How Does Semaglutide Work?
Mechanism of Action Explained

What is GLP-1?

Glucagon-like peptide-1 (GLP-1) is a hormone naturally produced by L-cells in the small intestine after you eat. It is part of a group of hormones known as incretins, which play a central role in regulating appetite, blood sugar and digestion.

When you consume food, GLP-1 is released into the bloodstream and acts on receptors throughout the body. Its natural functions include:

• Stimulating insulin release from the pancreas in a glucose-dependent manner (it only increases insulin when blood sugar is elevated)
• Suppressing glucagon, a hormone that raises blood sugar levels
• Slowing gastric emptying, which prolongs feelings of fullness after eating
• Acting on the brain to reduce appetite and food intake

The problem with natural GLP-1 is that it is broken down extremely rapidly by an enzyme called dipeptidyl peptidase-4 (DPP-4). Natural GLP-1 has a half-life of just 1 to 2 minutes, making it impractical as a medication in its natural form.

From natural hormone to medication

Semaglutide was developed by Novo Nordisk to overcome the rapid breakdown of natural GLP-1. It retains 94 per cent structural similarity to human GLP-1 but includes three key modifications:

1. Amino acid substitution at position 8: A change from alanine to alpha-aminoisobutyric acid (Aib) makes semaglutide resistant to breakdown by DPP-4.
2. Fatty acid chain attachment: A C-18 fatty diacid chain is attached via a linker to position 26. This allows semaglutide to bind to albumin (a protein in the blood), which acts as a carrier and dramatically slows its elimination from the body.
3. Amino acid substitution at position 34: Lysine is replaced with arginine to ensure the fatty acid chain attaches only at position 26.

These modifications extend the half-life from approximately 2 minutes to about 7 days, enabling once-weekly dosing. After injection, semaglutide is slowly released from the albumin-bound pool, maintaining steady therapeutic levels throughout the week.

The brain: appetite regulation

The brain is central to how semaglutide produces weight loss. GLP-1 receptors are found in several brain regions involved in appetite regulation, particularly the hypothalamus and the brainstem (specifically the nucleus tractus solitarius and the area postrema).

Hypothalamic effects

The hypothalamus is the brain's primary appetite control centre. Within it, two opposing groups of neurones regulate hunger:

• POMC/CART neurones: These promote satiety (feeling full) and reduce food intake. Semaglutide activates these neurones.
• NPY/AgRP neurones: These stimulate appetite and food-seeking behaviour. Semaglutide inhibits these neurones.

By shifting the balance between these two systems, semaglutide reduces hunger signals and increases feelings of fullness. Patients typically report feeling satisfied with smaller portions and having less desire to eat between meals.

Reward centre effects

Functional brain imaging studies have shown that semaglutide also influences the mesolimbic reward system — the brain circuitry involved in food cravings and the pleasure derived from eating. Research published in Nature Medicine demonstrated that semaglutide reduces activity in reward centres when participants view images of palatable foods, particularly high-fat and high-sugar items.

This helps explain why many patients report not just reduced hunger, but a genuine decrease in food cravings and preoccupation with food — a change that can feel quite dramatic, especially for those who have struggled with overeating.

Brainstem effects

The area postrema and nucleus tractus solitarius in the brainstem are involved in nausea and satiety signalling. Semaglutide acts on GLP-1 receptors in these areas, which contributes to both its appetite-reducing effects and the nausea that some patients experience, particularly during dose escalation.

The stomach: gastric emptying

Semaglutide slows the rate at which food passes from the stomach into the small intestine, a process known as delayed gastric emptying or gastroparesis. This has several practical effects:

• Food remains in the stomach for longer, prolonging the sensation of fullness after eating
• Nutrient absorption is more gradual, leading to lower and more stable blood sugar levels after meals
• The slower emptying may contribute to nausea and bloating, particularly during the early weeks of treatment

Studies using paracetamol absorption tests (a standard method for measuring gastric emptying) have shown that semaglutide delays gastric emptying by approximately 1 to 3 hours. This effect is most pronounced in the first few months and may partially attenuate over time, which aligns with the observation that nausea tends to improve with continued treatment.

The pancreas: insulin and glucagon

Semaglutide acts on GLP-1 receptors on pancreatic beta cells to enhance insulin secretion. Crucially, this effect is glucose-dependent: semaglutide only stimulates insulin release when blood glucose levels are elevated. This means the risk of hypoglycaemia (dangerously low blood sugar) is very low when semaglutide is used alone.

Effects on insulin

• Enhances first-phase insulin secretion (the rapid initial release of insulin after eating)
• Improves second-phase insulin secretion (sustained insulin release during meals)
• May improve beta-cell function over time by reducing glucotoxicity and lipotoxicity

Effects on glucagon

Semaglutide suppresses the release of glucagon from pancreatic alpha cells. Glucagon normally raises blood sugar by stimulating glucose production in the liver. By reducing glucagon levels, semaglutide helps lower fasting blood sugar and reduces post-meal blood sugar spikes.

This dual effect on insulin and glucagon is why semaglutide (Ozempic) is so effective for managing type 2 diabetes. It improves blood sugar control through complementary mechanisms without the weight gain associated with many other diabetes medications.

Additional metabolic effects

Beyond appetite, digestion and blood sugar, semaglutide has broader metabolic effects that contribute to overall health improvements.

Cardiovascular effects

Semaglutide reduces blood pressure, improves lipid profiles and has anti-inflammatory effects. The SELECT trial demonstrated a 20 per cent reduction in major cardiovascular events, with benefits potentially beyond those attributable to weight loss alone.

Liver effects

Semaglutide reduces liver fat content and has shown promise in treating non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). Read more in our guide to GLP-1 and fatty liver disease.

Anti-inflammatory effects

Semaglutide reduces C-reactive protein (CRP) and other inflammatory markers. Chronic low-grade inflammation is implicated in many obesity-related conditions, and reducing inflammation may contribute to the broad health benefits observed with GLP-1 therapies.

How semaglutide compares to tirzepatide

While semaglutide targets only GLP-1 receptors, tirzepatide (Mounjaro) is a dual agonist that activates both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. GIP is another incretin hormone that plays complementary roles in appetite regulation and glucose metabolism.

For a full comparison, see our guide to Mounjaro vs Wegovy.

The dose-escalation process

Semaglutide is started at a low dose and gradually increased over several months. This approach minimises gastrointestinal side effects by allowing the body to adapt to the medication.

For Wegovy (weight management), the standard escalation schedule is:

1. Weeks 1–4: 0.25 mg once weekly
2. Weeks 5–8: 0.5 mg once weekly
3. Weeks 9–12: 1.0 mg once weekly
4. Weeks 13–16: 1.7 mg once weekly
5. Week 17 onwards: 2.4 mg once weekly (maintenance dose)

Each dose step allows GLP-1 receptors in the gut and brain to gradually adapt. The most common side effects — particularly nausea — tend to peak at each new dose level and then subside within 1 to 2 weeks.

What happens when you stop semaglutide

Because semaglutide's effects on appetite and metabolism are mediated by the drug itself rather than permanent changes, most patients experience a gradual return of appetite and weight regain after stopping treatment. The STEP 1 extension trial showed that participants regained approximately two-thirds of their lost weight within one year of discontinuation.

This is why current NICE guidance recommends that semaglutide be used as long-term treatment for obesity, much like medications for high blood pressure or high cholesterol. Read more about what happens in our article on stopping Ozempic.

Frequently asked questions