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 Weight Loss Side Effects — What NHS Patients Should Know
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK and available through NHS specialist services and private prescribing. Like all medications, it can cause side effects — most commonly nausea, diarrhoea, and stomach discomfort. Understanding what to expect, which side effects are typical, and when to seek medical attention is essential for anyone considering this treatment. This guide covers the evidence from clinical trials and NHS prescribing guidelines to help you make an informed decision.
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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 and Serious Side Effects — NHS Evidence
In the SURMOUNT clinical trials, most side effects were mild to moderate and temporary, typically improving within a few weeks as your body adjusts. The most common side effects include:
- Nausea — affects more than 1 in 10 people, usually improves over time
- Diarrhoea and constipation — occur during dose titration phases
- Stomach pain and indigestion — may persist at higher doses
- Injection site reactions — redness, bruising, or tenderness at injection sites
- Fatigue and dizziness — reported in up to 1 in 10 people
- Hair loss — rare but documented during gradual weight management
Serious side effects requiring immediate medical attention are uncommon but include severe stomach pain (possible pancreatitis), changes in vision (diabetic retinopathy complications), and gallbladder problems. Your GP should be contacted immediately if you experience any of these. The MHRA monitors Mounjaro closely under additional surveillance, meaning new safety data is reviewed continuously.
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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
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NHS Prescribing and Managing Side Effects
The NHS tier 3 weight management services prescribe Mounjaro through a careful dose titration schedule, starting at 2.5 mg weekly and escalating every 4 weeks to allow your body to adjust. This gradual approach helps minimise side effects. If nausea or other effects are severe, your prescriber can pause escalation or stay at the current dose longer. Some side effects — particularly gastrointestinal symptoms — often improve as treatment continues, though this varies individually. Your GP should monitor you regularly during treatment, and you can report side effects through routine appointments or the yellow card adverse event system. Use our free GLP-1 weight loss calculator to understand projected outcomes at each dose level, and track your side effect experience with our progress tracker to share with your healthcare team.
- 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.
Is nausea with Mounjaro permanent?
No. Nausea is very common when starting Mounjaro but typically improves within 2–4 weeks as your body adjusts. It may return temporarily when doses are increased, but usually settles again. If nausea is severe or persistent, your prescriber can slow dose escalation or adjust your approach.
Can I stop Mounjaro if side effects are too bad?
Yes. Mounjaro can be discontinued at any time if side effects are intolerable. Discuss this with your GP or prescriber — they may also adjust your dose or timeline rather than stopping completely. Most people who experience significant side effects find they improve naturally over time.
Are Mounjaro side effects worse than Wegovy?
Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Side effect profiles are similar, though some people tolerate one better than the other. Individual responses vary — your prescriber can help choose the best option for you.
What should I do if I get severe stomach pain on Mounjaro?
Severe stomach pain spreading to the back may indicate pancreatitis, a rare but serious condition. Stop the medication immediately and seek emergency medical attention or call 999. Do not wait for a GP appointment — this requires urgent assessment.
Will Mounjaro side effects affect my daily life?
Most people can manage daily activities despite mild side effects. Nausea and diarrhoea may be uncomfortable but typically don't prevent work or normal routines. Severe effects are uncommon but should be reported to your GP immediately. Your prescriber can adjust treatment if side effects significantly impact your quality of life.
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.