Injection Pens for Weight Loss: What They Are and How to Use Them Well

Mounjaro (tirzepatide) and Wegovy (semaglutide) are both supplied as pre-filled injection pens. Each pen contains a single dose of the medicine and is designed for subcutaneous injection — meaning the medicine is delivered just beneath the skin, not into a muscle or vein.

The medicines inside the pens

Semaglutide (the active substance in Wegovy) is a GLP-1 receptor agonist, meaning it acts on pathways involved in appetite regulation and the feeling of fullness. Tirzepatide (Mounjaro) works on both GLP-1 and GIP receptors. Both are taken once weekly on the same day each week, as directed by your prescriber. Neither is insulin, and neither should be confused with an adrenaline auto-injector (sometimes called an EpiPen, which is used in allergic emergencies and is an entirely different device).

Common injection sites

The three sites most commonly used for these injections are the abdomen, the front of the thigh, and the upper arm. Your prescriber or the patient information leaflet that came with your pen will have the full guidance on site selection. One important general principle: rotating between sites — and between different areas within the same site — helps reduce the chance of skin reactions such as lumps or hardness building up at a single spot.

Handling your pen

Each pen has its own storage instructions; check the leaflet that came with your pen, as requirements can differ between brands and dose strengths. In general terms, unused pens are stored in a refrigerator, while a pen in use can typically be kept at room temperature for a period specified in the product information. Always refer to your own pen's leaflet or the electronic Medicines Compendium for the exact figures rather than relying on memory.

For a broader overview of self-injection for weight management, see our guide to weight-loss injections. You may also find it useful to read about how these injections work for weight loss and what to expect with the Mounjaro injection specifically.

Keeping a clear record of each injection — which site you used, the date, and the dose — is one of the most practical habits you can build on a weekly injection schedule. GLP Trackr's injection-site tracking feature logs every injection and suggests your next rotation automatically, so you never have to rely on memory to avoid overusing the same spot.

Alongside site rotation, the app's dose reminder feature sends a gentle alert on your chosen injection day, helping you stay on the same day each week without setting calendar reminders manually. If you ever miss a dose, the app can log that too, giving you something clear to discuss with your prescriber at your next appointment — dose decisions, including what to do after a missed injection, always belong with your prescriber or doctor.

If you want to understand whether your BMI affects your eligibility or progress, our BMI and weight-loss injections page has more context.

GLP Trackr is a companion app, not a medical service. It helps you record your journey clearly so you can have better-informed conversations with your healthcare professional — it is not a substitute for their guidance. If you experience any injection-site reaction that is severe, spreading, or not resolving, contact your healthcare professional promptly.

Ready to keep your injection schedule on track? Download GLP Trackr on the App Store or Google Play and explore it free for your first 7 days — log your sites, set your weekly reminder, and let the app handle the rotation for you.