How Do Weight Loss Injections Work?

Both Mounjaro and Wegovy belong to a class of medicines that mimic naturally occurring gut hormones involved in appetite regulation. Understanding how they work can help you make sense of the changes you may notice after your injections.

GLP-1 receptor agonists

Wegovy contains semaglutide, which acts on GLP-1 (glucagon-like peptide-1) receptors. GLP-1 is a hormone released after eating that signals fullness to the brain, slows the rate at which the stomach empties, and reduces appetite. By activating these receptors more consistently than the natural hormone does, semaglutide helps many people experience reduced hunger and what is often described as quieter food noise — the persistent thoughts about eating that can make calorie control difficult.

Dual GIP and GLP-1 action

Mounjaro contains tirzepatide, which acts on both GLP-1 receptors and GIP (glucose-dependent insulinotropic polypeptide) receptors. GIP is another gut hormone involved in energy metabolism and appetite. This dual mechanism is what distinguishes tirzepatide from semaglutide, though both medicines are prescribed for weight management and are taken once weekly.

Do they work without lifestyle changes?

These medicines are prescribed alongside dietary guidance and, where appropriate, increased physical activity — not as a replacement for them. Many people find that reduced appetite makes it easier to maintain a lower calorie intake and to be more active, but the medicines are intended to support a broader approach to weight management. Whether a particular regimen includes formal exercise is a question for your prescriber or doctor, who knows your individual health picture. For a broader overview of this medication category, visit our guide to weight loss injections.

It is also worth knowing that these are prescription-only medicines — their suitability, dosing, and duration of use are decisions made by a qualified prescriber based on your individual circumstances. According to NHS guidance, these treatments are recommended as part of a specialist weight management programme, not as over-the-counter solutions.

If you are curious about the detailed mechanism of how weight loss injections work, that page explores the physiology in more depth.

Knowing how these injections work is one thing — staying consistent with them week after week is another. GLP Trackr is built specifically for people already on Mounjaro or Wegovy, and the most immediately useful feature for injection days is injection-site tracking and rotation.

Each time you inject, you log the site — abdomen, thigh, or upper arm — and GLP Trackr keeps a rotation history so it can suggest where to inject next. Rotating sites is generally recommended to avoid irritation at any one spot, and having an automatic record removes the guesswork of trying to remember where you last injected.

Alongside site tracking, the app logs your dose strength and the date and time of each injection, so you always have a clear picture of your treatment history. Gentle weekly reminders mean a dose is far less likely to slip your mind. If you ever do miss a dose, you can log that too and bring the record to your next appointment — any decision about what to do belongs with your prescriber, not an app.

Many people also use the side-effect tracker to note how they feel in the days following an injection, which can reveal useful patterns over time — handy context to share with your prescriber at reviews. If you are weighing up what you might spend on this treatment, our page on the cost of weight loss injections has some neutral context.

GLP Trackr is a tracking and journaling tool — it is not a substitute for your prescriber's guidance. If you experience any symptoms that feel severe, persistent, or worrying, contact your healthcare professional promptly.

Ready to bring some order to your weekly injection routine? GLP Trackr is available on the App Store and Google Play — give it a try free for 7 days and see how simple staying on top of your journey can be.