How Do Weight Loss Jabs Work?

Both Mounjaro and Wegovy are prescription-only, once-weekly injectable medicines. Their active substances — tirzepatide and semaglutide respectively — work on receptor pathways involved in appetite regulation.

GLP-1 receptor agonism

Semaglutide (Wegovy) is a GLP-1 receptor agonist. GLP-1 (glucagon-like peptide-1) is a hormone released naturally after eating; it signals to the brain that you are full, stimulates insulin release in response to food, and slows the rate at which the stomach empties. By mimicking this hormone, semaglutide amplifies and prolongs these satiety signals.

Dual GIP and GLP-1 receptor agonism

Tirzepatide (Mounjaro) acts on both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors — making it a dual receptor agonist. GIP is another gut hormone involved in energy balance and appetite. Acting on both pathways is what distinguishes tirzepatide mechanistically from semaglutide, though both medicines share the core effect of reducing appetite and slowing gastric emptying.

What this means day to day

In practice, many people on these medicines report eating noticeably less without feeling deprived, feeling full more quickly, and thinking about food less frequently. It is important to note that individual responses vary — these effects are commonly reported, but not universal. The medicines are also not a substitute for the dietary and lifestyle guidance your prescriber has given you.

For a broader overview of these medicines and how they are used, see our guide to weight loss injections. You may also find it useful to read about whether weight loss jabs work and what the evidence generally shows, or explore our weight loss jabs overview for more context.

According to NHS guidance, these medicines are intended to be used alongside a reduced-calorie diet and increased physical activity, under clinical supervision.

Understanding how your jab works is one thing — staying consistent with it week after week is another. GLP Trackr is a companion app built specifically for people already on Mounjaro or Wegovy, and its injection-site tracking and rotation feature is one of the most practical tools on the platform.

Each time you inject, you can log the site — abdomen, thigh, or upper arm — and the app records your rotation history, then suggests your next site automatically. Rotating sites consistently is something most prescribers recommend, and having a clear log removes the guesswork entirely.

Alongside site tracking, dose logging lets you record every injection with its date, time, and dose strength, so you always have an accurate record of where you are in your treatment. Paired with dose reminders, it means a weekly injection is much harder to accidentally forget.

If you want to understand how your injection schedule connects to your broader progress — weight, energy, mood — the insights features bring all of that data together in one place.

For information on finding the medicine once prescribed, the page on weight loss jabs and pharmacies covers how access generally works. And for the full picture on managing your journey, our weight loss injections hub is a good starting point.

Always follow the guidance your prescriber or doctor has given you about your injections — they know your full medical history and their instructions take precedence over any general information. If you experience anything severe, persistent, or unexpected, contact your healthcare professional promptly.

Ready to keep your injection schedule on track? Download GLP Trackr on the App Store or Google Play and try it free for 7 days — log your sites, set your reminders, and let the app handle the rotation memory for you.