How Ozempic Works: GLP-1 Receptor Agonism, Blood Glucose Control, and Appetite Regulation

Glucagon-like peptide-1 (GLP-1) is an incretin hormone. Incretin hormones are released by the gut after you eat. Cells in the small intestine, called L-cells, release GLP-1 within minutes of a meal.

In a healthy body, GLP-1 helps keep blood glucose in check in several ways.

Natural GLP-1 actions:

• It tells the pancreas to release insulin, but only when blood glucose is high. Insulin comes from beta-cells in the pancreas. Because it works this way, the risk of low blood sugar (hypoglycaemia) stays low
• It lowers glucagon, a hormone made by alpha-cells in the pancreas. Glucagon tells the liver to release glucose, so less glucagon means less glucose from the liver
• It slows how fast the stomach empties. This means glucose from food enters the blood more slowly
• It acts on the hypothalamus, part of the brain, to make you feel full and eat less

The incretin defect in type 2 diabetes:

People with type 2 diabetes have a weaker incretin effect. When glucose is taken by mouth, it triggers GLP-1 release.

Studies show the insulin response to oral glucose is much smaller than the response to glucose given by drip, which skips the gut.

This weaker incretin effect is one reason blood glucose rises too high after meals.

Limitations of native GLP-1:

Natural GLP-1 does not last long in the blood. Its half-life is only about 2 minutes, because an enzyme called dipeptidyl peptidase-4 (DPP-4) breaks it down quickly.

This makes natural GLP-1 useless as a medicine. Semaglutide was designed to resist DPP-4 and to stick to albumin, a blood protein. This stretches its half-life to about 7 days.

Semaglutide is a lab-made copy of GLP-1, called a GLP-1 analogue. It shares 94% of its structure with the human GLP-1 found in the body. Three changes to the molecule make it work as a medicine.

Structural modifications:

• Amino acid swap at position 8: alanine is swapped for alpha-aminoisobutyric acid. This stops DPP-4 from cutting the molecule apart
• C-18 fatty diacid chain: this chain is added through a linker at position 26 (lysine). It lets the molecule cling loosely to albumin in the blood. This is what makes it last so much longer
• Amino acid swap at position 34: lysine is swapped for arginine. This stops the fatty chain from attaching in the wrong place

Pharmacokinetic profile (how the body handles the drug):

• Half-life: about 7 days (165 hours). This is why you only inject it once a week
• Time to peak level in the blood (Tmax): 1 to 3 days after the injection under the skin
• Steady levels in the blood are reached after 4 to 5 weeks of weekly doses
• How it is cleared: the body breaks semaglutide down by cutting the protein and burning off the fatty chain. The kidneys and liver are not the main route. This is why you do not need to change the dose if your kidneys or liver work poorly
• Bioavailability under the skin: about 89%. This is how much of the dose reaches the blood

The long half-life has a knock-on effect. After you stop Ozempic, semaglutide stays in the blood for about 5 weeks. That is 5 half-lives, the time it takes to clear almost fully.

This matters when you manage side effects or plan a pregnancy.

In type 2 diabetes, semaglutide mainly works on the pancreas. More precisely, it acts on the islet, the cluster of hormone cells.

It works on both beta-cells and alpha-cells to bring blood glucose back into balance.

Beta-cell effects (insulin release):

• Semaglutide binds to GLP-1 receptors on beta-cells. This switches on a signal called cyclic AMP, which helps the cells release more insulin when glucose is high
• The glucose-dependent part matters in the clinic. Insulin is released only when blood glucose is above about 4 to 5 mmol/L. Below that level, semaglutide does not push out more insulin. This is why, used on its own, it rarely causes low blood sugar
• The first-phase insulin response is the quick burst of insulin right after eating. Semaglutide brings part of it back. This burst is usually weak in type 2 diabetes
• Beta-cell function, measured by a score called HOMA-B, gets better with treatment. Animal studies hint that it may protect beta-cells from dying, but this is not yet proven in people

Alpha-cell effects (lowering glucagon):

• When blood glucose is high, semaglutide lowers the glucagon that should not be there. Glucagon normally tells the liver to make glucose, so lowering it cuts both fasting and after-meal glucose
• During low blood sugar, glucagon is left alone. This keeps the body's safety response working. That is a key safety edge over sulfonylureas and insulin

Net glycaemic impact:

In the SUSTAIN trials, Ozempic 1 mg lowered HbA1c by 16 to 18 mmol/mol (1.5 to 1.8%). A typical starting point was 64 mmol/mol (8.0%). Fasting plasma glucose dropped by about 2.0 mmol/L.

