3.75 mg Mounjaro from a 5mg Pen: How It Works and What to Decide

What 3.75mg actually means when you're using a 5mg pen

The Mounjaro 5mg KwikPen holds 3ml of solution and is pre-calibrated to deliver exactly 1.25mg per dose, four doses in total, giving a cumulative 5mg across the pen. There is no mechanism on the device to dial in a different amount. Each press of the button delivers the fixed dose; the pen locks once all four are used. So 3.75mg from a single 5mg pen would mean three presses across three weeks, except that Mounjaro is a once-weekly injection, and the pen is designed for exactly four consecutive weekly doses, not to be shared between strengths or paused mid-way through.

If you've read that 3.75mg is achievable from a 5mg pen, you may have come across suggestions about using a partial pen alongside a different one. That's where clinical oversight matters most. Combining doses from two pens of different strengths (for example, 2.5mg from one pen and 1.25mg from another) is a clinical arrangement, not a DIY option, and it requires a prescriber to plan it deliberately. The 5mg pen guide covers what that pen is and how it fits the standard schedule.

The practical upshot: if a prescriber has written a plan that arrives at 3.75mg Mounjaro, a dose that sits outside the standard licensed steps, they've almost certainly specified the combination of pens that achieves it, along with the injection schedule. That information will be in your clinical notes and your Patient Information Leaflet, not something to reconstruct from the pen label alone.

Why 3.75mg isn't a standard rung on the Mounjaro ladder

Mounjaro's licensed UK strengths are 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg. The titration schedule in the Summary of Product Characteristics moves in steps: 2.5mg for the first four weeks, then 5mg, then upward in 2.5mg increments at the prescriber's discretion. There is no licensed 3.75mg step. That doesn't make the figure meaningless (it may reflect a considered decision to hold at an intermediate level when a jump to 5mg has caused side effects) but it does mean it sits outside the standard path.

When a prescriber recommends something between two standard doses, it's usually a response to tolerability: nausea, vomiting, or other gastrointestinal symptoms that settled at 2.5mg but returned sharply at the full 5mg dose. Slowing the escalation is clinically reasonable. The evidence from SURMOUNT-1, published in the New England Journal of Medicine, showed that tirzepatide's benefits accumulate over weeks, getting to the highest tolerated dose matters more than speed. A careful climb is better clinical practice than a rushed one.

What this means practically: 3.75mg as a target almost always involves your prescriber specifying two pens of different strengths used together in a defined pattern. It's not a shortcut inside one pen. Our page on reaching 3.75mg using a 2.5mg pen goes into the clinical context in more detail.

Storage, routine and the real-life question of managing two pens

If your plan does involve drawing from more than one pen strength simultaneously, keeping track of which pen is current matters. Both pens should be stored in the fridge door at 2–8°C (not in the main body of the fridge where temperatures fluctuate more) and neither should be used past its expiry date. The exact room-temperature handling window for each pen is in the Patient Information Leaflet; refer to that rather than any general guidance, because the window differs by batch and storage history.

A two-pen arrangement also means your injection day stays fixed (weekly, same day each week) and that you note clearly in your records which pen you used and how many doses remain. Confusion between pens is one of the more common practical errors our prescribers hear about. If you're unsure which pen applies this week, that's the moment to contact your clinical team, not to work it out from the carton. For a full walkthrough of how the 5mg pen fits into a 3.75mg plan, including the practical steps for managing the injection schedule, our dedicated page covers the detail you'll need. For a broader look at what Mounjaro is and how the full dosing ladder works, the Mounjaro treatment page is the right starting point.

When to talk to your prescriber rather than adjust things yourself

The decision to change dose (up, down, or sideways to an intermediate level) belongs with a prescriber. This isn't bureaucratic caution; it's because your response to tirzepatide is shaped by factors a prescriber can see in your records and you may not be able to weigh alone: kidney function, concomitant medicines, prior GI history, blood-pressure readings, how much weight has shifted so far. The NHS patient-level guidance on tirzepatide, published on NHS.uk, is clear that dose changes should follow clinical advice.

If you're experiencing side effects at your current dose and wondering whether a slower climb would help, that conversation is exactly what a prescriber review is for. Similarly, if you've been told 3.75mg is your target and you're unsure whether your pens deliver it correctly, check before injecting, not after. A quick message to your clinical team is faster than unpicking a dosing error. If you'd like a prescriber to review your current plan, starting with a free consultation is straightforward. You can also find more context on weight-loss treatment options if you're still at the stage of deciding which route suits you. Our clinical team page gives you a sense of who reviews these decisions at nume.