How Does Semaglutide Work? Mechanism, Half Life and What the Trials Showed

Q: How does semaglutide work?

It is a GLP-1 receptor agonist, a long acting copy of a hormone the gut releases after eating. It acts on appetite regulating areas of the brain so people eat less, raises insulin and lowers glucagon only when blood glucose is high, and delays early stomach emptying. Binding to albumin gives it a half-life of about one week.

Q: How does semaglutide work for weight loss?

Mainly by reducing how much people eat. The Wegovy label says semaglutide decreases calorie intake and that the effect is likely mediated by appetite. In a 20 week trial of the 2.4 mg dose, people ate 35% less at a free choice lunch than on placebo and reported less hunger and fewer food cravings.

Q: How long does it take for semaglutide to start working?

Drug levels peak 1 to 3 days after an injection and take 4 to 5 weeks to reach steady state at each dose, and the Wegovy dose climbs for 16 weeks before 2.4 mg. In trials that measured it, mean weight loss was about 10% by week 20, and 14.9% by week 68 in STEP 1.

Q: What is the half-life of semaglutide?

About one week. The label attributes this mainly to albumin binding, which lowers kidney clearance and protects the peptide from breakdown, plus resistance to the DPP-4 enzyme. The Wegovy label says semaglutide stays in the circulation for about 5 to 7 weeks after the last dose.

Q: Does semaglutide slow digestion?

The labels say it delays gastric emptying, mainly in the early phase after a meal. The effect looks modest at the weight loss dose: a 20 week study of 2.4 mg found no evidence of delayed gastric emptying by the paracetamol method, while calorie intake still fell by 35%.

Q: Is oral semaglutide the same as the injection?

Same molecule, different delivery. Tablets are taken daily on an empty stomach and only about 1 to 2% is absorbed, so doses are far higher in mg. Wegovy 25 mg tablets gave 13.6% weight loss at 64 weeks against 2.4% on placebo. Rybelsus and Ozempic tablets are approved for type 2 diabetes.

Q: Does semaglutide reduce heart attack and stroke risk?

In SELECT, 17,604 adults with heart disease and overweight or obesity but no diabetes took 2.4 mg weekly or placebo. Cardiovascular death, heart attack or stroke occurred in 6.5% against 8.0%, a hazard ratio of 0.80, which is a 20% lower risk. The label says the mechanism has not been established.

Q: What happens when you stop taking semaglutide?

Weight tends to come back. In the STEP 1 extension, people regained two thirds of their prior weight loss within a year of stopping, and most cardiometabolic improvements moved back towards baseline. In STEP 4, those switched to placebo regained 6.9% while those who continued lost a further 7.9%.

By the Peptide Bureau editorial desk Mechanism 11 min read Last updated 14 October 2026

Short answer. Semaglutide is a GLP-1 receptor agonist: it copies a hormone the gut releases after meals. It acts on brain areas that regulate appetite, so people eat less; it raises insulin and lowers glucagon only when blood glucose is high; and it delays early stomach emptying. It binds to albumin in the blood, giving a half-life of about one week, so it is taken once weekly.

The detail behind each part of that answer, the trial numbers, and how tablets differ from injections are below. Everything is taken from the prescribing information on DailyMed or from trial abstracts, linked in place and listed at the end. For the escalation schedule see the semaglutide dosage guide, and for tolerability see semaglutide side effects. None of it is medical advice.

What semaglutide does, where, and the evidence

Effect	Where	How it shows up in trials or the label	Source
Less appetite, fewer calories eaten	Brain areas that regulate appetite; in rats, the brainstem, septal nucleus and hypothalamus	Free choice lunch 35% smaller than on placebo at week 20 on 2.4 mg (1,736 against 2,676 kJ), with less hunger, more fullness and fewer cravings	Wegovy label 12.1, 12.2; Friedrichsen 2021; Gabery 2020
More insulin, only when glucose is high	Pancreas, beta cells	First and second phase insulin secretion increased; in type 2 diabetes the insulin secretion rate became similar to that of healthy people	Ozempic label 12.1, 12.2
Less glucagon	Pancreas, alpha cells	Fasting glucagon 8% lower, after-meal glucagon response 14 to 15% lower, 24 hour glucagon 12% lower than placebo	Ozempic label 12.2
Slower early stomach emptying	Stomach	Label: a delay of early after-meal gastric emptying. At 2.4 mg after 20 weeks, no evidence of delayed emptying by paracetamol absorption	Ozempic label 12.1, 12.2; Friedrichsen 2021
Weight loss, more fat than lean mass	Whole body	STEP 1: -14.9% against -2.4% on placebo at 68 weeks; label: greater fat mass loss than lean mass loss	STEP 1; Wegovy label 12.2
Fewer heart attacks, strokes and cardiovascular deaths	Heart and blood vessels	SELECT: 6.5% against 8.0% over a mean 39.8 months, hazard ratio 0.80; mechanism not established	SELECT; Wegovy label 12.1
Long action, once weekly	Blood, bound to albumin	Over 99% bound to albumin; half-life about 1 week; still in the circulation about 5 to 7 weeks after the last dose	Wegovy label 12.3; Ozempic label 12.1

What semaglutide is built from

Native GLP-1 is released by the gut in response to a meal and, in the words of a standard review, "is extremely rapidly metabolized and inactivated by the enzyme dipeptidyl peptidase IV even before the hormone has left the gut" (Holst 2007). As a hormone that suits a signal meant to report a meal and then switch off. As a drug it is useless.

