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 Mounjaro?
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection licensed in the UK for weight management. Like all medicines, it can cause side effects. Most are mild and temporary, though some people experience no side effects at all. Understanding what to expect helps you manage treatment effectively and know when to contact your GP.
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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 Side Effects of Mounjaro
According to UK product information, the most common side effects of Mounjaro affect more than 1 in 10 people:
- Nausea — the most frequently reported effect, usually improves within days or weeks as your body adjusts
- Diarrhoea or constipation — often temporary and related to dose changes
- Vomiting — less common but can occur, especially early in treatment
- Stomach (abdominal) pain — usually mild and settles quickly
Other common effects include indigestion, acid reflux, injection site redness or bruising, fatigue, dizziness, and hair loss (alopecia). Most of these side effects are temporary and typically reduce as your body adapts to the weekly weight loss injection. Starting at a lower dose (2.5 mg) and increasing gradually helps minimise discomfort.
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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 Help
Serious side effects are rare but require urgent medical attention. Contact your GP or go to A&E immediately if you experience:
- Severe, persistent stomach pain that spreads to your back (possible pancreatitis)
- Serious allergic reactions (swelling of face, lips, tongue, or throat; difficulty breathing)
- Sudden changes in vision or eye problems (in people with existing diabetes)
- Gallbladder problems (pain in upper stomach area, yellowing of skin or eyes)
- Severe dehydration or fainting
These effects are uncommon but serious. Always inform your prescriber immediately if you develop any unexpected symptoms during treatment.
- 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.
What is the most common Mounjaro side effect?
Nausea is the most frequently reported side effect, affecting more than 1 in 10 people. It usually improves within days or weeks as your body adapts. Taking the injection at the same time each week and eating smaller meals can help manage nausea. If it persists or is severe, contact your GP — they may adjust your dose or recommend additional support.
How long do Mounjaro side effects last?
Most common side effects (nausea, stomach pain, diarrhoea) typically improve within 3–7 days as your body adjusts. Some people experience minimal effects. Serious side effects are rare. If side effects persist beyond two weeks or worsen, contact your GP. Most people find effects improve significantly after the first few doses.
Can I continue Mounjaro if I have side effects?
Yes — most side effects are mild and temporary. Your prescriber may recommend dose adjustments, dietary changes (eating smaller, low-fat meals), or over-the-counter remedies. However, if side effects are severe or you develop warning signs like severe stomach pain or allergic reactions, stop the injection and seek immediate medical advice from your GP or A&E.
Are Mounjaro side effects different from Wegovy side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cause similar common side effects — nausea, diarrhoea, constipation, stomach pain. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. Individual tolerance varies. Your GP can discuss which medication might suit you better based on your personal health history.
Should I stop taking Mounjaro if I feel sick?
Do not stop suddenly without advice from your GP. Nausea is usually temporary and improves with time. Your prescriber may recommend dietary adjustments, anti-nausea strategies, or dose timing changes. However, contact your GP if nausea is severe, persistent, or you cannot keep food or fluids down. They will advise whether to continue, pause, or adjust your treatment.
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.