What does Mounjaro do in the body?

Most people know Mounjaro as a highly effective weight loss injection. In studies, some participants lost over 20% of their starting weight (1), making it one of the most talked-about obesity treatments in recent years.

But what’s really happening inside your body when you use it? How does it help you feel fuller, think less about food, and gradually lose weight?

This article breaks down the science simply. Whether you’re researching your options or already taking Mounjaro and want to understand it better, we’ll walk you through what it does, how it works inside the body, and what you can expect along the way.

What is Mounjaro?

Mounjaro is a weekly, prescription-only injection that contains tirzepatide. During studies, researchers noticed that participants were also losing a significant amount of weight, leading to its approval for weight loss in the UK for those who meet specific criteria (2-3).

Unlike older weight loss medicines that suppress appetite or block the uptake of carbs and fats from your digestive tract, Mounjaro works in a much more natural, body-based way: by mimicking hunger hormones your body already produces to control hunger and digestion.

How Mounjaro works in the body

Tirzepatide belongs to a class of medicines called GLP-1 and GIP receptor agonists (3). This means it mimics two key hormones that help to regulate appetite and metabolism:

• GLP-1 (glucagon-like peptide-1)
• GIP (glucose-dependent insulinotropic polypeptide)

These hormones are part of what’s known as your body’s incretin system, which helps regulate how you digest food, manage blood sugar, and recognise when you’ve eaten enough. They signal your body to release insulin (which lowers blood sugar), tell your brain when you’re full, and slow down how quickly food leaves your stomach.

As a GLP1 and GIP receptor agonist, tirzepatide binds to hormone receptors in the pancreas, gut, and brain, triggering responses that mirror those of your body’s natural GLP-1 and GIP. This helps reduce your appetite by making you feel fuller (3-4).

Because it activates both receptors, Mounjaro has a dual action, giving it broader effects on appetite control than most other weight loss drugs that only target GLP-1.

What does this mean in practice?

The way in which Mounjaro works in the body translates into real, day-to-day changes in how you feel, eat, and think about food. While everyone responds differently, you can expect to:

1. Feel full sooner and for longer: Meals feel more satisfying because Mounjaro makes food move through your stomach more slowly (known as delayed gastric emptying).
2. Have less hunger and fewer cravings: It helps balance the hunger and fullness signals in your brain, so you’re less likely to snack between meals or overeat.
3. Have stabilised blood sugar levels: By controlling how your body releases insulin, Mounjaro helps prevent sugar spikes and crashes that can trigger cravings or tiredness.
4. Hear less “food noise”: Some people find they spend less time thinking about food, which makes it easier to make intentional, balanced choices and enjoy eating without overdoing it.

Because Mounjaro slows down digestion, side effects such as nausea, bloating, diarrhoea, or constipation are also fairly common at first (4). These are usually temporary and improve as your body adjusts. To minimise discomfort, treatment starts on a low dose that’s gradually increased until you find the level that works best for your body.

What can you expect while taking Mounjaro?

Because Mounjaro works with your hormones, results build slowly and sustainably:

• Most people see differences in appetite, cravings, and portion sizes within the first few weeks.
• Visible weight changes tend to appear over several months as your eating patterns stabilise.

It’s also normal to still feel hungry at times, especially around mealtimes. It doesn’t mean that Mounjaro isn’t working or that your current dose is too low. The goal isn’t to erase hunger completely, but to help you tune in to your body’s natural hunger cues, regulate feelings of fullness, and recognise when you’ve had enough.

To get the best results, it’s also worth noting that Mounjaro should be taken alongside healthy eating and regular physical activity (4). Think of it as a helping hand, not a replacement for positive lifestyle habits. You should still be prepared to eat well and move your body.

Mounjaro in summary: what it is (and what it isn’t)

Mounjaro IS:

• A prescription-only weekly injection containing tirzepatide
• A dual GLP-1 and GIP receptor agonist, mimicking natural hormones that help regulate appetite, digestion, and blood sugar
• A treatment that helps you feel fuller, eat less, and manage weight more effectively
• Clinically proven to support weight loss when used with healthy lifestyle changes

Mounjaro ISN’T:

• A stimulant or quick-fix appetite suppressant, like older medicines
• A fat, sugar, or carb blocker
• A replacement for balanced eating or regular movement
• Suitable for everyone (3) — it must be prescribed and monitored by a qualified doctor

Finding the right weight loss injection for you

Everyone’s weight loss journey is different. What works brilliantly for one person might not be the right fit for another, and that’s okay. That’s why if you’re considering Mounjaro or another GLP-1, it’s important to have proper medical guidance and ongoing support throughout your treatment.

At Oxford Online Pharmacy, our UK-registered doctors can help you decide whether Mounjaro or another weight loss treatment is right for you, and support you safely throughout your journey.

References

1. Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine [Internet]. 2022 Jun 4;387(3):205–16.
2. MHRA. MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss [Internet]. GOV.UK. 2023.
3. NICE. Tirzepaide: local formulary information. [Internet]. Nice.org.uk. 2025.
4. NHS. Treatment - Obesity [Internet]. NHS. NHS; 2023.