Semaglutide was designed to fix that. It has two amino acid substitutions compared with human GLP-1, Aib at position 8 and Arg at position 34, and a fatty acid chain attached at lysine 26 that binds albumin (Lau 2015). The label gives it 94% sequence homology with human GLP-1 and describes the result: "The principal mechanism of protraction resulting in the long half-life of semaglutide is albumin binding, which results in decreased renal clearance and protection from metabolic degradation. Furthermore, semaglutide is stabilized against degradation by the DPP-4 enzyme" (Ozempic label 12.1).

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The appetite part, which drives the weight loss

The Wegovy label puts it plainly: "GLP-1 is a physiological regulator of appetite and caloric intake, and the GLP-1 receptor is present in several areas of the brain involved in appetite regulation." It adds that animal studies show semaglutide reaching and activating neurons in brain regions that regulate food intake, and that "Semaglutide decreases calorie intake. The effects are likely mediated by affecting appetite."

The rodent work behind that line found that semaglutide reached the brainstem, septal nucleus and hypothalamus but "did not cross the blood-brain barrier"; it got in through the circumventricular organs and sites next to the ventricles, and it changed food preference and reduced food intake (Gabery 2020). In people, the clearest measure is the 20 week study by Friedrichsen and colleagues: on 2.4 mg, ad libitum energy intake was 35% lower than on placebo, hunger and prospective food consumption fell, fullness and satiety rose, and food cravings were fewer and weaker (Friedrichsen 2021).

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Insulin and glucagon: why it lowers blood sugar

The Ozempic label states the glucose mechanism: semaglutide "stimulates insulin secretion and lowers glucagon secretion, both in a glucose-dependent manner. Thus, when blood glucose is high, insulin secretion is stimulated, and glucagon secretion is inhibited." During induced low blood sugar it did not blunt the body's normal glucagon response. Low blood sugar can still happen, and the labels say the risk is increased when semaglutide is combined with insulin or a sulfonylurea.

This is the reason the molecule was developed for type 2 diabetes first. It is not the main weight loss mechanism; the label ties weight loss to lower calorie intake.

Gastric emptying: real, but smaller than people think

Both semaglutide labels say it delays gastric emptying, and the Ozempic label is specific that this is "a delay of early postprandial gastric emptying" and "a minor delay in gastric emptying in the early postprandial phase". The same label notes that in a separate study "no apparent effect on the rate of gastric emptying was observed with semaglutide 2.4 mg." The 20 week Friedrichsen study found the same at the weight loss dose: no effect on early paracetamol absorption, and an 8% rise in the 5 hour measure that was not significant once body weight was accounted for. Calorie intake still fell by 35%, which is why appetite, not a full stomach, is the better explanation for the weight loss.

Why it is once weekly: the label's pharmacokinetics

Property	Injection	Tablet
Bioavailability	89%	About 1% to 2% (Wegovy and Ozempic tablets); 0.4% to 1% (Rybelsus)
Time to peak level	1 to 3 days after a dose	About 1 hour after a dose
Steady state	After 4 to 5 weeks of weekly dosing	After 4 to 5 weeks of daily dosing
Albumin binding	Over 99%	Over 99%
Half-life	About 1 week	About 1 week
Still in the circulation after the last dose	About 5 weeks (Ozempic label); about 5 to 7 weeks after 2.4 mg or 7.2 mg (Wegovy label)	About 5 weeks (Rybelsus and Ozempic tablets label); about 5 to 7 weeks after 25 mg (Wegovy label)

Sources: section 12.3 of the Wegovy, Ozempic injection and Rybelsus and Ozempic tablets labels. Exposure is similar in the abdomen, thigh and upper arm, so site choice is about comfort and rotation; the detail is in where to inject semaglutide, and the semaglutide dosage calculator converts a milligram dose into syringe units.

What STEP-1 and SELECT showed

STEP 1 (NEJM 2021): 1,961 adults with obesity, or overweight with a weight related condition, and no diabetes, 2.4 mg weekly or placebo plus lifestyle intervention for 68 weeks. "The mean change in body weight from baseline to week 68 was -14.9% in the semaglutide group as compared with -2.4% with placebo." Half the semaglutide group (50.5%) lost 15% or more, against 4.9% on placebo (Wilding 2021).

SELECT (NEJM 2023): 17,604 adults aged 45 or older with existing cardiovascular disease and a BMI of 27 or more, without diabetes. "A primary cardiovascular end-point event occurred in 569 of the 8803 patients (6.5%) in the semaglutide group and in 701 of the 8801 patients (8.0%) in the placebo group (hazard ratio, 0.80; 95% confidence interval, 0.72 to 0.90; P<0.001)." A hazard ratio of 0.80 is a 20% lower risk of cardiovascular death, heart attack or stroke (Lincoff 2023).

