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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What Are Mounjaro Reactions and Side Effects?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medications, it can cause reactions and side effects — though not everyone experiences them. Most reactions are mild to moderate, particularly during the initial weeks of treatment or when your dose is increased. Understanding what to expect helps you recognise normal adjustment effects from reactions that need medical attention. Our GLP-1 weight loss calculator can help you explore potential outcomes alongside understanding the realistic side effect profile.
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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 Reactions and Side Effects
The most frequently reported reactions to Mounjaro occur during the first few weeks and often improve as your body adjusts. Very common reactions (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and abdominal pain. Many people find these reactions are manageable with dietary adjustments — eating smaller, lighter meals and staying hydrated can help significantly.
Common reactions (affecting up to 1 in 10 people) include indigestion, acid reflux, fatigue, dizziness, injection site reactions (redness or bruising), and hair loss. These typically occur as your dose increases and usually settle within a few days to a week. Some people experience reduced appetite, which is actually part of how the medication works — the tirzepatide activates GLP-1 and GIP pathways to help regulate appetite naturally.
- Nausea often peaks around days 2–3 after injection and diminishes over time
- Injection site reactions are usually minor and can be minimised by rotating injection sites weekly
- Constipation and diarrhoea often alternate as your digestive system adjusts
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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.
Serious Reactions and When to Seek Medical Attention
While serious reactions to Mounjaro are uncommon, you should seek urgent medical attention if you experience severe, persistent stomach or back pain — this could indicate pancreatitis and requires immediate evaluation. Contact your GP or call 111 if you notice signs of a serious allergic reaction (swelling of face, lips, tongue, or throat), severe changes in heart rate, vision changes, or persistent gallbladder symptoms (pain in upper stomach area).
Most reactions are temporary and manageable, but your prescriber needs to know about any concerning symptoms. If reactions are significantly impacting your quality of life, discuss dose adjustments or management strategies with your healthcare provider — you should never stop taking Mounjaro suddenly without medical guidance.
- 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 long do Mounjaro reactions typically last?
Most reactions like nausea and digestive symptoms peak during the first few days after an injection and typically improve within 3–7 days as your body adjusts. When your dose increases, you may experience mild reactions again, but they usually settle quickly. Reactions tend to become less frequent and severe as you continue treatment.
Can you prevent Mounjaro reactions?
While you cannot completely prevent reactions, you can reduce their severity by eating smaller, lighter meals, staying well hydrated, and avoiding fatty or rich foods — especially when starting or increasing your dose. Some people find ginger, peppermint tea, or slowly eating helps nausea. Your prescriber may also recommend timing strategies or dose adjustments.
What's the difference between normal reactions and a serious problem?
Normal reactions improve within a week and are manageable with dietary changes. Serious problems include severe stomach pain radiating to your back (pancreatitis risk), severe allergic reactions (swelling, difficulty breathing), gallbladder pain (persistent upper stomach pain), or vision changes. These require urgent medical attention.
Should you stop taking Mounjaro if you have reactions?
No — do not stop Mounjaro suddenly without medical guidance. Most reactions settle with time and adjustments. Contact your GP or prescriber if reactions are severe or persistent. They may recommend a slower dose increase, dietary modifications, or temporary symptom management strategies to help you continue safely.
Are Mounjaro reactions worse than Wegovy or other GLP-1 medications?
Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy is GLP-1 only. Clinical trials show similar side effect profiles — nausea and digestive symptoms are common across all GLP-1 and GIP/GLP-1 medications. Individual reactions vary significantly; some people tolerate Mounjaro better than others and vice versa.
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.