New Weight Loss Injections: What to Know If You're Already Prescribed

Tirzepatide (Mounjaro) and semaglutide (Wegovy) are both prescription-only, once-weekly injectable medicines. They work on appetite-regulating pathways — semaglutide acts on GLP-1 receptors, while tirzepatide acts on both GLP-1 and GIP receptors — which commonly leads to a reduced sense of hunger and increased feelings of fullness. These are not insulin, and they are not suitable for everyone; suitability is always a decision made by a prescriber.

Both medicines are given as a subcutaneous injection — meaning just under the skin — using a pre-filled pen device. The three most commonly used injection sites are the abdomen (stomach area), the front of the thigh, and the back of the upper arm. Your prescriber or the patient information leaflet that came with your pen will guide you on exact technique.

Why injection site rotation matters

Rotating between sites — and moving slightly within each site from week to week — helps reduce the chance of skin reactions such as redness, soreness, or changes in tissue at the injection area. Most clinical guidance recommends not using the exact same spot twice in a row. If you notice persistent swelling, lumps, or skin changes at an injection site, mention it to your healthcare professional.

For a broader overview of how these medicines are prescribed and managed, see our guide to weight loss injections. You may also find it useful to read about how weight loss injections are accessed through national health systems or explore eligibility criteria for weight loss injections if you have questions about your prescription pathway.

Keeping track of which site you used last week — and which side of your abdomen or thigh — is harder than it sounds, especially once weekly injections become routine. GLP Trackr's injection-site tracker logs each injection with the site and date, then suggests where to rotate next, so you're never guessing. Over time, you build a clear rotation history without having to remember anything manually.

Alongside site tracking, the app's dose logging feature lets you record the strength of each injection and when it was given — useful if you ever need to review your schedule or bring a clear summary to an appointment with your prescriber. Dose reminders can also be set so a weekly injection doesn't slip your mind.

If you want to see how your injection schedule fits into the bigger picture of your progress, our overview of managing weight loss injections covers what to expect across your treatment.

GLP Trackr is free to try for 7 days — download it on the App Store or Google Play and start logging your injections, rotation history and dose schedule from day one.

Important: GLP Trackr is a tracking tool, not a clinical service. Any questions about your injection technique, dose, or whether a site reaction needs attention should be directed to your prescriber or doctor — they know your full history and are best placed to advise you. If you experience a side effect you want to report, ask your healthcare professional about your national reporting scheme (such as the UK's Yellow Card scheme).