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 Injection Side Effects — UK NHS Access & Cost
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection available in the UK through NHS specialist services or private prescribing routes. Understanding both the potential side effects and how to access this medication — whether through the NHS or privately — helps you make informed decisions about whether it's right for you. This guide covers what to expect from side effects, how the NHS pathway works, and what private access typically involves.
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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 What to Expect
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means side effects are often related to how your digestive system adjusts to the medication. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain — most commonly during the initial dose titration phase. These often improve as your body adjusts over weeks.
- Nausea: Most common initially; typically reduces after the first 2–4 weeks
- Diarrhoea or constipation: Manage with hydration and dietary adjustments
- Stomach pain: Usually mild; severe persistent pain requires urgent medical attention
- Injection site reactions: Redness or bruising at injection site — rotate sites weekly to minimise
- Hair loss: Reported by some users; typically temporary
Serious side effects are rare but require immediate GP attention: severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, or signs of allergic reaction. Always report unexpected symptoms to your prescriber.
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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 Access — Cost, Timeline, and Process
NHS Access: Mounjaro is available through NHS tier 3 specialist weight management services, typically following GP referral. Access varies by region — some ICBs (Integrated Care Boards) have established pathways, whilst others have lengthy waiting lists of many months. NHS prescriptions cost £9.65 per item (standard prescription charge) if eligible, or free if you qualify for exemptions.
Private Access: Available through GPhC-registered online pharmacies following a clinical consultation with a UK-licensed prescriber. No NHS referral needed. Private costs typically range from £129–£320+ per month depending on dose. The process is faster — usually assessment and prescription within 1–7 days — making it an option when NHS waiting lists are long. Use our GLP-1 weight loss calculator to estimate potential results at different doses.
- 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?
Nausea, diarrhoea, constipation, and stomach pain are very common, especially during dose titration. Most side effects improve within 2–4 weeks as your body adjusts. Rotating injection sites reduces local reactions. If side effects persist or worsen, discuss with your prescriber about adjusting your dose schedule.
How do I access Mounjaro through the NHS?
Ask your GP for a referral to your local tier 3 specialist weight management service. You'll need a BMI of 30 or above (or 27+ with weight-related health conditions) to qualify. Assessment timelines vary by region — some areas have waiting lists of several months. NHS prescription cost is £9.65 per item if you're not exempt.
What does Mounjaro cost privately in the UK?
Private Mounjaro costs typically range from £129–£320+ per month depending on your dose (2.5mg to 15mg). This includes the prescription assessment and dispensing. Costs vary between providers, so it's worth comparing. Private access is usually faster than NHS, with prescriptions often issued within days.
How long does private Mounjaro prescription take?
Most GPhC-registered online services provide assessment and prescription within 1–7 days. You'll complete a clinical questionnaire, your health information is reviewed by a UK-licensed prescriber, and if approved, your prescription is sent to a pharmacy for dispensing. Delivery usually takes 1–3 working days.
Can I switch between NHS and private Mounjaro?
Technically yes, but it requires careful management. If you start privately and later access NHS services, your GP needs to be informed. Conversely, NHS patients can explore private options if NHS waiting times are long. Always inform your current prescriber before switching routes to ensure continuity of care and appropriate dose management.
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.