What 40 Units of Semaglutide Actually Means

Where the "40 units" figure comes from — and what the evidence actually says

Clinical trials for semaglutide, including the landmark STEP 1 study published in the New England Journal of Medicine, dosed participants using milligrams delivered via pre-filled injection pens. The terminology of "units" does not appear in that body of evidence. It enters the conversation through a parallel world: compounded semaglutide preparations, often drawn into insulin syringes, where the barrel is marked in units rather than milligrams.

On a standard U-100 insulin syringe, 40 units equals 0.4 ml of liquid. What that 0.4 ml contains in milligrams depends entirely on the concentration of the vial, and concentrations vary between products and suppliers. At 1 mg/ml, 40 units delivers 0.4 mg. At 2.5 mg/ml, it delivers 1 mg. The same syringe marking can therefore mean very different doses depending on what is in the vial. That ambiguity is precisely why licensed semaglutide products moved to fixed-dose pens with no unit markings at all.

If you have seen references to converting semaglutide between ml and units, that page explains the arithmetic in full. The short answer here: 40 units of semaglutide has no single milligram equivalent without knowing the concentration, which is one of many reasons why unlicensed preparations are difficult to dose accurately.

How licensed Wegovy dosing actually works in the UK

The Wegovy pen removes the unit question entirely. Each pen is pre-filled and pre-set: you dial to the dose your prescriber has specified, inject, and the pen delivers that exact milligram amount. No drawing up, no unit arithmetic, no concentration to check. The licensed schedule in the UK begins at 0.25 mg weekly and is increased in steps by the prescriber, typically every four weeks, up to a standard maintenance dose of 2.4 mg. Following the MHRA's approval of a higher-dose single-inject pen in April 2026, a 7.2 mg maintenance option is also now available for eligible patients.

The NHS patient-level guidance for semaglutide (available on the NHS medicines page for semaglutide) describes this titration pathway clearly and sets out what to do if a dose is missed. That page is worth bookmarking; it is written for patients rather than clinicians and gives practical storage advice too. One detail worth noting: the pen lives in the fridge between doses, kept at 2–8 °C, so a slot in the fridge door works well for most people.

The reason for the gradual titration is tolerability. The gastrointestinal side effects most commonly associated with semaglutide (nausea, loose stools, indigestion) tend to be most pronounced at the start of treatment or after a dose increase. Stepping up slowly gives the body time to adjust. A prescriber, not a unit calculator, manages that process.

The safety case against unlicensed preparations and self-dosed "units"

The appeal of compounded semaglutide is almost always price. But the risks attached to preparations sold without a licensed product's quality controls are not theoretical. The MHRA has issued repeated public warnings about fake and unlicensed weight-loss injectables, and enforcement through 2025 included seizures of approximately 20 million doses of illegally traded weight-loss medicines worth around £45 million. Some seized products contained substances entirely different from what was on the label.

Dosing by units from a vial of unknown provenance compounds that risk. An error in concentration, or a mislabelled vial, means 40 units could deliver a fraction of the intended dose or a dangerous excess. Neither outcome is safe. For context on how private pricing for licensed products compares across providers, the Wegovy price comparison page sets out what a legitimate private prescription typically covers.

If you are currently using a compounded preparation and want to move to a licensed product, or if you are trying to work out what your current dose equates to in milligrams, that is a conversation for a prescriber rather than a conversion table. The semaglutide overview covers how the licensed medicine works in practice, and the team at nume includes GPhC-registered prescribers who review consultations the same working day.

Who licensed semaglutide is approved for in practice

Whether 40 units or any other dose of semaglutide is appropriate for a given person is a clinical question, not a mathematical one. The licensed product, Wegovy, is approved for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, raised cholesterol, or type 2 diabetes. Lower thresholds apply for some ethnic backgrounds under UK guidance.

Semaglutide is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. It is not licensed for people under 18. Certain conditions (including a personal or family history of medullary thyroid carcinoma) require specific prescriber discussion before starting. If you are trying to map a compounded dose back to licensed equivalents, our guide to what 15 units of semaglutide means in practice may also help illustrate how the same unit figure can represent different milligram amounts depending on concentration, and our page on what 5 units of semaglutide represents shows how even a small syringe marking carries the same concentration-dependent uncertainty, though the starting point is always a clinical review.

If you want to explore licensed semaglutide treatment through a regulated UK pharmacy, check your eligibility with our prescribers, it is free, takes a few minutes, and the clinical review happens the same day.