Semaglutide helps with weight loss mainly by acting on the brain. It works on the hypothalamus and on brainstem centres. These are the parts of the brain that control appetite and energy.

Hypothalamic signalling:

• GLP-1 receptors sit in two parts of the hypothalamus: the arcuate nucleus and the paraventricular nucleus
• Semaglutide switches on POMC neurons (pro-opiomelanocortin neurons). These make alpha-melanocyte-stimulating hormone, which strongly curbs appetite
• At the same time, it quietens NPY and AgRP neurons (neuropeptide Y and agouti-related peptide). These neurons drive hunger
• The result is less hunger, feeling full sooner, and less interest in food

Brainstem effects:

• GLP-1 receptors in two brainstem areas, the area postrema and the nucleus tractus solitarius, affect both nausea and fullness. This overlap explains why nausea is the most common side effect. The pathways that curb appetite and the ones that cause nausea share the same receptors
• Semaglutide also changes signals from the vagus nerve. This adds to the slowing of the stomach

Reward pathway modification:

• Brain scans (functional MRI) of people on semaglutide show less activity in the insula and putamen when they look at high-calorie foods. These brain regions are tied to food reward
• People report fewer cravings, above all for fatty and sugary foods
• Some report less interest in alcohol. This is not an approved use, and trial data on it are limited

These brain effects explain why semaglutide leads to more weight loss than its effects outside the brain alone would suggest.

Semaglutide protects the heart by more than just lowering glucose and weight. This extra benefit is one of the main reasons NICE NG 28 rates GLP-1 agonists well.

SUSTAIN 6 cardiovascular outcomes trial:

• 3,297 people with type 2 diabetes at high risk of heart problems
• Median follow-up: 2.1 years
• Primary endpoint (MACE: death from heart causes, non-fatal heart attack, non-fatal stroke): semaglutide cut the relative risk by 26% versus placebo (HR 0.74, 95% CI 0.58 to 0.95)
• Most of this came from a 39% drop in non-fatal stroke
• Death from heart causes was lower in number, but on its own this did not reach statistical significance

SELECT trial (semaglutide 2.4 mg in obesity without diabetes):

• 17,604 people with a BMI of 27 or more and known heart disease, but no diabetes
• A 20% drop in MACE events (HR 0.80, 95% CI 0.72 to 0.90)
• This was the first time a GLP-1 agonist showed heart benefit in people without diabetes

Proposed mechanisms of cardiovascular protection:

• It calms inflammation. Semaglutide lowers C-reactive protein and other markers of inflammation
• It improves the lipid profile, with small drops in LDL cholesterol and triglycerides
• It lowers blood pressure by about 2 to 5 mmHg systolic, on top of any weight loss
• It may act directly on blood vessels. GLP-1 receptors on the cells lining vessels may help them work better
• It cuts visceral fat, the fat around the organs, which is closely linked to heart risk

It helps to see how Ozempic compares with other glucose-lowering medicines. This puts its mechanism and its role into context.

Versus metformin:

Metformin mainly cuts the glucose made by the liver and helps the body respond better to insulin. It does not touch the incretin system, and it causes little weight loss (1 to 2 kg).

Ozempic and metformin work on different pathways, so they are often used together. NICE NG 28 advises metformin first, with GLP-1 agonists as an option when you need more control.

Versus sulfonylureas (gliclazide, glimepiride):

Sulfonylureas push out insulin no matter what the blood glucose is. This glucose-independent effect brings a real risk of low blood sugar. Ozempic only acts when glucose is high, so it avoids this.

Sulfonylureas also lead to weight gain.

Versus DPP-4 inhibitors (sitagliptin, linagliptin):

DPP-4 inhibitors stop the body's own GLP-1 from breaking down. This raises GLP-1 levels by a small amount (2 to 3 fold).

Ozempic switches on far more GLP-1 receptors, because it is given at medicine-level doses. This is why GLP-1 agonists lower HbA1c and weight more.

Versus SGLT2 inhibitors (dapagliflozin, empagliflozin):

SGLT2 inhibitors block the kidney from taking glucose back, so glucose leaves in the urine (glycosuria). They add their own benefits: they protect the heart in heart failure and protect the kidneys.

NICE supports using GLP-1 agonists together with SGLT2 inhibitors in people with heart or kidney disease.

Versus insulin:

Insulin lowers glucose the most, but it causes weight gain and low blood sugar. Ozempic can delay or reduce how much insulin you need.

The SUSTAIN 5 trial showed that adding semaglutide to basal insulin lowered HbA1c by 13 mmol/mol and body weight by 3.7 kg.