What Is Semaglutide? GLP-1 Drug Guide (Ozempic, Wegovy)

What Is Semaglutide?

Semaglutide is a GLP-1 receptor agonist: a type of medication that mimics glucagon-like peptide-1 (GLP-1), a hormone your gut naturally releases after you eat. It's used to treat type 2 diabetes and obesity, and it's the active ingredient in the brand-name drugs Ozempic, Rybelsus, and Wegovy. For a full overview of the semaglutide family, visit the semaglutide hub.

GLP-1 is a hormone your intestines produce in response to food. It tells your pancreas to release insulin, tells your liver to stop releasing sugar into the blood, slows down digestion, and signals your brain that you're full. In people with type 2 diabetes or obesity, these signals are often weaker than they should be.

Semaglutide amplifies all of those signals. It has a structure that is 94% identical to native human GLP-1, close enough to activate the same receptors, but with key modifications that make it far more durable. Natural GLP-1 breaks down in minutes; semaglutide lasts approximately seven days in your body, which is what makes once-weekly dosing possible.

The drug was developed by Novo Nordisk. It was first approved by the FDA in December 2017 for type 2 diabetes and has since been approved for chronic weight management and cardiovascular risk reduction. Its clinical trial programme spans the SUSTAIN (diabetes), STEP (obesity), and SELECT (cardiovascular) programmes, each enrolling thousands of patients.

Brand Names: Ozempic, Rybelsus, and Wegovy

Semaglutide comes in three distinct FDA-approved formulations. Each has different doses, delivery methods, and approved uses. The drug itself is the same; what differs is how it's formulated and what it's prescribed for.

Ozempic is prescribed for adults with type 2 diabetes to improve blood sugar control and, in patients with established cardiovascular disease, to reduce the risk of heart attack, stroke, or cardiovascular death. It's injected once a week using a pre-filled pen.

Rybelsus is the oral version. It uses a special formulation (with a compound called SNAC) that allows semaglutide to be absorbed through the stomach wall. The oral route means lower bioavailability, so doses are higher than the injectable forms. It's taken once daily on an empty stomach.

Wegovy uses the same molecule as Ozempic but at a higher dose (2.4 mg recommended maintenance, 7.2 mg maximum, vs. 2 mg) and is specifically approved for chronic weight management. It was approved for cardiovascular risk reduction in 2024, following results from the SELECT trial showing a 20% reduction in major heart events in obese patients without diabetes.

How Does Semaglutide Work?

Semaglutide works by binding to GLP-1 receptors, proteins found on cells throughout your body, including your pancreas, liver, gut, and brain. Activating these receptors triggers several effects at once.

Blood Sugar Control

When blood sugar rises after a meal, semaglutide signals the insulin-producing cells in your pancreas (called beta cells) to release insulin. Crucially, this signal only fires when blood sugar is actually elevated. At normal blood sugar levels, the signal is weak. This glucose-dependent mechanism is why semaglutide doesn't cause hypoglycaemia (dangerously low blood sugar) on its own, unlike insulin injections.

At the same time, semaglutide suppresses glucagon, a hormone released by your pancreas that tells your liver to dump stored sugar into the bloodstream. Less glucagon means less unnecessary glucose release between meals.

Appetite Suppression and Weight Loss

Semaglutide slows the rate at which your stomach empties food into your small intestine. This keeps you feeling full for longer after each meal and blunts the spike in blood sugar that normally follows eating. A 2021 pharmacodynamic study found significant gastric emptying delays at weight-management doses.

It also crosses the blood-brain barrier and activates GLP-1 receptors in the hypothalamus, the part of your brain that regulates hunger. This reduces appetite signals and lowers caloric intake independent of what's happening in your gut.

Slower digestion, reduced hunger signals, and lower blood sugar peaks together drive the significant weight loss seen in clinical trials. For a detailed breakdown of the signalling pathways, see our companion article How Does Semaglutide Work?

Why It Lasts a Week

Natural GLP-1 is broken down by an enzyme called DPP-4 within minutes of being released. Semaglutide was engineered to resist this. It has three modifications to its amino acid sequence (at positions 8, 26, and 34) that block DPP-4 from degrading it. It also has a fatty acid chain attached that lets it bind to albumin, a protein in the blood that acts as a molecular carrier. This keeps semaglutide circulating rather than being filtered out by the kidneys. The result is a half-life of approximately seven days. That stability allows for consistent, predictable weekly dosing.

What Is Semaglutide FDA-Approved For?

The FDA has approved semaglutide for three distinct clinical purposes, each tied to a specific brand and dose.

