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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Worst Side Effects of Mounjaro and How to Access It in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection that works by activating two natural hormone pathways. While clinical trials showed significant weight loss results, understanding the potential side effects and knowing how to access it in the UK is essential. This guide covers the most common adverse effects, how they typically develop, and the realistic costs and routes to prescription — whether through NHS specialist services or private online assessment with a UK-licensed prescriber.
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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.
Most Common Side Effects and What to Expect
The most frequent side effects reported during Mounjaro clinical trials were gastrointestinal, particularly during dose titration when increasing from one strength to the next. Nausea affects more than 1 in 10 people and is often the most challenging side effect, especially in the first 4 weeks. Vomiting and diarrhoea occur in similar proportions, whilst constipation also affects many users — the medication slows how quickly food moves through your stomach, which can initially cause unpredictable digestive changes.
Other common effects include stomach pain, indigestion, fatigue, and reduced appetite (which is actually the intended mechanism but can feel unpleasant initially). Injection site reactions such as redness or bruising are mild and temporary. Most side effects peak during the first 1–2 weeks at each new dose and often improve as your body adapts. However, some people experience persistent nausea throughout treatment, which is why staying in close contact with your prescriber during dose titration is important.
Serious side effects are rare but require urgent medical attention: severe, persistent stomach pain radiating to your back (possible pancreatitis), serious allergic reactions, or changes in vision. Always contact your GP immediately if you experience these symptoms.
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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.
NHS vs Private Routes: Cost, Access, and Timeline
NHS route: Mounjaro is now approved by NICE for use within NHS tier 3 specialist weight management services. However, access is severely limited by regional availability and lengthy waiting lists — in many areas, waits exceed 12–18 months. You'll need a GP referral to your local integrated care board's specialist weight management service. Once accepted, the NHS covers the full cost, but availability depends entirely on your region.
Private route: Available through GPhC-registered online pharmacies following a consultation with a UK-licensed prescriber. Private Mounjaro costs approximately £150–£280 per month depending on your dose (higher doses cost more). The consultation process is typically quick (24–48 hours approval), and prescriptions can be dispensed within days. No NHS referral is needed. This route is faster but requires you to cover the full cost.
Use our free GLP-1 weight loss calculator to understand potential outcomes at different doses, and check your BMI with our free calculator to assess eligibility before pursuing either route.
- 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.
Can I get Mounjaro on the NHS?
Yes, NICE has approved Mounjaro (tirzepatide) for use within NHS tier 3 specialist weight management services for adults with a BMI of 30 or above, or 27+ with weight-related health conditions. However, NHS availability is very limited by region, and waiting lists often exceed 12 months. Access requires a GP referral to your local integrated care board.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs vary by dose and provider, typically ranging from £150–£280 per month. The starting 2.5mg dose is usually cheaper; costs increase with 5mg, 7.5mg, 10mg, 12.5mg, and maximum 15mg doses. Some private providers offer monthly or annual pricing plans.
What's the difference between Mounjaro and Wegovy for side effects?
Both are once-weekly injections with similar side effect profiles (nausea, diarrhoea, vomiting). Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist; Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro may produce stronger appetite suppression but also reports slightly higher nausea rates in early trials. Individual responses vary significantly.
How long do Mounjaro side effects last?
Most side effects peak in the first 1–2 weeks after starting or increasing your dose and often improve within 2–4 weeks as your body adapts. However, some people experience persistent nausea throughout treatment. Side effect duration varies between individuals; always discuss ongoing symptoms with your prescriber.
What happens if I experience severe side effects on Mounjaro?
Contact your GP immediately if you experience severe, persistent stomach pain (possible pancreatitis), serious allergic reactions, or changes in vision. For moderate side effects like persistent nausea, speak with your prescriber — they may recommend staying at your current dose longer, adjusting your diet, or switching to a different weight management medication.
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.