3.75 mg Mounjaro: the half-dose explained

What the clinical programme tells us about starting low

The SURMOUNT-1 trial (published in the New England Journal of Medicine and involving 2,539 adults with obesity) tested tirzepatide at 5 mg, 10 mg and 15 mg maintenance doses after a structured escalation from 2.5 mg. The 2.5 mg starting period was not designed to produce weight loss. It was there so participants' gastrointestinal systems could adjust before the dose climbed. That design choice carried into the UK licence. Nausea and digestive side effects are most common early in treatment, and a gradual start measurably reduces how many people stop because of them.

The NHS tirzepatide information page reflects this: treatment begins at 2.5 mg for the first four weeks, then steps upward at the prescriber's direction. There is no clinical evidence that 3.75 mg (half of a 2.5 mg pen stretched over two weekly doses) produces a better-tolerated or more effective start. The SURMOUNT data were gathered on the licensed dose ladder, not on improvised increments between rungs.

One misconception worth putting down gently: some people assume a smaller starting dose means slower, safer progress toward their goal weight. The 2.5 mg pen's job is to settle your system, not to delay results. The therapeutic work begins as the dose rises.

What splitting a pen actually involves, and why it matters

Each Mounjaro KwikPen is pre-filled and factory-calibrated to deliver a single, fixed dose. To reach something close to 3.75 mg you would need to use roughly half the pen's contents in week one and the remainder in week two. Questions about how many millilitres corresponds to 3.75 mg and how many clicks that equates to circulate online precisely because people are attempting this at home.

The problems are practical as well as regulatory. Mounjaro pens are not designed for partial delivery. Storing a used pen between doses raises questions about sterility and the stability of the remaining solution. Dose accuracy without calibrated equipment is genuinely uncertain. And none of this appears in the SmPC (the authoritative product document) because it was never tested this way. For anyone curious about what the 2.5 mg pen can and cannot do, our page on reaching 3.75 mg from a 2.5 mg pen walks through the mechanics in detail.

If Mounjaro's standard starting dose feels too strong, that is a conversation for your prescriber, who can review timing and supporting measures. It is not a problem that pen-splitting solves.

The licensed schedule: what comes after 2.5 mg

After the first four weeks on 2.5 mg, the prescriber typically moves the dose to 5 mg for another four weeks, then continues upward in 2.5 mg steps at roughly monthly intervals. Most people reach a maintenance dose somewhere between 5 mg and 15 mg depending on response and tolerability. The full 5 mg dose is where the weight-management effect genuinely begins for most people, which is why the clinical trials reported their headline results at 5 mg and above.

Context on cost is worth a note here: dose increases mean moving through different pen strengths over time, which affects what you pay month to month. If that is on your mind, the Mounjaro cost guide covers how UK private pricing works across the dose range and what a legitimate quote should include.

NICE's appraisal of tirzepatide (TA1026) endorses the licensed titration schedule and recommends reviewing whether treatment should continue if less than 5% weight loss is achieved after six months at the highest tolerated dose. That review is a clinical conversation, not something to pre-empt by adjusting doses independently.

Getting the dose decision right from the start

If you are wondering whether a 3.75 mg Mounjaro dose is right for you, the most direct answer is that it is not an option your prescriber will offer, because it does not exist within the licensed framework. What your prescriber can do is discuss whether 2.5 mg is the right starting point for your circumstances, how quickly to step up, and what to do if side effects make the early weeks uncomfortable. Those are the questions that actually shape your experience.

Our Mounjaro overview covers the full picture of how treatment works, eligibility, and what supervised care looks like. For questions about whether the 2.5 mg pen can practically be used to approximate a smaller dose, the page on whether you can take 3.75 mg of Mounjaro addresses that directly. And if you are already on treatment and wondering about the step after your current pen, the guide to the fifth Mounjaro dose may be useful context.

A prescriber at nume reviews every consultation personally, a real clinician, same day. If you would like that clinical conversation about where to start, speak to our prescribers through a free consultation.