Type 2 Diabetes (Ozempic, Rybelsus)

Semaglutide is approved as an adjunct to diet and exercise for improving blood sugar control in adults with type 2 diabetes. It's not approved for type 1 diabetes. For Ozempic, it's also approved to reduce the risk of major adverse cardiovascular events (heart attack, stroke, or cardiovascular death) in adults with type 2 diabetes and established cardiovascular disease. Rybelsus received an expanded first-line indication in 2023.

Chronic Weight Management (Wegovy)

Wegovy is approved for long-term weight management in adults with an initial BMI of 30 kg/m² or greater (obesity), or 27 kg/m² or greater (overweight) with at least one weight-related condition such as high blood pressure, type 2 diabetes, or high cholesterol. It must be used alongside a reduced-calorie diet and increased physical activity.

In 2024, the FDA expanded Wegovy's label to include cardiovascular risk reduction following the SELECT trial results. Wegovy is now approved to reduce the risk of serious heart events in obese adults with established cardiovascular disease, regardless of diabetes status.

Who Semaglutide Is Not For

Semaglutide carries a black box warning (the FDA's most serious category). It should not be used by anyone with a personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia type 2 (MEN2). These are rare but serious conditions where thyroid tumour risk may be elevated. It's also not approved for type 1 diabetes or for people under 18 years old (for weight management).

What Do the Clinical Trials Show?

Semaglutide's approvals are backed by large, well-designed clinical trials. Here are the three most important ones.

STEP 1: Weight Loss in Obesity

The STEP 1 trial enrolled 1,961 adults with obesity (or overweight with a weight-related comorbidity) without diabetes. Participants received either semaglutide 2.4 mg weekly or a placebo for 68 weeks, alongside lifestyle intervention. The semaglutide group lost an average of 14.9% of their body weight (approximately 15.3 kg), compared to 2.4% in the placebo group. 86.4% of participants on semaglutide lost at least 5% of their body weight, and more than half lost 15% or more.

SUSTAIN-6: Cardiovascular Safety in Type 2 Diabetes

The SUSTAIN-6 trial followed 3,297 adults with type 2 diabetes over two years. It was initially designed to prove semaglutide was safe for the heart. Instead, semaglutide demonstrated superiority: it reduced the risk of major adverse cardiovascular events (MACE: heart attack, stroke, or cardiovascular death) with a hazard ratio of 0.74. This means semaglutide users had a 26% lower rate of these serious events than the placebo group, which became part of the Ozempic label.

SELECT: Cardiovascular Benefit in Obesity Without Diabetes

SELECT is the most recent landmark trial. It enrolled 17,604 adults who had obesity and established cardiovascular disease but no diabetes. Over a median follow-up of just over three years, Wegovy (2.4 mg weekly) reduced the risk of MACE by 20% compared to placebo (hazard ratio 0.80, p<0.001). This was the first trial to demonstrate that a weight-loss drug reduces serious heart events in people without diabetes. It formed the basis of Wegovy's cardiovascular label expansion in 2024.

What Are the Side Effects?

The most common side effects are gastrointestinal: nausea, vomiting, diarrhoea, and constipation. These are a class effect of all GLP-1 receptor agonists, and they're most pronounced during dose escalation. For most people, they improve significantly after the first few weeks at each dose level.

The slow, stepwise titration schedule (starting at 0.25 mg weekly for four weeks, then increasing every four weeks) is specifically designed to minimise GI side effects by letting your body adapt gradually. Starting at the full dose makes side effects much worse.

Serious side effects are less common but include pancreatitis (inflammation of the pancreas) and gallbladder disease. The black box warning covers medullary thyroid carcinoma risk. Semaglutide should not be used if you have a personal or family history of MTC or MEN2.

For a full breakdown of how common each side effect is, when it typically appears, and how to manage it, see the semaglutide side effects guide.

Semaglutide, Compounded Versions, and Dosing

Many people access semaglutide through compounding pharmacies rather than Ozempic or Wegovy pens. Compounded semaglutide uses the same peptide but comes as a lyophilised (freeze-dried) powder that you reconstitute with bacteriostatic water and inject using an insulin syringe.

The dosing process is the same (once weekly, subcutaneous injection), but the concentration of your vial determines how many units you draw. Getting this calculation right matters: a dosing error with a concentrated vial can mean taking several times your intended dose.

Our semaglutide dose calculator shows your projected drug levels over time, and our reconstitution calculator tells you exactly how many units to draw on an insulin syringe for your target dose. For a complete guide to compounded semaglutide covering sourcing, FDA regulatory status, and storage, see the compounded semaglutide guide.

The semaglutide dosing guide covers the full titration schedule in detail, including conservative protocols for people who are sensitive to side effects.