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 the Symptoms and Side Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that works through a dual GIP and GLP-1 mechanism. Like all medications, Mounjaro can cause side effects — some very common, others less frequent. Understanding what to expect helps you decide whether this treatment is right for you and know when to contact your GP. Most side effects are mild and often improve as your body adjusts to the medication over weeks or months.
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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 Mounjaro Side Effects in the UK
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are typically mild to moderate and often settle within the first few weeks as your dose gradually increases through the dose titration schedule. Nausea is one of the most frequently reported symptoms but tends to improve significantly once you reach your maintenance dose.
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, fatigue, dizziness, headache, and injection site reactions such as redness or bruising. Hair loss has also been reported by some users. Injection site symptoms usually resolve within a few hours. Most gastrointestinal side effects are related to how Mounjaro slows stomach emptying — this effect often becomes less noticeable over time.
These side effects are generally manageable and do not require stopping treatment, though your prescriber may adjust your dose or timing if symptoms are troublesome.
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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 Symptoms Requiring Medical Attention
Whilst uncommon, certain symptoms require urgent medical assessment. Seek immediate help if you experience severe, persistent stomach or back pain — this may indicate pancreatitis. Similarly, signs of a serious allergic reaction (swelling of the face, lips, tongue, or throat, difficulty breathing) require emergency treatment.
Other serious symptoms needing GP review include gallbladder problems (pain in the upper stomach area), changes in vision or eye symptoms (particularly if you have diabetes), rapid or unusual changes in heart rate, or severe dehydration. If you are unsure whether a symptom needs medical attention, contact your GP or call NHS 111 for advice. Always inform your prescriber of any concerning symptoms when you attend appointments or have your dose adjusted on your Mounjaro KwikPen schedule.
- 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.
Will I definitely get nausea with Mounjaro?
No. While nausea is very common (affecting more than 1 in 10 people), not everyone experiences it. Severity varies — some people have mild nausea, others none at all. Nausea often improves within weeks as your body adapts. Starting at a low dose (2.5 mg) and increasing gradually helps minimise this symptom.
How long do Mounjaro side effects last?
Most gastrointestinal side effects improve significantly within 2–4 weeks as your body adjusts. Some people find symptoms ease faster if they eat smaller, blander meals and stay well hydrated. Injection site reactions typically resolve within hours. If side effects persist beyond 4 weeks at the same dose, discuss this with your prescriber.
Can I manage Mounjaro side effects without stopping treatment?
Yes, in most cases. Your prescriber may suggest dietary changes, slower dose increases, or different meal timing to manage symptoms. Staying hydrated and eating smaller portions often helps. Only in rare cases where side effects are severe or serious is treatment stopped — this decision would be made with your healthcare provider.
Is Mounjaro vs Wegovy different in terms of side effects?
Both are GLP-1-based medications with similar gastrointestinal side effects, though Mounjaro (tirzepatide) uses a dual GIP and GLP-1 mechanism. Clinical trial data shows slightly higher weight loss with Mounjaro, but side effect profiles are comparable. Individual tolerance varies significantly between people.
When should I contact my GP about Mounjaro symptoms?
Contact your GP if side effects are severe, persistent beyond 4 weeks, or worsening. Seek urgent help (NHS 111 or A&E) for severe stomach pain, allergic reactions, vision changes, or difficulty breathing. Always report any concerning symptoms at your next appointment or dose review with your prescriber.
Ready to take control of your weight loss journey?
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