What Are Weight Loss Injections?

The two most widely prescribed weight loss injections are Mounjaro (active substance tirzepatide) and Wegovy (active substance semaglutide). Both are given as a subcutaneous injection — meaning just under the skin — once a week, using a pre-filled pen device.

How they act on the body

Semaglutide is a GLP-1 receptor agonist, meaning it mimics a naturally occurring hormone involved in regulating appetite and blood sugar. Tirzepatide acts on both GLP-1 and GIP receptors, making it a dual agonist. Both medicines are commonly reported to increase the feeling of fullness, reduce appetite, and slow the rate at which the stomach empties — an effect that often eases over time. These are factual descriptions of how the medicines work; individual experience varies, and neither medicine is a quick fix or suitable for every person.

How often are weight loss injections given?

Both Mounjaro and Wegovy are taken once a week, on the same day each week where possible. Treatment typically begins at a low starting dose, which may be adjusted over time — always under the direction of a prescriber, never independently. The dose schedule is set by the prescriber based on individual circumstances.

Where injections are given

Common injection sites include the abdomen, front of the thigh, and upper arm. Rotating between sites each week is generally recommended to avoid irritation at any one spot. Your prescriber or the patient information leaflet that came with your pen is the right source for technique guidance specific to your medicine.

For more detail on safety considerations, see our page on whether weight loss injections are safe and a closer look at how safe weight loss injections are in practice. You can also read about weight loss injection pricing factors if cost is on your mind. For a broader overview, visit our guide to weight loss injections.

Both Mounjaro and Wegovy carry a black triangle (▼) status in the UK, meaning they are subject to additional monitoring. Any suspected side effects should be reported to a healthcare professional and to your national reporting scheme — such as the UK's Yellow Card scheme — or its equivalent in your country.

Once you are prescribed a weekly injection, keeping track of where and when you inject quickly becomes part of your routine — and it is easier to manage than it might sound with the right tool.

GLP Trackr's injection-site tracking and rotation feature lets you log each site you use — abdomen, thigh, or upper arm — and the app keeps a history so it can suggest your next rotation automatically. No more trying to remember which side you used last week. Alongside this, the dose logging feature records each injection with the date, time, and dose strength, giving you a clear, timestamped record of your treatment.

If you ever need to discuss your injection history with your prescriber — for a check-in appointment or to flag a concern — the app's doctor report feature pulls your logged data into a clear summary you can bring to the conversation.

GLP Trackr is not a substitute for your prescriber's guidance. Any questions about your injection technique, dose, or schedule should always go to your healthcare professional — they have the full picture of your health. The app is there to help you stay organised and consistent alongside that clinical support.

Ready to take the guesswork out of injection-site rotation? GLP Trackr is free to try for seven days — download it on the App Store or Google Play and start logging your very next shot.