You know that weight loss injections like Mounjaro and Wegovy can help people lose a significant amount of weight. But how do they actually do it? What's happening in your body when you inject once a week that makes you less hungry and helps you eat less?
The answer involves gut hormones, brain signalling, and the mechanics of digestion. Let's break it down.
It starts with a natural hormone
After you eat a meal, your gut releases a hormone called GLP-1 (glucagon-like peptide-1). This hormone does several things simultaneously: it tells your brain you've eaten and should stop feeling hungry, it slows down how quickly food moves from your stomach into your intestines, and it stimulates insulin release to help manage blood sugar.1
The problem is that natural GLP-1 is broken down very quickly — within a few minutes. Its effects are brief. Weight loss injections contain synthetic versions of GLP-1 that have been chemically modified to resist breakdown, allowing them to stay active in your body for days rather than minutes. This produces a sustained, powerful appetite-suppressing effect.
Three mechanisms working together
1. Appetite suppression in the brain
GLP-1 receptors exist in the hypothalamus — the part of your brain that regulates hunger, satiety, and energy balance. When the medication activates these receptors, it reduces the signals that make you feel hungry and increases the signals that make you feel full. The result is that you simply want to eat less. Many patients describe it as the "food noise" — that constant background thinking about food — going quiet.1
2. Slowed gastric emptying
The medication slows down how quickly food leaves your stomach and moves into your small intestine. This means that after eating a meal, you feel physically full for much longer. A portion size that might previously have satisfied you for two hours might now keep you feeling full for four or five. This naturally reduces how much you eat throughout the day.2
This mechanism also explains the most common side effect — nausea. When food sits in the stomach longer, it can cause that queasy feeling, particularly when you eat more than your newly slowed digestive system can comfortably handle.
3. Improved insulin response
GLP-1 medications enhance how your body releases insulin in response to food, which helps maintain stable blood sugar levels after meals. This reduces the blood sugar spikes and crashes that can trigger cravings and reactive hunger. For people with Type 2 diabetes or insulin resistance, this has the dual benefit of improving blood sugar control alongside weight loss.1
What makes Mounjaro different?
Mounjaro (tirzepatide) is the only medication in its class that targets two hormone receptors rather than one. In addition to GLP-1, it also activates GIP receptors (glucose-dependent insulinotropic polypeptide).3
GIP is another gut hormone released after eating. It works alongside GLP-1 to enhance insulin secretion, and research suggests it may also play a role in how the body stores and processes fat. The combination of activating both pathways simultaneously — sometimes called "twincretin" therapy — appears to produce slightly greater weight loss than targeting GLP-1 alone, which is what we see in the clinical trial data comparing Mounjaro to Wegovy.
Why do they cause weight loss specifically?
It's worth being clear about this: GLP-1 medications don't "burn fat" directly. They don't speed up your metabolism or melt fat cells. What they do is make it dramatically easier for you to eat less by reducing appetite and increasing satiety. The weight loss comes from the calorie deficit that results from eating less food.
This is why healthcare guidelines consistently recommend using these medications alongside dietary changes and physical activity — not as a replacement for them. The medication handles the hardest part (suppressing the hunger that makes dieting miserable), while lifestyle changes contribute to and maintain the weight loss.
How is the medication actually taken?
All current GLP-1 weight loss injections are administered via pre-filled pen devices — similar to an insulin pen. You inject under the skin (subcutaneously) of your abdomen, thigh, or upper arm. The needle is very small and the process takes seconds. Most people describe it as no more painful than a slight pinch.
Mounjaro and Wegovy are injected once per week, on the same day each week. Saxenda requires a daily injection. All use a gradual dose escalation over several weeks to allow your body to adjust and minimise side effects.
For step-by-step injection guidance, see our injection guide. For dosing schedules, see our dosing explainer.
Why doesn't the effect last after stopping?
Once you stop the medication, the synthetic GLP-1 clears from your system within days. Your body's natural GLP-1 goes back to being broken down in minutes as before, appetite signals return to their pre-treatment levels, and most people gradually regain weight. This is why many clinicians now view obesity as a chronic condition requiring ongoing treatment, rather than something that can be "cured" with a short course of medication.4
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