Semaglutide Mix Ratio: What the Evidence Says About Compounded vs Licensed Doses

Where the term "mix ratio" comes from, and what it signals

In countries where compounded semaglutide is more widely available, patients and providers sometimes calculate a mix ratio: typically how many milligrams of semaglutide base or salt to dissolve in a set volume of bacteriostatic water, producing a concentration that is then drawn up in a syringe. The arithmetic matters because getting the ratio wrong by even a small margin changes the effective dose. A solution that is twice as concentrated as intended delivers double the dose from the same drawn volume; too dilute, and the patient effectively under-doses.

In the UK, this calculation has no place in the routine use of authorised semaglutide. The MHRA-licensed Wegovy injection pen contains a precisely validated, ready-to-use formulation, the ratio has already been established during pharmaceutical development and quality-controlled in manufacture. If you have been directed toward a source that asks you to mix semaglutide yourself, that is a reliable signal you are looking at a product outside the licensed UK supply chain, and our guide to self mix semaglutide explains in detail why that practice sits outside the regulated pathway. Our guide on what not to mix with semaglutide covers related safety considerations worth reading alongside this page.

The MHRA has warned publicly about unlicensed semaglutide products (including compounded versions) being sold online, sometimes with instructions for self-preparation. The risks include dose inaccuracy, microbial contamination from improper aseptic technique, and entirely unknown impurities if the starting material comes from an unverified supplier.

What the evidence base actually measures, and which formulation it measured it in

The clinical results that made semaglutide a significant advance in weight management were produced in the STEP trials, using the licensed subcutaneous formulation at validated concentrations. The STEP 1 trial (68 weeks, adults with obesity or overweight and at least one weight-related condition, no diabetes) reported an average body-weight reduction of around 15% at the 2.4 mg weekly maintenance dose, compared with roughly 2.4% for placebo, as published in the New England Journal of Medicine. That figure is not transferable to a compounded product where the actual delivered dose is uncertain.

This is not a minor technicality. Semaglutide's dose-response relationship is well established: moving from 0.25 mg to 2.4 mg produces progressively greater appetite suppression and weight loss, but also a dose-dependent increase in gastrointestinal side effects. A poorly calculated mix ratio that overshoots the intended concentration could produce severe nausea, vomiting and dehydration from a dose the body was not titrated up to. One that undershoots could stall progress and lead a patient to wrongly assume the medicine is not working for them.

NICE's appraisal of semaglutide for weight management, published as TA875, is based entirely on evidence from the licensed formulation. Any clinical assessment of whether semaglutide is appropriate for you (including the benefit-risk judgement a prescriber makes) draws on that evidence base, not on data from compounded alternatives.

The licensed route in the UK: no mixing required

Wegovy arrives as a pre-filled pen. There is nothing to reconstitute. Each pen contains four weekly doses, and the pen itself does the measuring. The titration schedule (starting at 0.25 mg and escalating over roughly 16 weeks to the 2.4 mg maintenance dose, or now up to 7.2 mg following the MHRA's 2026 approval of that higher strength) is managed by a prescriber, not calculated by the patient at home with a syringe and a vial.

For people who prefer not to inject, the Wegovy tablet (approved by the MHRA in June 2026) is equally pre-dosed: each tablet contains a fixed quantity of semaglutide co-formulated with an absorption enhancer. Again, no ratio, no mixing. You can read more about what to expect with the injection version on our first day on Wegovy page, and a fuller overview of the medicine is on our Wegovy page.

Accessing either form legally in the UK requires a prescription following a clinical assessment. At nume, a GPhC-registered prescriber (a real clinician, not automated software) reads your consultation the same day. Once approved, your order is dispatched from our GPhC-registered pharmacy and arrives via DPD the next working day in plain, unbranded packaging, tracked to your door. That is the licensed supply chain: manufacturer, licensed wholesaler, regulated pharmacy, named prescriber, patient.

If you have been using or considering a compounded product

People sometimes arrive at compounded semaglutide because they cannot access the licensed product quickly enough, or because they have seen a lower apparent price. The cost context for licensed semaglutide is worth understanding clearly, and our Wegovy price comparison page sets out what market prices actually look like and what they include.

A prescriber's concern with compounded semaglutide is not about the molecule (semaglutide is semaglutide) but about whether the patient is actually receiving the dose they believe they are, in a sterile formulation, from a source whose quality can be verified. Our semaglutide overview explains how the medicine works and what a legitimate treatment pathway looks like. If you are currently using a compounded product and want to transition to the licensed version, a prescriber can discuss how that might work for your situation. The starting point is a free consultation, with no obligation to proceed.

If you have questions about the process or want to understand your options before starting, our FAQs cover the most common queries, and you can reach our clinical team directly through our contact page. Check your eligibility for licensed semaglutide treatment by starting a free consultation with our prescribers.