Semaglutide is a GLP-1 receptor agonist, and the mechanism that has actually been measured in humans is a large reduction in how much people eat. In a crossover trial at 1.0 mg, total ad libitum energy intake across a day was 24% lower than on placebo (PMID 28266779); at the 2.4 mg weight-management dose, intake at a test lunch was 35% lower (PMID 33269530).
The receptor pharmacology is the explanation underneath that, not a substitute for it. Below is what each mechanism trial measured, what the gastric emptying evidence does and does not support, and why one injection lasts a week.
- The measured effect is reduced energy intake: 24% lower across a day at 1.0 mg and 35% lower at a test lunch on 2.4 mg.
- Participants reported less hunger, fewer cravings and a lower preference for high-fat foods, with nausea ratings no different from placebo.
- The label states semaglutide delays gastric emptying, but a week-20 paracetamol study found no evidence of delay once corrected for body weight.
- Resting metabolic rate adjusted for lean mass did not change, so the drug works on intake rather than expenditure.
- Albumin binding above 99% and a DPP-4 resistant modification give an elimination half-life of about one week.
What the mechanism trials measured
Three small crossover and parallel-group trials were run specifically to find out why people on semaglutide lose weight, rather than how much they lose. They are the best direct evidence there is, and they all point at intake.
| Trial | Dose and duration | Participants | What it measured | Result |
|---|---|---|---|---|
| Blundell 2017 (PMID 28266779) | Semaglutide 1.0 mg weekly, 12 weeks | 30 adults with obesity | Ad libitum energy intake across a day | 24% lower total intake than placebo; body weight down 5.0 kg |
| Friedrichsen 2021 (PMID 33269530) | Semaglutide 2.4 mg weekly, 20 weeks | 72 adults with obesity | Ad libitum lunch, appetite ratings, gastric emptying | Intake 35% lower (1736 vs 2676 kJ); body weight down 9.9% vs 0.4% |
| Gibbons 2021 (PMID 33184979) | Oral semaglutide 14 mg daily, 12 weeks | 15 adults with type 2 diabetes | Energy intake across lunch, dinner and a snack box | Total daily intake 38.9% lower than placebo |
The mechanism, in the plainest terms the data supports: semaglutide makes people eat substantially less, and weight follows. The Wegovy label says the same thing in its own words, that semaglutide decreases calorie intake and the effects are likely mediated by affecting appetite.
Appetite, cravings and what people chose to eat
The intake figures came with participant-reported measures, and those add detail a calorie count does not.
In Blundell's trial, participants on semaglutide reported less hunger and fewer food cravings, better control of eating and a lower relative preference for high-fat, energy-dense foods. Nausea ratings were similar between semaglutide and placebo, which matters because it rules out the simplest alternative explanation: people were not eating less because they felt sick.
Friedrichsen's 2.4 mg trial found the same pattern at the weight-management dose. Hunger and prospective food consumption fell, fullness and satiety rose, and the Control of Eating Questionnaire showed fewer and weaker food cravings, all against placebo.
The gastric emptying question
Semaglutide is widely described as working by slowing the stomach. The evidence is more specific than that.
The Wegovy prescribing information states flatly that semaglutide delays gastric emptying. That statement drives two practical parts of the label: a drug interaction note that absorption of other oral medicines may be affected, and a warning about pulmonary aspiration during general anaesthesia or deep sedation.
Friedrichsen's trial measured it directly at week 20 using paracetamol absorption after a standard breakfast. The area under the paracetamol concentration curve over five hours was 8% higher on semaglutide 2.4 mg than placebo, which reached significance at p = 0.005 but not once corrected for week-20 body weight. There was no effect on the one-hour area, on peak paracetamol concentration or on time to peak. The authors concluded there was no evidence of delayed gastric emptying at week 20 by that indirect measure.
The honest reading is that the delay is real enough to carry label warnings and is most relevant early and around dose increases, but it is not what is still driving weight loss at month five. Appetite and intake are.
What does not change
Blundell's trial also measured resting metabolic rate, adjusted for lean body mass, and found no difference between semaglutide and placebo. Weight loss in that trial came predominantly from body fat mass, and the Wegovy label makes the same point, that semaglutide lowers body weight with greater fat mass loss than lean mass loss.
So the drug is not speeding metabolism up. It is lowering the input side of the equation. That also explains why the effect does not persist after stopping: nothing about the body's energy expenditure has been permanently reset.
Why it only has to be injected once a week
Native GLP-1 is broken down within minutes. Semaglutide is engineered around that problem in three ways, all described in the label's own chemistry section:
- The lysine at position 26 carries a hydrophilic spacer and a C18 fatty di-acid, which binds the molecule to plasma albumin. More than 99% of circulating semaglutide is albumin-bound, which cuts renal clearance and shields it from degradation.
- Position 8 is modified to resist dipeptidyl peptidase-4, the enzyme that clears natural GLP-1.
- The result is an elimination half-life of about one week, with semaglutide present in the circulation for roughly five to seven weeks after a last dose.
Subcutaneous bioavailability is 89%, maximum concentration arrives one to three days after a dose, and exposure is the same whether the injection goes into the abdomen, thigh or upper arm, which is why injection site does not change the dose. Tirzepatide solves the same durability problem differently and adds a second receptor, which we go through in how tirzepatide works.
Glucose-dependent, which shows up in the overdose reports
Semaglutide stimulates insulin secretion and reduces glucagon secretion in a glucose-dependent manner, meaning the insulin response scales with how high blood glucose actually is.
