Can you get 3.75mg of Mounjaro from a 5mg pen? Here's the honest answer

The myth: a 5mg pen can be adjusted to deliver 3.75mg

It can't. Mounjaro's KwikPen is a pre-filled, single-strength device. Each pen contains four weekly doses at a fixed concentration, 5mg per injection, set at manufacture. There is no adjustable plunger, no volume markings visible to the user and no mechanism to intercept delivery mid-dose. Attempting to extract a partial volume by any means would compromise the pen's sterility, risk air injection and almost certainly produce a dose you cannot measure accurately. The device simply isn't built for it.

Where does 3.75mg come from, then? It appears on some forums as a rough midpoint between the licensed 2.5mg starter dose and the 5mg step above it. The idea is that a smaller incremental increase might reduce side effects during titration. That logic isn't unreasonable in principle, but it has nothing to do with how Mounjaro is supplied or approved. The NHS tirzepatide guidance is clear that licensed strengths are 2.5, 5, 7.5, 10, 12.5 and 15mg, nothing between them.

If your prescriber thinks a slower titration is appropriate, that conversation happens at the clinical stage. It might result in staying at 2.5mg for longer, for example. It does not result in an unlicensed fractional dose.

What the 5mg pen is actually for, and what comes before it

Treatment begins at 2.5mg for the first four weeks. That starting pen's job is to let your body adjust: the GI side effects that many people notice (nausea, changes in appetite, digestive shifts) are typically most pronounced early on and tend to settle. The 5mg pen is the next step up, prescribed once the prescriber is satisfied with tolerance at 2.5mg.

Some patients do find the step to 5mg uncomfortable. That is a legitimate clinical conversation (one our prescribers have regularly) and the answer to it is a properly supervised, evidence-based titration plan, not a DIY dose modification. If you're currently on 2.5mg and wondering about reaching a 3.75mg equivalent, the right place to raise that is with whoever holds your prescription.

The 5mg to 7.5mg step follows a similar pattern. Patients on 5mg who want to understand how that next increase works will find the same principle applies: licensed steps, prescriber-led timing, no partial doses. Unsurprisingly, the same is true going in the other direction, moving from 7.5mg back to 5mg is also a clinical decision, not something managed pen-side.

The only legal route to Mounjaro in the UK, and how to verify it

Mounjaro is a Prescription-Only Medicine. In the UK that means a qualified prescriber must assess your suitability before any supply. There is no legal route that skips that step. The MHRA has warned repeatedly about fake weight-loss pens being sold online and via social media, some containing insulin rather than tirzepatide. In October 2025, enforcement teams raided the first known illicit UK site manufacturing counterfeit tirzepatide pens; roughly 20 million doses of illegally traded weight-loss medicines, worth around £45 million, were seized across 2025. Report any suspect seller via the MHRA Yellow Card scheme.

Before using any online pharmacy, check it on the GPhC register at pharmacyregulation.org, nume's registration number is 9012878. Red flags include sellers offering Mounjaro without clinical assessment, prices that look too far below the market and anything framed as a discount code for a prescription medicine.

For those thinking about the GP route, our guide on getting Mounjaro through a GP covers the NHS criteria honestly, they're strict and phased, and not everyone qualifies yet. Private prescribing through a regulated pharmacy is the alternative for people who don't meet the NHS thresholds or who'd rather not wait.

What eligibility for Mounjaro actually looks like

For private prescribing, the licensed criteria are: adults with a BMI of 30 or above, or 27 and above alongside at least one weight-related condition (type 2 diabetes, hypertension, high cholesterol, obstructive sleep apnoea and others qualify). BMI alone doesn't guarantee approval, a prescriber weighs your full health history, current medicines and any contraindications. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance.

At nume, the process runs like this: you complete a free consultation online, a named prescriber reads your answers the same day, and ID plus live weight verification happen before any prescription is issued. If approved, your pen ships the same day for next-working-day delivery, a DPD tracking notification on your phone, then a plain cardboard box at the door with the pen inside, ready to go in the fridge.

You can read more about our full range of weight-loss treatment options or go straight to a free consultation. For anyone considering buying Mounjaro online, our guide to doing that safely covers what to look for in a provider. And if your question is more specifically about the 2.5mg starting pen and how titration from there works, the guide to partial doses from the 2.5mg pen addresses the same category of question for that strength. The answer is consistent: licensed doses, prescriber-led decisions, no workarounds.