What 25 Units of Tirzepatide Refers To — and Why It Matters

Why 'units' and milligrams are not interchangeable for tirzepatide

The confusion is understandable. For decades, 'units' has been the standard language of injectable medicines in the UK, largely because insulin is prescribed and drawn in international units (IU) using marked syringes. People arriving at tirzepatide from a diabetes background (or from a household where insulin is used) naturally carry that language across.

But Mounjaro works entirely differently at the device level. The KwikPen is pre-filled and pre-set: you dial the pen, press the button, and the mechanism delivers the full dose in a fixed volume. There is no syringe to draw, no conversion to perform, no units to count. The milligram figure printed on the pen label is the complete and only measurement a patient needs to know, and even then, the prescriber sets it.

The MHRA-approved Mounjaro SmPC on the eMC makes no reference to unit measurements for tirzepatide dosing. Volume per injection varies by concentration across the different-strength pens, but patients are never expected to calculate or measure it. If you have seen a number described as '25 units of tirzepatide' on a forum or social media, it most likely reflects a mistaken translation from an insulin context, or refers to an unlicensed compounded preparation that does not exist in the UK licensed supply chain. For more on how tirzepatide units and milligrams relate, that page goes into the arithmetic in detail.

What the licensed dose schedule actually looks like, and what the trial data tell us

NICE's appraisal of tirzepatide, TA1026, drew heavily on the SURMOUNT-1 trial, which tested doses up to 15 mg in 2,539 adults with obesity. The trial reported average body-weight reductions of around 20–21% at the 15 mg dose over 72 weeks, results that informed NICE's December 2024 recommendation.

Treatment starts at 2.5 mg for the first four weeks. The starting dose exists to allow the body to adjust (not to deliver weight loss) and is then increased in steps under prescriber supervision, typically every four weeks, through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. The pace is clinical, not fixed: if side effects are significant at any rung, the prescriber may hold the dose there longer. Some patients reach their therapeutic dose at 5 mg or 7.5 mg; others titrate all the way to 15 mg.

None of those steps correspond to a unit figure. A dose of '25 units' maps to no licensed Mounjaro strength, and our tirzepatide 20 units page explains why similar figures circulating online do not correspond to anything in the licensed dose schedule. If you want to understand how unit conversions are sometimes applied to tirzepatide, the context there explains the arithmetic, but the clinical takeaway is the same: the milligram label on your pen is the number that counts, and changing or adjusting it is a prescriber decision, not a self-managed calculation.

The safety reason this distinction cannot be waved away

This is not pedantry. Insulin syringes calibrated in units have been used (sometimes fatally) to draw and inject GLP-1 medicines in healthcare settings and, more worryingly, with counterfeit pens sourced outside the licensed supply chain. The MHRA has issued warnings about fake weight-loss pens, including cases where counterfeit devices were found to contain insulin rather than the labelled GLP-1 medicine. Injecting insulin in place of tirzepatide can cause dangerous hypoglycaemia.

The licensed supply chain matters for exactly this reason. Medicines dispensed through a GPhC-registered pharmacy arrive in the manufacturer's original sealed pen, with the strength clearly printed, alongside the full Patient Information Leaflet. There is no measuring, no drawing, no conversion. If you have a question about storage, injection technique, or what to do after a missed dose, the leaflet and your prescriber are the right places, not an approximation in units lifted from an insulin chart.

For context on what private tirzepatide treatment typically costs through a regulated UK pharmacy, the Mounjaro cost page covers how pricing is structured and what a transparent price should include.

What to do if you have a question about your specific dose

Prescribers set doses based on the whole clinical picture: starting weight, tolerance, comorbidities, and response at each stage. No figure (whether in milligrams or the 'units' language carried over from insulin) should be self-adjusted.

If you are already on tirzepatide and have a question about your pen, your dose, or a side effect you have noticed, the right first call is your prescribing team. The general FAQs at nume cover many common practical questions. For anything clinical, the contact page connects you with our aftercare team, available seven days a week.

If you are exploring whether tirzepatide could be appropriate for you and want to understand how Mounjaro works more broadly, or how it sits within the wider range of weight-loss treatment options, those pages lay out the evidence and eligibility picture clearly. Our prescribers review every consultation personally, speak to them when you are ready to take the next step.