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 weight management injection licensed in the UK. Common side effects include nausea, diarrhoea, and stomach discomfort, which typically improve over time as your body adjusts to each dose. Understanding these effects, alongside UK access routes and costs, helps you make an informed decision about whether this medication is right for you. This guide covers what to expect, how to access Mounjaro in the UK, and the cost differences between NHS and private routes.
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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 and Management
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These often occur during the first few weeks after starting or increasing your dose. Common side effects (up to 1 in 10 people) include indigestion, acid reflux, injection site reactions, fatigue, dizziness, and hair loss. Most side effects are mild to moderate and tend to decrease as your body adjusts to the Mounjaro KwikPen injection.
- Nausea and vomiting typically peak in the first 1–2 weeks of each dose increase
- Eating smaller, frequent meals and avoiding fatty foods may help manage gastrointestinal symptoms
- Stay hydrated and increase fibre intake gradually to manage constipation or diarrhoea
- Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder problems, and changes in vision
- If you experience severe or persistent symptoms, contact your GP or seek urgent medical attention
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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.
Accessing Mounjaro in the UK: NHS vs Private Routes
In the UK, you can access Mounjaro through two main routes: NHS specialist services or private prescription. NHS access is through tier 3 or tier 4 specialist weight management services, typically requiring a GP referral. Availability varies significantly by region, and waiting lists can be lengthy. Private access is faster — you can have an online consultation with a UK-licensed prescriber within days, receive a prescription if clinically appropriate, and have your Mounjaro dispensed by a registered pharmacy. Private costs typically range from £150–£320 per month depending on your dose and provider. With the BMI calculator on this page, you can check your starting point and discuss eligibility with a healthcare professional.
- 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 are the most common Mounjaro side effects?
The most common are nausea, diarrhoea, constipation, and stomach pain. These affect more than 1 in 10 people and usually improve within a few weeks as your body adapts to the tirzepatide dose. Starting at a low dose and increasing gradually (dose titration) helps minimise side effects.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs typically range from £150–£320 per month, depending on your dose (2.5 mg to 15 mg) and the provider. Each KwikPen contains 4 weeks of doses. Prices vary between online prescription services, so it's worth comparing options before starting treatment.
Can I get Mounjaro on the NHS?
Yes, but access is limited to specialist tier 3 or tier 4 weight management services. You'll need a GP referral and must meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). NHS availability varies by region and waiting lists can be long.
How do I access Mounjaro privately?
You complete an online consultation with a UK-licensed prescriber, who assesses your eligibility. If approved, you receive a prescription that's dispensed by a registered pharmacy. The process typically takes 2–7 days from consultation to receiving your first Mounjaro KwikPen.
Are Mounjaro side effects permanent?
No. Most side effects are temporary and improve within weeks as your body adjusts. Serious side effects are rare. If you experience severe symptoms (severe stomach pain, gallbladder problems, or vision changes), contact your GP immediately.
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.