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 Side Effects of Monjuro?
Monjuro (tirzepatide) is a prescription-only injection used for weight management in UK adults. Like all medications, it can cause side effects. Most are mild and temporary, but some require medical attention. Understanding what to expect helps you use this medication safely and recognise when to contact your GP. This UK health guide covers the full range of side effects, how long they typically last, and practical management strategies.
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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 Side Effects of Monjuro
Very common side effects (affecting more than 1 in 10 people):
- Nausea and vomiting — most frequently reported in the first 4–8 weeks, often improves over time
- Diarrhoea and constipation — digestive changes as your body adjusts
- Stomach (abdominal) pain — usually mild and temporary
Less common but still frequent reactions include indigestion, acid reflux, fatigue, dizziness, and injection site redness or bruising. Hair loss (alopecia) has been reported but is not fully understood. Most people find that nausea and digestive side effects improve significantly after 4–12 weeks as the body adapts. Starting at a lower dose (2.5 mg) and increasing gradually helps your system adjust. If side effects persist, your prescriber may adjust your dose or suggest timing strategies — for example, taking anti-nausea medication or adjusting meal timing.
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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 Side Effects — When to Seek Urgent Help
Seek immediate medical attention if you experience severe, persistent stomach pain spreading to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), gallbladder problems (pain in upper stomach area), changes in vision, or unusual heart rate changes. These are rare but require urgent evaluation by your GP or A&E. Do not delay — if you're unsure, contact NHS 111 or your GP immediately. Always inform your prescriber of any new or worsening symptoms, even if you're not certain they're medication-related. Your medical history and current medications affect your individual risk profile, which is why prescription-only status ensures professional oversight from the start.
- 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 monjuro side effects usually last?
Most common side effects like nausea and digestive changes improve significantly within 4–12 weeks as your body adjusts. Some people experience mild side effects throughout treatment, while others have none. Individual variation is normal. If side effects persist beyond 12 weeks or worsen, contact your GP or prescriber.
Can you prevent monjuro side effects?
Several strategies help reduce side effects. Start at the lowest dose (2.5 mg) and increase gradually — rushing escalation increases side effect risk. Eat smaller, lower-fat meals. Stay hydrated. Avoid very hot or very cold foods. Some people find anti-nausea medication helpful. Your prescriber can advise on personalised management strategies based on your symptoms.
Is monjuro safe for people with type 2 diabetes?
Monjuro is approved for weight management in people with and without type 2 diabetes. However, if you take diabetes medications like insulin or sulphonylureas, your doses may need adjustment — Monjuro can lower blood sugar. Always inform your prescriber of all medications before starting. Regular monitoring by your GP is essential.
What's the difference between monjuro and wegovy side effects?
Monjuro (tirzepatide) is a dual GIP and GLP-1 agonist; Wegovy (semaglutide) is GLP-1 only. Both cause similar common side effects (nausea, digestive changes), but Monjuro may cause these effects more frequently in early weeks due to its dual mechanism. Clinical trials show both are generally well-tolerated with proper dose escalation.
Should I stop monjuro if I develop side effects?
Do not stop suddenly without GP advice — abrupt discontinuation can cause problems. Contact your prescriber immediately about side effects. They may adjust your dose, slow escalation, or suggest management strategies before considering stopping. Only your GP or prescriber should advise on continuing, pausing, or stopping treatment.
Ready to take control of your weight loss journey?
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