Semaglutide injection ingredients: what's in the pen and why it matters

The misconception worth correcting first: it's not just the hormone

A common assumption is that a semaglutide injection is essentially a single molecule dissolved in water. The reality is more considered than that. The Wegovy pre-filled pen contains semaglutide as the active substance, but the formulation around it is carefully engineered to keep the molecule stable, adjust the solution's acidity to match the body's tissues, and ensure the liquid behaves predictably through the pen mechanism.

The excipients in Wegovy's injection solution are: disodium hydrogen phosphate dihydrate (a phosphate buffer that maintains pH stability), propylene glycol (a solubiliser that keeps the active molecule evenly distributed), phenol (a preservative, standard in multi-dose injectable formulations), hydrochloric acid and sodium hydroxide (used to adjust pH to the right level for injection), and water for injections as the solvent. None of these are unusual in pharmaceutical injectables. They are present in very small quantities and are not pharmacologically active at those concentrations. For the exact quantities, the Wegovy SmPC on the electronic Medicines Compendium gives the full specification, this is the right place to check if you have a documented sensitivity to any component.

What Wegovy does not contain: insulin, growth hormone, steroids, gluten or animal-derived proteins. These are misconceptions our prescribers encounter often enough to be worth stating plainly here.

What semaglutide itself actually is, and why the molecule is designed the way it is

Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist. The human body produces GLP-1 naturally after eating, it signals fullness to the brain, slows the rate at which the stomach empties, and plays a role in blood-sugar regulation. Natural GLP-1 is broken down within minutes by an enzyme called DPP-4.

Semaglutide is engineered to be resistant to that breakdown. It has a fatty-acid chain attached to its structure via a linker, which allows it to bind to albumin in the bloodstream and extend its half-life to roughly one week, the reason a single weekly injection is enough. This structural design is what gives the injection its once-weekly schedule, and it is also why the molecule cannot simply be swallowed in the same form (stomach acid degrades it). For anyone curious about oral alternatives to the injection, that trade-off is worth understanding.

The NHS semaglutide medicines page gives a plain-language summary of how the medicine works for patients who want a reliable starting point before or after reading the leaflet.

Why the excipients affect storage, and one practical detail worth knowing

Phenol, the preservative in the formulation, and the pH-sensitive buffer system both contribute to why Wegovy must be stored in a refrigerator at 2–8°C. The pen should not be frozen (freezing degrades both the active molecule and the buffer system), and it should be kept away from direct light. Once in use, there is a limited period during which the pen can be kept at room temperature, but the precise window depends on the specific product and pack, so this is one detail to read from your Patient Information Leaflet rather than from any secondary source. Many people keep their current pen in the fridge door, where it stays consistently cool but is also easy to spot on the day of their weekly injection. For a full guide on how to store a semaglutide injection correctly, including what to do if the pen has been left out, that page covers the specifics.

If you are comparing how Wegovy's formulation sits alongside other GLP-1 injection options, it is worth noting that Ozempic contains the same active molecule but is licensed for type 2 diabetes rather than weight management, a distinction the MHRA is clear about. The formulations are similar but the licences are not interchangeable.

Ingredient questions that come up at consultation, and when they matter clinically

For most people, the excipients in Wegovy present no specific concern. Phenol allergy is rare, and the quantities present are far below levels associated with sensitivity reactions. Propylene glycol is also generally well-tolerated. That said, if you have a history of hypersensitivity to any injectable medicine, or if you are on medicines that interact with phenol-preserved injectables, your prescriber needs to know.

People who are pregnant, breastfeeding or actively trying to conceive are not candidates for semaglutide treatment, the medicine is not licensed for use in these circumstances, and this applies regardless of the ingredient profile. Treatment is also not licensed for anyone under 18. These are clinical boundaries, not formulation ones.

If you are exploring semaglutide injection treatment more broadly (including who it suits and what the process of getting started looks like) that overview covers the clinical context without diving into formulation detail. For questions about injection technique, how to inject semaglutide and where to put the shot are both practical reads before your first dose. And if you want to understand the broader landscape of weight-loss treatments available in the UK, that page sets out the options clearly.

At nume, every consultation is read by a real GPhC-registered prescriber (not automated software) before any treatment is issued. If you have specific questions about the Wegovy ingredients in the context of your own health history, check your eligibility and start a free consultation to get a considered clinical view.