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 and blood sugar regulation.

GLP-1 receptor agonists

Wegovy contains semaglutide, which acts on GLP-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 the drive to eat. By activating these receptors, semaglutide helps many people experience a significant reduction in appetite — sometimes described as a quietening of "food noise".

Dual GIP and GLP-1 action

Mounjaro contains tirzepatide, which acts on both GLP-1 and GIP receptors. GIP is a second gut hormone that also plays a role in appetite regulation and energy balance. This dual action is what distinguishes tirzepatide from earlier GLP-1 medicines, though both types are prescribed on an individual basis by a qualified clinician.

What this means in practice

For many people, the effects build gradually. The medicines are typically started at a low dose, with the prescriber adjusting it over time — so noticeable changes in appetite or weight often become more apparent across weeks rather than days. It is common to find that smaller portions feel satisfying and that the urge to snack reduces. Slower stomach emptying can also contribute to these feelings of fullness, though this effect often lessens as the body adjusts.

On the question of belly fat specifically: weight loss from these medicines tends to reflect overall body fat reduction rather than targeting one area. Where fat is lost — and how fast — varies between individuals and is influenced by starting weight, diet, activity, and other factors.

For a broader look at these medicines, see our guide to weight loss injections. You may also find it useful to read about how weight loss injections work in more detail and the practical side of taking weight loss injections.

Understanding how these medicines work is one thing — staying consistent week after week is another. GLP Trackr is built specifically for people already on Mounjaro or Wegovy, and its injection-site tracking and rotation feature is one of the most practical tools it offers.

Each time you inject, you can log the site (abdomen, thigh, or upper arm), and the app tracks your rotation history so it can suggest where to inject next. Rotating sites is generally recommended to keep skin in good condition, and having a clear record makes it easy to follow that pattern without having to remember where you last injected.

Alongside site tracking, dose logging lets you record every injection — the date, time, and dose strength — so you always have an accurate picture of your schedule. Dose reminders mean a weekly injection is less likely to slip your mind during a busy week.

Over time, this data forms a useful record you can bring to appointments with your prescriber, giving them a clear view of your consistency and any patterns you have noticed. GLP Trackr also includes a side-effect log, so if you experience nausea or other commonly reported effects as your dose is adjusted, you can note them alongside your injection entries.

For more on what to expect from these medicines, visit our weight loss injections hub.

A note on safety: if you experience any symptoms that feel severe, persistent, or worrying at any point during your treatment, contact your healthcare professional promptly. GLP Trackr helps you record what you observe — it is not a substitute for clinical advice, and any questions about your dose or how the medicine is working for you should be discussed with your prescriber or doctor.

Ready to bring some order to your weekly injection routine? Download GLP Trackr on the App Store or Google Play — your first 7 days are completely free.