Two context points from the same sources. In people with type 2 diabetes the weight result is smaller: STEP 2 reported -9.6% on 2.4 mg against -3.4% on placebo at 68 weeks (Davies 2021). And in SELECT, adverse events led 16.6% of the semaglutide group to stop the drug permanently, against 8.2% on placebo.

Timeline: when the effects show up

When	What the label or trial reports	Source
1 to 3 days after an injection	Peak blood level	Wegovy label 12.3
Weeks 1 to 16	Wegovy dose escalation: 0.25 mg, 0.5 mg, 1 mg and 1.7 mg, 4 weeks each, then the maintenance dose from week 17; each dose takes 4 to 5 weeks to reach steady state	Wegovy label 2.2; Ozempic label 12.3
Week 20	Mean weight loss 10.6% after a 20 week run-in to 2.4 mg in STEP 4; 9.9% against 0.4% on placebo in a separate 20 week study	STEP 4; Friedrichsen 2021
Week 68	-14.9% against -2.4% on placebo	STEP 1
About 3 years	20% lower risk of cardiovascular death, heart attack or stroke over a mean 39.8 months	SELECT
After stopping	Present in the circulation about 5 to 7 weeks; two thirds of the lost weight regained within a year	Wegovy label 12.3; STEP 1 extension

None of these sources gives a week when weight loss begins for an individual; the early weeks are a dose escalation for tolerability, and the label says if a dose is not tolerated during escalation, escalation can be delayed by 4 weeks. Holding a step is covered in the dosage guide.

Oral vs injectable semaglutide

Product	How it is taken, per the label	Approved for	Weight result on the label
Wegovy injection	Once weekly; 0.25 mg start, 1.7 mg or 2.4 mg maintenance, up to 7.2 mg for weight in adults	Weight reduction (adults and age 12 and older), cardiovascular risk reduction, MASH with liver fibrosis	-14.9% against -2.4% at 68 weeks (2.4 mg)
Wegovy tablets	Once daily; 1.5 mg for days 1 to 30, 4 mg, 9 mg, then 25 mg from day 91	Weight reduction and cardiovascular risk reduction in adults	-13.6% against -2.4% at 64 weeks (25 mg)
Ozempic injection	Once weekly; 0.25 mg start, then 0.5 mg, 1 mg or 2 mg	Type 2 diabetes, cardiovascular and kidney risk reduction	Not a weight loss product
Rybelsus tablets	Once daily; 3 mg for 30 days, then 7 mg, then 14 mg if needed	Type 2 diabetes, cardiovascular risk reduction	Not a weight loss product
Ozempic tablets	Once daily; 1.5 mg for 30 days, then 4 mg, then 9 mg if needed	Type 2 diabetes, cardiovascular risk reduction	Not a weight loss product

Ozempic tablets are real and on DailyMed: they share one label with Rybelsus, and the label says the two "are not substitutable on a mg-to-mg basis." The tablets are co-formulated with SNAC, which helps absorption in the stomach, and they have to be taken on an empty stomach in the morning with up to 4 ounces of water, waiting at least 30 minutes before eating, drinking or taking other oral medicines. That is why the tablet doses are so much larger in milligrams: only about 1% to 2% gets absorbed. The Wegovy label predicts that, in people without type 2 diabetes, 25 mg daily gives blood levels comparable to the 2.4 mg weekly injection, with more variation from person to person. More on the tablets is in oral semaglutide.

What happens when you stop

In the STEP 1 extension, 327 participants were followed for a year after treatment ended. They had lost 17.3% on semaglutide by week 68, regained 11.6 percentage points by week 120, and ended 5.6% below their starting weight; "participants regained two-thirds of their prior weight loss", and most cardiometabolic improvements reverted towards baseline (Wilding 2022). STEP 4 tested it prospectively: after a 20 week run-in, people switched to placebo regained 6.9% over 48 weeks while those who continued lost a further 7.9% (Rubino 2021). The effect lasts while the drug is present. The head to head with the dual agonist is in semaglutide versus tirzepatide, and the wider category in weight loss peptides.

Compounded and research grade semaglutide

Everything above describes the branded products and the trials run with them. Compounded semaglutide and research grade vials share the molecule, so the mechanism is the same, but none of the label figures on dose, exposure or results was generated with them, and research grade vials are sold for research use only. The differences between the supply routes are in compounded semaglutide.

Key Takeaways

Semaglutide is a GLP-1 receptor agonist with 94% sequence homology to human GLP-1, made long acting by albumin binding and DPP-4 resistance.
Weight loss comes mainly from eating less: 35% lower intake at a free choice meal in a 20 week study.
Insulin rises and glucagon falls only when blood glucose is high.
The gastric emptying delay is mostly early after a meal and looked small at 2.4 mg.
STEP 1: -14.9% against -2.4% at 68 weeks. SELECT: 20% lower risk of cardiovascular death, heart attack or stroke.
Tablets (Wegovy, Rybelsus, Ozempic) are daily and absorb only 1% to 2%; Wegovy 25 mg tablets gave -13.6% at 64 weeks.

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