Using a Weight Loss Injection Pen: What to Expect

Both Mounjaro and Wegovy come as pre-filled, single-use pens. Each pen contains one dose of the medicine, and injections are typically taken once a week on the same day, as directed by your prescriber or doctor.

Common injection sites

The three most commonly used sites for subcutaneous injections are the abdomen (stomach area), the front of the thigh, and the upper arm. Your prescriber or the patient information leaflet that came with your pen will guide you on the exact technique — GLP Trackr supports the tracking of sites rather than providing clinical injection instruction.

Why site rotation matters

Rotating injection sites — moving to a different spot or area with each weekly dose — is widely recommended to help reduce the risk of localised skin reactions such as lumps or thickened tissue at the injection site. Many people find it easy to lose track of which site they used last, especially over months of treatment. Keeping a simple log takes the guesswork out of rotation.

Storing and handling your pen

Storage requirements vary slightly between products, so always check the leaflet that came with your pen — but as a general guide, unopened pens are typically stored in a refrigerator. Once in use, some pens can be kept at room temperature for a period; your patient information leaflet or the electronic Medicines Compendium (eMC) entry for your specific medicine will give the definitive guidance for your pen.

If you are new to injecting or have questions about technique, your prescriber or a pharmacist is the right person to ask. You can find further practical context in our overview of weekly weight loss injections and in our broader guide to weight loss injections.

Staying on top of a weekly injection schedule is easier when you have a record of every dose. GLP Trackr's injection-site tracking is built specifically for this — log each injection, including the site you used (abdomen, thigh, or arm), and the app tracks your rotation history and suggests where to inject next, so you never have to remember on your own.

Alongside site tracking, the dose logging feature records the date, time, and dose strength of every shot in one place. Pair this with dose reminders and you have a reliable system that keeps your weekly schedule on track without relying on memory alone.

If you notice any reaction at an injection site — redness, swelling, or discomfort — the side-effect log lets you record it and spot whether it follows a pattern, which can be a useful detail to bring to your next prescriber appointment.

If you are thinking about the wider cost and availability of your pen, our page on getting your weight loss injection covers what to consider, and our look at Novo Nordisk injection pens goes into more detail on semaglutide devices specifically.

A note on safety: any decision about your injection schedule, dose, or technique should be made with your prescriber — they know your full history. If you experience severe or persistent reactions at an injection site, or any symptom that concerns you, contact your healthcare professional promptly. GLP Trackr helps you record what is happening so you can discuss it clearly; it is not a substitute for clinical advice.

Ready to take the guesswork out of injection-site rotation? Download GLP Trackr on the App Store or Google Play and explore site tracking, dose logging, and reminders — all free for your first 7 days.