Semaglutide is a lab-made peptide, a short chain of 31 amino acids designed to mimic a natural gut hormone called GLP-1. The active ingredient is the same whether you take it as an injection (Ozempic, Wegovy) or a tablet (Rybelsus), but the inactive ingredients surrounding it differ significantly between the two forms. Here’s a closer look at what’s actually in each formulation and why those ingredients matter.
The Active Ingredient: A Modified Gut Hormone
Your body naturally produces GLP-1 (glucagon-like peptide-1) after you eat. It signals your pancreas to release insulin, slows digestion, and tells your brain you’re full. The problem is that natural GLP-1 breaks down in minutes, far too fast to be useful as a medication.
Semaglutide solves this by taking the basic structure of human GLP-1 and modifying it in two key ways. First, one amino acid in the chain is swapped out, making the molecule harder for enzymes to chop apart. Second, a long fatty acid chain is attached to the peptide through a chemical linker at position 26 in the amino acid sequence. This fatty acid chain latches onto a protein in your blood called albumin, which acts like a slow-release shuttle, carrying semaglutide through your bloodstream for days instead of minutes. The result is a half-life of about 7 days, meaning it takes roughly 4 to 5 weeks of weekly dosing before levels stabilize in your body.
Despite these modifications, semaglutide retains 94% structural similarity to natural human GLP-1. That’s close enough to activate the same receptors but different enough to last roughly 1,000 times longer.
How Semaglutide Is Made
Semaglutide isn’t synthesized purely in a chemistry lab. It starts with living yeast cells, specifically a strain of baker’s yeast called Saccharomyces cerevisiae, engineered through recombinant DNA technology to produce a precursor version of the peptide. The yeast cells are grown in large fermentation tanks, where they churn out this precursor protein.
After fermentation, the precursor is isolated and purified, then chemically modified. This is the step where the fatty acid chain and its linker are attached to the peptide backbone. Finally, a two-amino-acid segment is joined to the front end of the chain to complete the finished molecule. This combination of biological production and chemical modification is part of what makes semaglutide difficult to replicate, and a key reason compounding pharmacies have faced scrutiny over their versions.
Inactive Ingredients in the Injection
The injectable form, sold as Ozempic for diabetes and Wegovy for weight management, is a clear liquid solution. Each milliliter contains semaglutide plus a short list of supporting ingredients:
- Disodium phosphate dihydrate (1.42 mg per mL): A buffering agent that keeps the solution at a stable pH of about 7.4, close to the pH of your blood.
- Propylene glycol (14 mg per mL): A solvent that helps keep semaglutide evenly dissolved in the solution.
- Phenol (5.5 mg per mL): A preservative that prevents bacterial growth. This is common in multi-dose injection pens.
- Water for injections: The base of the solution.
Small amounts of hydrochloric acid or sodium hydroxide may also be added during manufacturing to fine-tune the pH. Ozempic pens come in concentrations of 1.34 mg/mL or 2.68 mg/mL depending on the prescribed dose. Wegovy pens are single-dose and come in five strengths ranging from 0.25 mg to 2.4 mg, matching the gradual dose escalation schedule most people follow.
Inactive Ingredients in the Tablet
The oral form, Rybelsus, faces a unique challenge: stomach acid and digestive enzymes would normally destroy a peptide before it could be absorbed. To get around this, the tablet contains an absorption enhancer called salcaprozate sodium, often referred to as SNAC. This compound creates a localized protective environment around the semaglutide in your stomach, raising the pH near the stomach lining and helping the peptide cross into the bloodstream intact. Absorption happens primarily in the stomach, not the intestines.
The rest of the tablet is straightforward: microcrystalline cellulose (a plant-based filler that gives the tablet its structure), povidone (a binder that holds everything together), and magnesium stearate (a lubricant used during manufacturing). Rybelsus comes in 3 mg, 7 mg, and 14 mg tablets. The requirement to take it on an empty stomach with no more than 4 ounces of water exists because food and excess liquid interfere with SNAC’s ability to protect the semaglutide during absorption.
What Semaglutide Does in the Body
Once semaglutide reaches your bloodstream, it activates GLP-1 receptors in multiple organs. In the pancreas, it triggers insulin release, but only when blood sugar is elevated. This glucose-dependent mechanism is why semaglutide carries a lower risk of dangerous blood sugar drops compared to some older diabetes medications. It also suppresses glucagon, a hormone that tells your liver to release stored sugar, and promotes the growth of insulin-producing beta cells in the pancreas.
In the digestive system, semaglutide slows gastric emptying, meaning food stays in your stomach longer. This is a major reason people feel full faster and sometimes experience nausea, especially during the first weeks of treatment.
In the brain, semaglutide acts on GLP-1 receptors in the hypothalamus, the region that regulates hunger and satiety. This reduces appetite, decreases food cravings, and amplifies feelings of fullness. The combination of slower digestion and central appetite suppression is what drives the significant weight loss seen in clinical trials, even in people without diabetes.
Compounded Versions Differ
Compounding pharmacies have marketed their own semaglutide products, often as semaglutide sodium salt or semaglutide acetate. These are not the same as the finished drug substance in FDA-approved products. The approved manufacturing process, from yeast fermentation through precise chemical modification, produces a specific molecular form with a defined purity profile. Compounded versions may use different salt forms, lack the fatty acid attachment step, or contain different inactive ingredients. The multi-step biological and chemical process required to produce pharmaceutical-grade semaglutide is one reason the FDA has raised concerns about compounded alternatives.