Mounjaro (tirzepatide) is a once-weekly injectable that works on two appetite-regulating pathways — GLP-1 and GIP — to reduce hunger and help your body use energy more effectively. If you want to understand the science behind how it works, our article on how GLP-1 medications work goes deeper. This guide is about the practical side: how to use it confidently and safely, week after week.
Mounjaro comes in a pre-filled, single-use injection pen. Pen designs can vary depending on the supply your clinic uses, so follow the specific instructions that come with your pen and the guidance your clinician gave you at initiation. Before your first injection, make sure you've watched or read through those instructions — the mechanism, the needle activation, and the click that confirms the dose is delivered can differ slightly between pen types.
A few universal rules apply regardless of which pen you have:
Mounjaro must be kept refrigerated at the temperature specified on the packaging. Do not freeze it. Keep pens in their original carton away from direct light.
Some pens may be stored at room temperature for a limited period once removed from the fridge — check the product information or ask your care team for the exact window. If you have any doubt about whether a pen has been stored correctly (left out too long, accidentally frozen, or exposed to heat), contact your care team before using it rather than assuming it's fine.
Mounjaro is injected subcutaneously — into the layer of fat just under the skin. There are three recommended sites:
Rotate sites with each injection. Using the same spot repeatedly can cause skin changes that affect how the medication absorbs. A simple system is to move around the same site (e.g., different areas of the abdomen) for a few weeks before switching to another site entirely.
Avoid injecting into skin that is bruised, tender, scarred, or has any visible skin condition.
Mounjaro is most effective — and easiest to manage — when taken on the same day each week. Pick a day that suits your schedule and stick to it. Many patients tie it to a specific event: Sunday evening, Monday morning before work, or whichever day feels consistent.
If you miss a dose: do not double up. Contact your care team to confirm the best course of action based on how much time has passed. As a general rule, if it's within a few days of your usual injection day, take it as soon as you remember and then continue on your regular schedule.
Your dose and titration schedule are set by your clinician, not this article. Mounjaro is introduced at a starting dose and stepped up gradually — this pacing exists specifically to minimise side effects while your body adjusts. Do not change the dose or schedule without guidance from your care team.
In the early weeks, the most common experience is a noticeable reduction in appetite. Some patients also experience mild gastrointestinal side effects — nausea, loose stools, or a feeling of fullness that lingers. For most people these settle over time as the dose stabilises. Our article on managing side effects covers practical strategies for getting through the adjustment phase more comfortably.
When to contact your care team:
You don't need to wait for your scheduled appointment to reach out — that's what the care team is there for.
If anything about your medication, your pen, or how you're feeling doesn't seem right, reach out to your care team — a quick message is always the right call.