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
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Mounjaro Side Effects: What Reviews and Clinical Data Show
If you're considering Mounjaro (tirzepatide) for weight management in the UK, understanding potential side effects is essential. Real-world reviews and clinical trial data show that most side effects are mild to moderate and often improve over time. This guide covers what the evidence tells us about Mounjaro side effects, how common they are, and what you can expect at different doses during treatment.
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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.
Common Mounjaro Side Effects and What the Evidence Shows
Clinical trials and real-world reviews consistently show that nausea, diarrhoea, vomiting, and constipation are the most frequently reported side effects of Mounjaro (tirzepatide). These gastrointestinal effects affect more than 1 in 10 people starting the medication. According to the SURMOUNT trial data, nausea typically peaks in the first 2–4 weeks after starting or increasing the dose, then often improves as your body adjusts.
Other commonly reported effects include stomach pain, indigestion, and fatigue. Many UK users report that side effects reduce significantly after 4–8 weeks at the same dose. Less common but notable side effects include headaches, dizziness, injection site reactions (redness or bruising), and hair loss. Use our free BMI calculator to check your starting point, and our GLP-1 weight loss calculator can help you understand what to expect across different dose levels.
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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.
Managing Side Effects and When to Seek Medical Help
Most Mounjaro side effects can be managed through dietary adjustments—eating smaller portions, avoiding greasy or high-fibre foods initially, and staying hydrated helps significantly. Your prescriber may recommend starting at the lowest dose (2.5 mg) and increasing gradually to allow your body to adapt. Seek immediate medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions, or gallbladder symptoms. Minor side effects like nausea are generally not a reason to stop treatment—discuss any concerns with your GP or prescriber, as dose adjustments or timing changes often resolve them.
- 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.
Are Mounjaro side effects permanent?
No. Most side effects are temporary and improve within weeks as your body adjusts to the medication. Nausea and gastrointestinal symptoms typically peak early and reduce significantly after 4–8 weeks at the same dose. If side effects persist, your prescriber may adjust your dose or timing.
How do Mounjaro side effects compare to Wegovy?
Both medications (Mounjaro uses tirzepatide, Wegovy uses semaglutide) cause similar gastrointestinal side effects. Clinical data suggests Mounjaro may produce nausea slightly more frequently, but individual experiences vary widely. Your prescriber can help you choose based on your health profile.
Can you take anti-nausea medication with Mounjaro?
Some anti-nausea medications may interact with Mounjaro, particularly metoclopramide. Always inform your prescriber of all medications you're taking before starting Mounjaro. They can recommend safe options or suggest dietary management strategies instead.
What's the difference between dose-related side effects and allergic reactions?
Dose-related side effects (nausea, diarrhoea) develop gradually and improve with time. Allergic reactions are sudden—swelling of the face, lips, tongue, or throat, difficulty breathing, or severe rash. Seek emergency care immediately if you suspect an allergic reaction. These are rare but serious.
Does hair loss from Mounjaro grow back?
Hair loss is reported in some UK users and may be related to gradual weight management or the medication itself. Hair typically regrows once the body stabilises. If hair loss concerns you, discuss it with your GP—it's not a reason to stop Mounjaro, but your prescriber can monitor and advise.
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.