That is not an abstraction. In a 2024 poison-centre case series of three unintentional semaglutide overdoses, including one patient who injected 2.4 mg instead of 0.25 mg, all three developed gastrointestinal symptoms and none became hypoglycaemic. The authors attributed that directly to the glucose-dependent mechanism (PMID 38470137). The label's hypoglycaemia warning is reserved for the situation where the mechanism is bypassed: concomitant insulin or an insulin secretagogue such as a sulfonylurea.
How quickly does semaglutide work?
Four measured reference points, in order of duration:
- 12 weeks, 1.0 mg: mean weight down 5.0 kg in Blundell's mechanism trial, mostly fat mass.
- 20 weeks, 2.4 mg: mean weight down 9.9% against 0.4% on placebo in Friedrichsen's trial.
- 3 and 6 months, mixed doses: 5.9% and 10.9% in a retrospective cohort of 175 patients treated for weight loss (PMID 36121652).
- 68 weeks, 2.4 mg: 14.9% in STEP 1, against 2.4% on placebo (PMID 33567185).
Appetite effects appear before any of that, because intake is what the drug changes first and weight is a lagging measure of intake. The dose escalation schedule stretches the early part of the curve further, which we cover in what the first month looks like.
What the mechanism does not explain
Half the semaglutide group in STEP 1 lost 15% or more of their body weight, and roughly one in seven did not reach 5%, on the same dose for the same 68 weeks. None of the mechanism trials above explains that spread. They establish what the drug does on average to appetite and intake; they do not establish why the size of that effect varies so much between people. Dose reached, diabetes status, duration of treatment and the lifestyle intervention running alongside all account for part of it, and a good deal is simply unexplained.
Turn the mechanism into a number
See what the published semaglutide percentages work out to from your own starting weight.
Mechanism explains the direction, not the size. Putting a starting weight through the GLP-1 weight loss calculator converts STEP 1's 14.9% into pounds, which is the version of the number that is actually usable. If you want the trial-by-trial breakdown, we set it out in how much weight you can lose on Wegovy.
FAQ
How does semaglutide cause weight loss?
By reducing food intake. Trials run specifically to measure the mechanism found ad libitum energy intake 24% lower than placebo at 1.0 mg over 12 weeks and 35% lower at a test lunch after 20 weeks on 2.4 mg, alongside less hunger, fewer cravings and a lower preference for high-fat foods.
Does semaglutide slow down your stomach?
The Wegovy label states that semaglutide delays gastric emptying, and that statement underpins its oral drug interaction note and its anaesthesia aspiration warning. A trial measuring gastric emptying indirectly at week 20 found an 8% increase in paracetamol absorption area that was not significant once corrected for body weight, and no change in peak concentration or time to peak.
How quickly does semaglutide work?
Appetite and intake change first, and weight follows. Measured points are 5.0 kg at 12 weeks on 1.0 mg, 9.9% at 20 weeks on 2.4 mg, 5.9% at three months in a real-world cohort on mixed doses, and 14.9% at 68 weeks on 2.4 mg in STEP 1.
Why is semaglutide injected only once a week?
Its lysine at position 26 carries a C18 fatty di-acid that binds it to plasma albumin, more than 99% of it, which cuts renal clearance and protects it from breakdown. A separate modification at position 8 resists the DPP-4 enzyme. The result is an elimination half-life of about one week.
Does semaglutide speed up metabolism?
No. Resting metabolic rate adjusted for lean body mass did not differ between semaglutide and placebo in the mechanism trial that measured it. Weight loss came predominantly from fat mass, and the label notes greater fat mass loss than lean mass loss.
Why does semaglutide rarely cause low blood sugar?
It stimulates insulin and suppresses glucagon in a glucose-dependent way, so the insulin response scales with blood glucose. In a published series of three accidental semaglutide overdoses, all three had gastrointestinal symptoms and none became hypoglycaemic. The labelled hypoglycaemia risk applies when it is combined with insulin or a sulfonylurea.
Sources
- Blundell J, Finlayson G, Axelsen M, et al. Effects of once-weekly semaglutide on appetite, energy intake, control of eating, food preference and body weight in subjects with obesity. Diabetes Obes Metab. 2017;19(9):1242-1251. PMID: 28266779
- Friedrichsen M, Breitschaft A, Tadayon S, Wizert A, Skovgaard D. The effect of semaglutide 2.4 mg once weekly on energy intake, appetite, control of eating, and gastric emptying in adults with obesity. Diabetes Obes Metab. 2021;23(3):754-762. PMID: 33269530
- Gibbons C, Blundell J, Tetens Hoff S, Dahl K, Bauer R, Baekdal T. Effects of oral semaglutide on energy intake, food preference, appetite, control of eating and body weight in subjects with type 2 diabetes. Diabetes Obes Metab. 2021;23(2):581-588. PMID: 33184979
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002. PMID: 33567185
- Ghusn W, De la Rosa A, Sacoto D, et al. Weight Loss Outcomes Associated With Semaglutide Treatment for Patients With Overweight or Obesity. JAMA Netw Open. 2022;5(9):e2231982. PMID: 36121652
- Wiener BG, Gnirke M, Vassallo S, Smith SW, Su MK. Challenges with glucagon-like peptide-1 (GLP-1) agonist initiation: a case series of semaglutide overdose administration errors. Clin Toxicol (Phila). 2024;62(2):131-133. PMID: 38470137
- Wegovy (semaglutide) injection and tablets, US prescribing information, Novo Nordisk, revised June 2026. Sections 11 Description, 12.1 Mechanism of Action, 12.2 Pharmacodynamics and 12.3 Pharmacokinetics.
This article is for information only and is not medical advice. Dosing, suitability and any change to treatment are decisions for your prescriber.