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 Side Effects — What NHS Patients Should Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK and approved by NICE for use within NHS specialist services. Like all medications, it can cause side effects — most commonly nausea, diarrhoea, and stomach discomfort, especially when starting or increasing doses. Understanding what to expect helps you decide whether this treatment is right for you and when to contact your GP. This guide covers the most common side effects, serious warning signs, and how the NHS manages tirzepatide treatment.
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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 and NHS Management
The most frequently reported side effects of Mounjaro occur during the dose escalation phase and often improve over time as your body adjusts. 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 tend to reduce after the first few weeks on each new dose.
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss (alopecia). Your GP or specialist can offer practical strategies to manage these — such as eating smaller meals for nausea, staying hydrated for digestive issues, or adjusting injection timing.
- Start on the lowest dose (2.5 mg) to minimise initial side effects
- Increase gradually at 4-week intervals as tolerated
- Report persistent side effects to your NHS specialist team
- Never skip doses or stop Mounjaro without medical advice
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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
Whilst most side effects are mild, contact your GP or call NHS 111 immediately if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), sudden changes in vision, or severe chest pain. These are rare but require urgent assessment.
Additionally, if you have a personal or family history of thyroid cancer or medullary thyroid carcinoma, Mounjaro is not suitable for you — your GP will identify this during the assessment. Using our free BMI calculator and tracking your health context helps your NHS team make the right decision about whether tirzepatide is appropriate for your individual circumstances.
- 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 side effect of Mounjaro on the NHS?
Nausea is the most frequently reported side effect, especially when starting or increasing doses. It typically improves within a few weeks as your body adjusts. Your GP can recommend smaller meals, staying hydrated, and avoiding rich or fatty foods to manage nausea. If nausea is severe or persistent beyond 4 weeks on a dose, contact your NHS specialist team about potentially slowing your dose escalation.
How long do Mounjaro side effects usually last?
Most side effects peak in the first 1–2 weeks after starting or increasing a dose, then gradually improve over the following 2–4 weeks. Some patients report side effects resolving completely after 4–6 weeks on a stable dose. However, individual experiences vary significantly. Your NHS specialist will monitor your progress and adjust your treatment plan if side effects are significantly impacting your quality of life.
Can I take Mounjaro if I have a sensitive stomach?
If you have a history of stomach problems, pancreatitis, or severe gastroparesis, discuss this with your GP before seeking Mounjaro on the NHS. Your medical history will be carefully assessed during your specialist appointment. In some cases, a slower dose escalation or a different weight management approach may be recommended instead.
Is Mounjaro safe to use long-term according to NHS guidance?
Mounjaro is approved for ongoing use in the UK, and many patients continue it long-term under NHS specialist supervision. However, the longest completed clinical trial (SURMOUNT-1) ran for 72 weeks. Your NHS team will regularly review whether to continue, pause, or stop treatment based on your individual health outcomes and side effect tolerance.
What's the difference between Mounjaro and Wegovy side effects?
Both are weight loss injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may have a slightly higher incidence of gastrointestinal side effects due to its dual mechanism, but both medications commonly cause nausea and digestive changes. Individual tolerability varies greatly between patients.
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