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: What to Expect and How to Access Treatment in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection that's widely discussed in the UK, but understanding both the side effects and how to actually access it is essential before starting. This guide covers what side effects you may experience, realistic costs, and the two main routes to getting Mounjaro in the UK—whether through NHS specialist services or private prescription. If you're considering this dual GIP and GLP-1 receptor agonist, it's important to know what you're signing up for and what options are available to you.
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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: What You May Experience
Common side effects of Mounjaro are mainly gastrointestinal and tend to improve over time as your body adjusts. The most frequently reported include nausea, diarrhoea, vomiting, and constipation—affecting more than 1 in 10 people in clinical trials. Stomach pain and indigestion are also common, alongside fatigue and dizziness. Injection site reactions like redness or bruising can occur, though these are usually mild. Hair loss has been reported by some users during gradual weight management, though this is typically temporary.
- Nausea is the most common side effect, especially in the first 4 weeks
- Gastrointestinal symptoms often improve as you stay on the medication
- Dose escalation may temporarily increase side effects before settling
- Serious side effects are rare but require immediate medical attention—including severe stomach pain (possible pancreatitis), gallbladder problems, or serious allergic reactions
Individual tolerance varies significantly. Some people experience minimal side effects, whilst others find them quite bothersome initially. Your prescriber will monitor you throughout the dose titration process to assess how you're tolerating the medication. If side effects are severe, your dose may be adjusted or you may be switched to an alternative weight loss medication like semaglutide (Wegovy).
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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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How to Access Mounjaro in the UK: NHS vs Private Routes and Real Costs
In the UK, there are two distinct pathways to accessing Mounjaro: NHS specialist services and private prescription. The NHS route involves GP referral to a tier 3 or 4 specialist weight management service, where you'll undergo assessment and potentially receive Mounjaro free at point of use—if you meet strict eligibility criteria and your local integrated care board has funded it. However, NHS waiting lists for specialist services can be extremely long in many areas, sometimes 12–24 months or more.
The private route offers faster access through an online clinical assessment with a UK-licensed prescriber, typically available within days. Private costs range from approximately £129–£320 per month depending on your dose—the KwikPen is dispensed by a GPhC-registered pharmacy following your prescription. You'll need to cover costs yourself, though some people use private health insurance. Both routes require a prescription; Mounjaro cannot be purchased without one. Use our GLP-1 weight loss calculator above to explore potential outcomes at different dose levels.
The online assessment process for private prescription typically involves: completing a detailed health questionnaire, providing current weight and height (use our free weight converter to ensure accurate metric measurements), and having a video or phone consultation with a prescriber who assesses your eligibility based on BMI and health conditions. If approved, your prescription is sent to a pharmacy and the Mounjaro KwikPen is posted to you with full guidance. The whole process usually takes 3–7 days. Check your BMI using our free BMI calculator to understand where you fall on the NHS eligibility thresholds—adults with a BMI of 30 or above, or 27–29.9 with weight-related comorbidities, may be candidates. Private prescribers will also assess whether tirzepatide is suitable given your medical history, medications, and any contraindications.
- 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 side effects are nausea, diarrhoea, vomiting, and constipation, affecting more than 1 in 10 people. Stomach pain, indigestion, fatigue, and dizziness are also reported. Most gastrointestinal side effects improve within a few weeks as your body adjusts. Injection site reactions like mild redness or bruising occasionally occur. Serious side effects such as pancreatitis or severe allergic reactions are rare but require immediate medical attention.
How much does Mounjaro cost on the NHS in the UK?
Mounjaro is free through the NHS if prescribed by a specialist weight management service, as you'll receive it at point of use. However, NHS access varies significantly by region and involves lengthy referral and assessment processes. Private prescription costs range from approximately £129–£320 per month depending on your dose, paid directly to the pharmacy. The 2.5mg starting dose is typically less expensive than higher maintenance doses like 15mg.
Can I access Mounjaro privately without seeing my GP first?
Yes. Private online prescribers can assess and prescribe Mounjaro without involving your GP, provided you meet eligibility criteria (BMI 30 or above, or 27–29.9 with comorbidities). However, it's good practice to inform your GP that you're starting weight loss medication. If you're already taking other medications, your prescriber will check for interactions. The private route typically takes 3–7 days from initial consultation to receiving your KwikPen.
Is Mounjaro or Wegovy better for minimising side effects?
Both are GLP-1-based medications with similar side effect profiles, though Mounjaro is a dual GIP and GLP-1 agonist. Some people tolerate one better than the other, but there's no universal answer—it's individual. Clinical trials show comparable nausea and gastrointestinal side effects between them. Your prescriber may recommend one based on your health history, other medications, and personal tolerance if you've previously taken GLP-1 medication.
What should I do if I experience severe side effects from Mounjaro?
Contact your prescriber immediately if you experience severe stomach pain (possible pancreatitis), persistent vomiting, serious allergic reactions, or gallbladder symptoms. For NHS patients, call 111 or contact your specialist service. For private patients, your prescriber should have emergency contact details. Do not stop the medication abruptly without guidance—your prescriber may adjust your dose or recommend an alternative. Side effect severity varies; mild nausea often resolves without intervention.
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