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 Pen Side Effects: Cost and Access in the UK
Mounjaro (tirzepatide) is a prescription-only injection for weight management available in the UK through both NHS specialist services and private prescription routes. Understanding the potential side effects, costs, and how to access it helps you make an informed decision about whether this medication is right for you. This guide covers what to expect, how to get a Mounjaro KwikPen prescription, and the typical costs involved in UK private prescribing.
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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.
Mounjaro Side Effects and Cost Access Routes
Mounjaro KwikPen can cause side effects ranging from mild gastrointestinal symptoms to more serious reactions. Most common side effects include nausea, diarrhoea, vomiting, and constipation—typically more pronounced when starting or increasing doses. These often improve within days to weeks as your body adjusts. Injection site reactions (redness, bruising, mild pain) are also common. More serious side effects like severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions require immediate medical attention.
- Very common: Nausea, diarrhoea, vomiting, constipation, stomach pain
- Common: Indigestion, acid reflux, injection site reactions, fatigue, dizziness, hair loss
- Serious (seek urgent help): Severe abdominal pain, gallbladder symptoms, allergic reactions, vision changes
Because side effects and responses vary individually, a qualified prescriber assesses your full health history before issuing a prescription. Use our GLP-1 weight loss calculator to explore potential weight loss outcomes alongside discussing side effect tolerability with a 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.
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Private vs NHS Access: How to Get Mounjaro in the UK
NHS route: Mounjaro is available through NHS tier 3 specialist weight management services following referral from your GP. However, access is limited and varies significantly by region (ICB). Waiting lists can be months to years in many areas, and not all ICBs currently offer tirzepatide. Private route: GPhC-registered online pharmacies dispense Mounjaro following an online consultation with a UK-licensed prescriber. This typically costs £129–£320 per month depending on your dose. The prescriber assesses your eligibility based on BMI, health conditions, and medical history. Results vary between individuals; our BMI calculator helps you understand your current status before consultation.
The online consultation process is straightforward: complete a health questionnaire, consult with a licensed prescriber (often via video), and receive a prescription if appropriate. Your medication is then dispatched by a registered pharmacy. Important: Mounjaro requires a prescription and is not available to buy without clinical assessment. Ensure any provider is GPhC-registered to verify legitimacy. Costs may vary by dose (2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, or 15mg weekly), with higher doses typically more expensive.
- 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 in the UK?
The most common side effects are nausea, diarrhoea, vomiting, and constipation, particularly when starting or increasing doses. These often settle within a few days to weeks. Injection site reactions (redness, bruising) are also common. Most people find these manageable, though some may need to adjust their dose or timing.
How much does Mounjaro cost privately in the UK?
Private Mounjaro prescriptions typically cost between £129 and £320 per month, depending on your dose and the pharmacy provider. Lower starting doses (2.5mg, 5mg) are cheaper than higher maintenance doses. Costs may include both the prescription fee and the medication cost, so clarify the total with your provider.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is approved by NICE for NHS use through tier 3 specialist weight management services. However, availability varies by region and waiting lists can be lengthy. You'll need a referral from your GP to access NHS services. Private prescription is an alternative if NHS access is limited in your area.
What's the difference between Mounjaro vs Wegovy for side effects?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both cause similar gastrointestinal side effects, though clinical data suggests slightly different profiles. Individual tolerance varies significantly, so what one person experiences may differ from another.
Do I need a prescription to buy Mounjaro in the UK?
Yes, Mounjaro is prescription-only in the UK. You cannot buy it without a valid prescription from a UK-licensed prescriber. Access is through either NHS specialist services or private online prescribers who conduct a health assessment before issuing a 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.