Using a 2.5mg Mounjaro pen to reach 3.75mg: what the dose schedule actually shows

What the clinical trial programme established about Mounjaro's starting dose

Tirzepatide's dose structure was shaped directly by the SURMOUNT-1 trial, published in the New England Journal of Medicine, which tested participants at 5mg, 10mg and 15mg maintenance doses over 72 weeks. The starting point in practice (2.5mg) was not a therapeutic target in its own right; it was calibrated to reduce the nausea and gastrointestinal discomfort that tend to peak when a new medicine like this begins slowing gastric emptying. Every strength in the licensed ladder exists because regulators and trial designers chose fixed, whole-dose increments that could be manufactured into pre-filled, single-dose pens. That engineering decision is not incidental. It means the pen physically cannot deliver a fractional amount between two steps. The first pen's job, in short, is to settle your system, not to produce significant weight loss on its own.

The NHS tirzepatide medicines page confirms the approved UK strengths as 2.5, 5, 7.5, 10, 12.5 and 15mg. There is no 3.75mg listed because no such pen exists in the licensed UK supply chain.

Where the 3.75mg figure comes from, and why it misleads

The number 3.75 occasionally surfaces in online discussions for two reasons, neither of which applies to Mounjaro in the UK. First, some compounded or unlicensed preparations sold abroad (or, in some cases, illicitly within the UK) use multi-dose vials that patients draw up themselves, making arbitrary doses possible. These are not the same product, and the MHRA has warned repeatedly about the risks of medicines obtained outside the regulated pharmacy supply chain. Second, 3.75mg appears in older literature about liraglutide (Saxenda), which uses a different escalation schedule entirely.

Neither source has any bearing on what a 2.5mg Mounjaro KwikPen should or can do. If a seller is offering tirzepatide at doses that do not match the licensed UK strengths, that is a red flag about the product's legitimacy, not evidence of a clinical innovation. Genuine Mounjaro pens, dispensed by a GPhC-registered pharmacy through the licensed supply chain, come in six fixed strengths and are incapable of delivering anything in between. Worth checking the GPhC register before you buy from any online source.

What actually happens between the 2.5mg and 5mg steps

For most people on Mounjaro, the 2.5mg period lasts four weeks. During that time the body adapts to the medicine's effect on gastric motility and appetite signalling. Clinically, prescribers are looking for two things: that you have tolerated the starting dose reasonably well, and that nothing in your response suggests the dose should not be increased. Some people stay at 2.5mg for longer if gastrointestinal side effects are significant, that is a clinical judgement, not a dose split.

If you are wondering what results to expect at the starter dose, our guide to 2.5mg Mounjaro weight-loss results covers the evidence honestly, including why the 2.5mg phase is not where the headline trial figures come from. The move from 2.5mg to 5mg is the first step that begins to approach therapeutic effect, and if you have seen references to drawing 3.75mg from a 5mg pen, our page explains exactly why that approach falls outside the licensed framework and what the pen is actually designed to deliver. Splitting that gap into a 3.75mg intermediate is not something the pen allows, not something the SmPC describes, and not something a prescriber working within the licensed framework would instruct.

Questions about your second injection, timings or what to do if your schedule slips are addressed in our second-dose guidance. For a broader look at how the full Mounjaro dose ladder works, the Mounjaro overview is a good place to start.

What this means if you are considering treatment or a dose review

If you are at the 2.5mg stage and wondering whether a mid-step option would ease your side effects, the right conversation is with your prescriber, who can decide whether extending the 2.5mg period is appropriate, rather than reaching for a dose that does not exist in the licensed schedule. Mounjaro is a prescription-only medicine, and the titration plan is part of the prescription, not a setting you adjust yourself.

For people looking at what private treatment looks like from the start, including how the 2.5mg pen is used in practice, our page on the 2.5mg Mounjaro pen covers storage, the injection process and what to expect in the first four weeks. If you are partway through treatment and want to understand the 5mg step, the 5mg Mounjaro page goes into the evidence for that phase. And if cost is on your mind (many people time their first order around payday or the start of a new month) the treatment pricing page sets out exactly what is included, with no hidden extras.

A prescriber at a GPhC-registered service will review your full picture before approving any treatment or dose change. At nume, that review happens the same working day your consultation is submitted. Start your free consultation if you would like a clinician to assess whether Mounjaro is right for you.