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, Cost & How to Access in the UK
Mounjaro (tirzepatide) is a prescription-only weight loss jab that's increasingly popular in the UK for managing excess weight. This guide explains the common side effects you may experience, how much it costs through different routes, and the practical steps to access this weekly weight loss injection. Whether you're considering NHS specialist services or private prescription options, understanding both the benefits and potential effects is essential before starting treatment.
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Calculate Your BMI Instantly
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 What to Expect
Mounjaro is a dual GIP and GLP-1 receptor agonist — a weekly weight loss injection that works differently from other weight management medications. Most side effects occur early in treatment and often improve as your body adjusts.
Very common side effects (affecting more than 1 in 10 people):
- Nausea and vomiting (especially in the first 2–4 weeks)
- Diarrhoea or constipation
- Stomach pain or indigestion
- Loss of appetite
Common side effects (affecting up to 1 in 10 people):
- Acid reflux (GORD)
- Injection site reactions (redness, bruising)
- Fatigue or dizziness
- Hair loss (alopecia) — reported in some users
- Headache
Most gastrointestinal effects reduce significantly after the first 4–8 weeks as your body adapts to the KwikPen dose. Your prescriber will monitor you and adjust your dose if needed. Use our free GLP-1 weight loss calculator to understand potential outcomes and discuss realistic expectations with your healthcare provider.
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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.
Cost and Access Routes in the UK
Accessing Mounjaro in the UK depends on your eligibility and preferred route. NHS access: NICE has approved tirzepatide for specialist weight management services. You'll need a GP referral to a tier 3 specialist service, though availability and waiting times vary significantly by region. Many areas have waiting lists of several months or longer.
Private access: Available through GPhC-registered online pharmacies following an online clinical consultation with a UK-licensed prescriber. Costs typically range from £129–£320 per month depending on your dose and provider. Private prescription offers faster access without NHS referral.
Both routes require a clinical assessment to confirm you meet eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Track your progress with our free progress tracker once you start treatment, and use our BMI calculator to monitor your health metrics throughout your weight loss journey.
- 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 Mounjaro and how does it work?
Mounjaro (tirzepatide) is a prescription-only, once-weekly injection that mimics two natural hormones — GIP and GLP-1. It helps reduce appetite, supports feeling fuller faster, and slows gastric emptying. Clinical trials showed participants lost an average of 15–21% of their body weight when combined with diet and exercise changes.
Are Mounjaro side effects permanent?
Most side effects are temporary and improve within the first 4–8 weeks as your body adjusts. Nausea, diarrhoea, and constipation typically subside. However, some effects like hair loss or appetite changes may persist longer. Always report persistent or concerning side effects to your prescriber or GP.
How much does Mounjaro cost on the NHS?
Mounjaro is available on the NHS through specialist tier 3 weight management services at no cost to eligible patients. However, NHS access requires a GP referral and availability varies significantly by region. Many areas have substantial waiting lists. Your GP can advise on your local NHS pathway.
How much does private Mounjaro cost in the UK?
Private Mounjaro costs typically range from £129–£320 per month depending on your dose (starting at 2.5 mg, up to 15 mg weekly). Prices vary by provider and dose escalation. Private access offers faster treatment initiation without NHS referral, following an online clinical consultation.
What's the difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1-only medication. Clinical trials showed Mounjaro achieved slightly higher average weight loss (up to 21% vs 15% for Wegovy), though individual results vary significantly. Both are prescription-only, once-weekly injections available through NHS specialist services or private prescription.
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.