of Glucagon-Like Peptide 1 Receptor Agonists on Glycemic Control Is Predominantly Related to Weight Loss
In a 2.5-year retrospective study of 256 people with type 2 diabetes on GLP-1 drugs, long-term blood sugar improvement was strongly tied to weight loss: those who lost the most weight (average 12.2%) saw HbA1c drop by 1%, while those who gained weight had no significant blood sugar change.
Once‐weekly semaglutide for obesity or overweight: A systematic review and meta‐analysis
This meta-analysis of 7 randomized controlled trials (4,824 participants) found that semaglutide led to an average 10.6% greater weight loss than placebo, with 4.3 times higher odds of losing at least 10% of body weight, but also increased nausea (2.7 times) and diarrhea (2 times).
Comparative Effects of GLP-1 and GLP-2 on Beta-Cell Function, Glucose Homeostasis and Appetite Regulation
In mouse and cell studies, GLP-1 directly stimulated insulin secretion from pancreatic beta-cells by raising cAMP levels, while both GLP-1 and GLP-2 suppressed appetite and improved blood sugar disposal—but only GLP-1 did so via enhanced insulin release.