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 (Tirzepatide) in the UK — Complete Information
Mounjaro is a prescription-only weight management injection containing tirzepatide, licensed in the UK for adults with obesity or overweight with health conditions. It's a dual GIP and GLP-1 receptor agonist — meaning it works differently from other weight loss injections by targeting two hormone pathways simultaneously. Administered once weekly via a pre-filled KwikPen, Mounjaro may help reduce appetite and support weight loss alongside diet and physical activity. This guide covers how tirzepatide works in the UK, eligibility criteria, available doses, potential side effects, and how to access it through both NHS specialist services and 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.
How Mounjaro Tirzepatide Works and UK Eligibility
Mechanism of action: Mounjaro activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help regulate appetite, reduce food cravings, and support feeling fuller after smaller meals. It also slows gastric emptying, though this effect often reduces over time. Unlike insulin, tirzepatide does not directly lower blood sugar in non-diabetic individuals — it works by modifying appetite signals in the brain.
- UK eligibility: Adults with BMI ≥30 kg/m², or BMI 27–29.9 with weight-related health conditions (type 2 diabetes, prediabetes, high blood pressure, abnormal blood fats, sleep apnoea, or cardiovascular disease history)
- Ethnicity adjustment: South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds (BMI ≥27.5) due to higher health risks at lower weights
- Not suitable: Under 18, pregnant, breastfeeding, or with a history of thyroid cancer or multiple endocrine neoplasia
Your GP or a licensed prescriber will assess whether tirzepatide is appropriate for your individual health.
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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
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Mounjaro Doses, Weight Loss Results, and Side Effects
Dose range: Mounjaro starts at 2.5 mg weekly and increases gradually to a maximum of 15 mg weekly, with a minimum of 4 weeks at each dose level. Clinical trials (SURMOUNT-1) showed average weight loss of approximately 15–21% of body weight over 72 weeks at maximum dose, alongside reduced-calorie diet and increased physical activity — though individual results vary significantly.
Common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain, particularly during dose increases. Most settle within days or weeks. Serious side effects are rare but may include severe abdominal pain (possible pancreatitis), gallbladder problems, or serious allergic reactions — seek immediate medical attention if these occur. Track your progress with our free progress tracker to monitor your response over time.
Storage: Unopened pens must be refrigerated at 2–8°C and kept in their original carton. After first use, pens can be stored at room temperature (below 30°C) for up to 30 days. Missed doses: If you miss a dose within 4 days, take it as soon as you remember and continue your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next injection on the scheduled day — never take a double dose.
Prescription requirement: Tirzepatide (Mounjaro) is prescription-only in the UK. Access is available through NHS specialist weight management services (tier 3) following GP referral, or privately through GPhC-registered online prescribing services following a clinical assessment. NHS availability varies significantly by region and waiting lists can be lengthy. Our GLP-1 weight loss calculator can help you estimate potential weight loss outcomes 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 is the difference between Mounjaro and Wegovy?
Mounjaro contains tirzepatide (dual GIP + GLP-1 agonist), while Wegovy contains semaglutide (GLP-1 agonist only). Both are once-weekly injections licensed for weight management in the UK. Mounjaro starts at 2.5 mg up to 15 mg maximum; Wegovy starts at 0.25 mg up to 2.4 mg. Clinical trial results show similar average weight loss (~15–21% body weight), but individual responses vary. Your prescriber will recommend the most suitable option for your health.
Can I get Mounjaro tirzepatide on the NHS in the UK?
Yes, Mounjaro is approved by NICE for use within NHS specialist weight management services (tier 3). You'll need a GP referral to a specialist service, and eligibility will be assessed based on your BMI and health conditions. Availability and waiting times vary significantly by region (ICB). Private prescribing through online services is also available without NHS referral, following a clinical assessment.
What are the main side effects of tirzepatide (Mounjaro)?
Most common side effects are nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. These often settle within days or weeks. Serious side effects are rare but include severe abdominal pain (possible pancreatitis), gallbladder problems, or allergic reactions. Always report any concerning symptoms to your prescriber immediately.
How much weight can I expect to lose on Mounjaro tirzepatide?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight over 72 weeks at maximum dose, alongside diet and physical activity changes. Individual results vary significantly — some people lose more, some less, depending on genetics, lifestyle adherence, and other factors. Your prescriber can give you realistic expectations for your situation.
How do I inject Mounjaro tirzepatide (KwikPen)?
Mounjaro comes as a pre-filled KwikPen with simple instructions. Inject once weekly under the skin in your stomach, upper thigh, or upper arm (arm requires assistance). Rotate injection sites each week. Each KwikPen contains 4 weekly doses. Never share pens, and dispose of used pens according to your prescriber's guidance — usually in a sharps bin.
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