Weight Loss Medication Injections: A Practical Guide for People Already Prescribed

Both tirzepatide and semaglutide are delivered as once-weekly subcutaneous injections — meaning the medicine goes into the layer of fat just beneath the skin, rather than into a muscle or vein. They come as pre-filled pen devices, which are designed to make self-injection straightforward once your prescriber or a healthcare professional has shown you how.

Common Injection Sites

Three areas are commonly used for these injections: the abdomen (belly), the front of the thigh, and the upper arm. Each site has a reasonably large usable area, and it is generally recommended to move around within and between these areas — a practice known as site rotation. Rotating injection sites helps reduce the chance of skin irritation, lumps, or changes in tissue at any one spot.

Why Rotation Matters

Repeatedly injecting into exactly the same small area can cause localised skin changes over time. Spreading injections across different sites — and varying the precise spot within each area — supports better skin health and may help the medication absorb more consistently. Your prescriber or the patient information leaflet that came with your pen will give you specific guidance on technique; this page is about understanding the principle, not replacing that instruction.

Side Effects Associated with the Injection

The most commonly reported side effects of weight loss medication injections are gastrointestinal — nausea, vomiting, diarrhoea, constipation, and indigestion are frequently noted, particularly in the early weeks or after a dose adjustment. Injection-site reactions such as redness, itching, or mild discomfort are also reported by some people. These medicines carry a black triangle (▼) symbol, meaning they are subject to additional monitoring; any side effects worth noting should be reported to your healthcare professional and, if appropriate, through your national reporting scheme (such as the UK's Yellow Card scheme). For a broader look at the names and types of weight loss injections available, that page covers the landscape in more detail. You may also find it useful to read about how weight loss injections are generally used as part of a managed treatment plan.

If you experience severe, persistent, or worrying symptoms — including severe abdominal pain, signs of an allergic reaction, or anything that concerns you — contact your healthcare professional promptly. Do not wait for your next scheduled appointment.

Keeping a clear record of every injection is one of the most practical things you can do while on a weekly injectable medication — and it is easier than it sounds when the right tool is in your pocket.

GLP Trackr's injection-site tracking feature lets you log each injection: which site you used (abdomen, thigh, or upper arm), the date, and the dose strength. As your history builds up, the app suggests your next rotation site automatically, so you never have to remember where you injected last week. For a weekly medication where consistency matters, that kind of quiet structure in the background makes a real difference.

Alongside site tracking, the app's side-effect logging lets you note how you felt after each injection — nausea, fatigue, injection-site discomfort, or anything else — and surfaces patterns over time. That record can be genuinely useful to bring to your prescriber, especially if you want to discuss how your body has been responding at different dose stages.

GLP Trackr is a tracking and journaling tool, not a source of medical advice. Any questions about your injection technique, dose, or side effects should always go to your prescriber or doctor — they know your full history and are best placed to guide you.

Explore the full picture of managing your weight loss injection journey on our hub page, or read about what to consider when evaluating injectable weight loss treatment providers.

Ready to keep every injection logged and every rotation on track? Download GLP Trackr on the App Store or Google Play — your first seven days are completely free.