What's in Wegovy — the active ingredient, excipients and how it all fits together

The myth worth correcting first: Wegovy isn't a new or exotic compound

A lot of people assume Wegovy must contain some novel, barely-understood chemical that's only just been invented. It hasn't. Semaglutide is a close structural relative of GLP-1 (glucagon-like peptide-1), a hormone your small intestine releases naturally after a meal. Scientists modified the molecule to slow its breakdown in the bloodstream, giving it a half-life long enough for once-weekly dosing. The underlying biology it works with has been studied for decades.

What semaglutide does is bind to GLP-1 receptors in the brain, gut and pancreas. In the hypothalamus it dials down appetite signals. In the stomach it slows the rate at which food moves through, so you feel full on less. In the pancreas it helps regulate insulin in a glucose-dependent way. None of this is alien to the body, it's an amplified version of something that happens every time you eat. The NHS medicines page for semaglutide explains the mechanism in plain language and is worth reading alongside the patient leaflet in your pen.

Understanding this also explains why Wegovy works gradually: the titration schedule gives your system time to adjust rather than flooding those receptors from day one. It's a considered design, not a limitation.

What the inactive ingredients in Wegovy are actually for

Beyond semaglutide itself, Wegovy's solution for injection contains a small list of excipients. These aren't filler, each one has a specific function. Disodium phosphate dihydrate and propylene glycol act as stabilisers, keeping the pH of the solution in the narrow range where semaglutide remains stable and comfortable to inject. Phenol is included as a preservative, which matters because each pen holds four weekly doses drawn from the same cartridge. Water for injection makes up the remainder of the solution.

None of these excipients are unusual in injectable medicines. If you have a known allergy to any of them, that's something to raise during a clinical assessment before treatment starts, it's exactly the kind of detail a prescriber needs to know. The full excipient list, including quantities, is set out in the Wegovy SmPC on the electronic Medicines Compendium (eMC), which is the authoritative reference for this level of detail.

If you want to understand more about what semaglutide actually is, including how it compares to other molecules in its class, there's a dedicated page that covers the pharmacology in more depth.

Wegovy vs Ozempic: same molecule, different product

This is where confusion is most common. Ozempic and Wegovy both contain semaglutide, so in that narrow sense they are chemically identical. The differences lie in the licensed doses and what each medicine is approved to treat. Ozempic is licensed in the UK for type 2 diabetes management, at doses up to 1mg. Wegovy is licensed specifically for weight management, at doses up to 2.4mg, and from January 2026, a 7.2mg maintenance dose was also approved by the MHRA for eligible adults with obesity.

Using Ozempic for weight loss is outside its licence, and any prescriber doing so takes on significant professional and clinical responsibility. The distinction isn't bureaucratic pedantry, the clinical trials, safety data and dose ranges behind each product are specific to their licensed use. Semaglutide goes by several brand names, and knowing which is which matters when you're making a decision about treatment.

If you're weighing up Wegovy against other weight-loss medicines and want a broader comparison, the weight-loss treatment overview covers the licensed options available in the UK.

Storage and formulation: what the solution's physical form means in practice

Wegovy is supplied as a clear, colourless solution in a pre-filled, multi-dose pen. Each pen is designed for four once-weekly injections from the same device. The solution must be kept refrigerated at 2–8°C until opened; after that, the in-use storage window and conditions are set out in the patient information leaflet, and the leaflet's instructions take precedence over anything you read elsewhere, including here. Room-temperature stability is time-limited in a way that matters if you're travelling, worth planning ahead if you're going away over a bank holiday or collecting your pen the Friday before a long weekend.

The injection sites are the abdomen, thigh or upper arm, rotated week to week to avoid tissue reactions at any single spot. The pen mechanism is designed for self-injection and doesn't require the user to handle a needle directly in the traditional sense. For more on the practical side of when to take Wegovy, including timing relative to meals and other medicines, there's a separate page that goes into the specifics. If you're also curious about how semaglutide stacks up against newer molecules, the comparison between semaglutide and retatrutide is worth a read before making any decisions about treatment.

At nume, every prescription is reviewed by a GPhC-registered Independent Prescriber before it's issued, a real clinician reads your consultation, not software. If Wegovy is clinically suitable, the pen is dispensed from our GPhC-registered pharmacy (registration number 9012878, verifiable at the GPhC register) and delivered free to your door the next working day.