Semaglutide mixing charts: what they are and why the licensed UK product doesn't need one

Picture the scenario: you've seen a mixing chart and a vial for sale

You're researching semaglutide and you come across what looks like a practical guide: a table of volumes, concentrations and units to draw up into a syringe. Perhaps it comes with a small vial of white powder and a separate bottle of bacteriostatic water. If you want to understand what those numbers actually represent, our semaglutide units chart page explains how the figures are calculated and why getting them wrong carries real risk.

This is compounded semaglutide, a preparation mixed outside the licensed pharmaceutical supply chain. It circulates on social media, through online marketplaces, and occasionally through clinics operating in legal grey areas. The appeal is usually price. The problem is substantial. Compounded GLP-1 preparations are not licensed by the MHRA for use in the UK, meaning they have not passed the same manufacturing, purity or dosing-accuracy standards that a licensed medicine must meet. The MHRA's Yellow Card scheme exists partly so that patients can report adverse reactions from exactly these kinds of unlicensed products, and reports have been made.

A question our prescribers hear most weeks is some version of: "I've seen semaglutide much cheaper online, is it the same thing?" Almost always, the cheaper product is either a compounded vial or, more seriously, a counterfeit. Neither is the same thing as a licensed Wegovy pen.

What the licensed UK product actually requires you to do

Wegovy (the brand name for licensed semaglutide in the UK, made by Novo Nordisk) arrives as a pre-filled injection pen. Each pen contains a set dose. There is no powder to reconstitute, no bacteriostatic water to measure and no unit chart to consult. You inject once a week, rotating between the abdomen, thigh and upper arm.

The titration schedule is part of the medicine's UK licence and is managed by your prescriber, not worked out by the patient. Treatment begins at a low starting dose to allow your system to adjust, then steps upward at roughly four-week intervals under clinical supervision. The NHS medicines page for semaglutide covers what to expect at each stage. The prescriber sets the pace based on how you're tolerating the current dose, not a chart you've downloaded.

If you want to understand how the Wegovy schedule is structured in general terms, our Wegovy dose guide covers the licensed titration steps factually. Storage is straightforward too: pens are kept refrigerated until use, with a limited room-temperature window set out in the Patient Information Leaflet.

Why the distinction between vials and pens matters for safety

Dosing errors with self-mixed solutions are a documented risk. When a patient draws up their own syringe from a reconstituted vial, the concentration calculation has to be exactly right; a small arithmetic error produces either an underdose (no effect) or an overdose (serious GI distress, and in severe cases, hypoglycaemia-like symptoms). Pre-filled pens eliminate this variable entirely.

There is also the question of what is actually in an unlicensed vial. The MHRA has conducted large-scale seizures of fake and unlicensed weight-loss products sold online, some of which contained substances quite different from the label. Counterfeit injectable pens have been found to contain insulin, a medicine that, in someone without diabetes, can cause life-threatening hypoglycaemia. For anyone thinking about semaglutide for weight management, sourcing through a GPhC-registered pharmacy that dispenses only licensed medicines is the one check that rules out this risk entirely.

You can verify any online pharmacy on the GPhC pharmacy register in under a minute. It is worth doing before handing over payment details to any online provider.

The right frame for thinking about semaglutide treatment in the UK

If a mixing chart is what brought you here, you're probably at the beginning of researching weight-loss treatment, which means you still have every option open. The full overview of Wegovy as a licensed UK medicine is a useful place to build from. For questions about what treatment costs through a regulated UK service, our Wegovy pricing page sets out what a transparent price should include.

Eligibility for licensed semaglutide in the UK is based on BMI (at least 30, or from 27 with a qualifying weight-related condition) alongside a full clinical assessment, the prescriber looks at your medical history, current medicines and overall health picture, not just a single number. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

Treatment is a prescription-only medicine requiring clinical assessment each time. That assessment is what protects you: it checks for interactions, monitors progress and adjusts the plan if something isn't working. If you have come across larger vial sizes in your research, our page on 10ml semaglutide explains what those products are and why they fall outside the licensed UK supply chain. A free consultation with a GPhC-registered prescriber is the right starting point, and the only legal one.