The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
What Are the Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection that works differently from other weight loss medications available in the UK. It's a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and support weight loss. Understanding what Mounjaro actually does — and what it doesn't — is essential before discussing it with your GP or a prescriber. This guide covers the clinical effects, side effects, and how Mounjaro compares to other options like Wegovy in the UK market.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro Produces Weight Loss Effects
Mounjaro's effects come from activating two hormone pathways simultaneously: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism is what distinguishes tirzepatide from other weight loss injections like Wegovy, which target GLP-1 alone.
The main effects include:
- Appetite reduction: Mounjaro signals to your brain that you're satisfied, reducing hunger and food cravings
- Increased fullness: You may feel fuller after smaller meals, naturally eating less
- Slowed gastric emptying: Food moves more slowly through your stomach (though this effect typically reduces over time)
- Improved fat utilisation: The dual mechanism helps your body process fat more effectively
These effects work together to create a calorie deficit without requiring extreme restriction. Clinical trial data showed that Mounjaro users lost an average of 15–21% of their body weight over 72 weeks, depending on the final dose used (5mg, 10mg, or 15mg weekly). However, individual results vary significantly, and weight loss only occurred alongside a reduced-calorie diet and increased physical activity.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Common Side Effects and Safety Considerations
Like all medications, Mounjaro produces side effects in many users. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These effects are typically mild to moderate and often improve as your body adjusts.
Common side effects include indigestion, acid reflux, injection site reactions, fatigue, dizziness, and hair loss. Less common but serious effects requiring urgent medical attention include severe stomach pain (possible pancreatitis), gallbladder problems, and serious allergic reactions.
Use our free BMI calculator above to check your baseline weight and health status before discussing Mounjaro with your prescriber. Track any weight changes and side effects using our progress tracker once you start treatment. Always inform your GP or prescriber of all existing medications, as Mounjaro may affect the absorption of oral contraceptives and other tablets.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
How quickly do you see the effects of Mounjaro?
Weight loss effects typically become noticeable after 2–4 weeks, though appetite reduction may occur within days. The tirzepatide dose is increased gradually (every 4 weeks), so effects increase over the first 5–6 months. Maximum effects are usually seen at weeks 16–20 when the final therapeutic dose is reached.
Are the weight loss effects of Mounjaro permanent?
Weight loss achieved with Mounjaro is not automatically permanent. Clinical evidence suggests that when people stop taking Mounjaro, weight may be regained over time. Long-term use alongside lifestyle changes (diet and exercise) provides the best results. Your prescriber will discuss maintenance strategies with you.
How does Mounjaro's effect compare to Wegovy?
Both are once-weekly injections approved for weight management in the UK. Mounjaro (tirzepatide) uses a dual GIP+GLP-1 mechanism, while Wegovy (semaglutide) uses GLP-1 alone. Clinical trials showed Mounjaro produced average weight loss of 15–21%, whilst Wegovy showed approximately 15%. Individual results vary considerably — your prescriber can discuss which is more suitable for you.
Do the appetite-suppressing effects of Mounjaro wear off over time?
Some users report that appetite suppression effects may slightly diminish after several months on a stable dose. This is why Mounjaro doses can be increased up to a maximum of 15mg weekly. If appetite returns, discuss with your prescriber about adjusting your dose or exploring alternative options.
What are the effects of Mounjaro on blood sugar and diabetes?
Mounjaro was originally developed for type 2 diabetes management, so it naturally improves blood sugar control. For diabetic users, this can reduce reliance on other diabetes medications — your GP must monitor this carefully. For non-diabetic users, Mounjaro still provides weight loss effects, though the blood sugar benefit is less pronounced.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.