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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How to Start Mounjaro in the UK
Starting Mounjaro (tirzepatide) involves meeting eligibility criteria, obtaining a prescription, and following a carefully managed dose titration schedule. In the UK, Mounjaro is available through NHS specialist weight management services or private online prescribing services following a clinical assessment. Understanding the steps to start—from initial eligibility checks through your first injection—helps you prepare for this prescription-only weight management treatment. This guide covers everything you need to know about beginning Mounjaro in the UK.
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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.
UK Eligibility and Getting a Prescription
To start Mounjaro in the UK, you must meet specific eligibility criteria set by NICE and the NHS. You may be eligible if you have a BMI of 30 kg/m² or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, hypertension, dyslipidaemia, or sleep apnoea). For South Asian, Chinese, Black African, and Black Caribbean adults, lower BMI thresholds may apply due to higher health risks at lower weights.
You cannot start Mounjaro if you are under 18, pregnant, breastfeeding, or have a severe allergy to tirzepatide. Use our free BMI calculator above to check where you fall on the scale.
NHS route: Ask your GP for a referral to your local tier 3 or tier 4 specialist weight management service. Availability and waiting times vary significantly by region (ICB).
Private route: Book an online consultation with a UK-licensed prescriber through a GPhC-registered service. A clinical assessment will determine if Mounjaro is appropriate for you.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Mounjaro Dose Titration and What to Expect
Once you start Mounjaro, you'll follow a dose titration schedule—this means your dose increases gradually over time. You begin with 2.5 mg once weekly for 4 weeks, then increase to 5 mg, then to 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg (the maximum dose). Each dose is administered as a once-weekly injection using a pre-filled KwikPen device.
Most people stay on each dose for at least 4 weeks before increasing, allowing your body to adjust. Side effects like nausea typically improve as your body adapts. Common early side effects include nausea, diarrhoea, and constipation—these often reduce over the first few weeks. Track your progress and side effects using our free progress tracker to discuss with your prescriber at follow-up appointments.
- 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.
Can I start Mounjaro through my GP?
Not directly. In the UK, Mounjaro prescriptions typically come through NHS specialist tier 3/4 weight management services (which your GP can refer you to) or through private online prescribing services. Standard GP surgeries do not usually prescribe Mounjaro. Waiting times for NHS specialist services vary widely by region—some areas have significant waiting lists.
What's the difference between Mounjaro and Wegovy when starting?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both once-weekly GLP-1-type injections, but Mounjaro is a dual GIP + GLP-1 agonist, whereas Wegovy is GLP-1 only. Mounjaro starts at 2.5 mg, while Wegovy starts at 0.25 mg. Clinical trials showed Mounjaro may achieve slightly higher average weight loss (~20% vs ~15%), though individual results vary significantly. Your prescriber will assess which is more suitable for you.
How do I inject Mounjaro when I start?
Mounjaro comes as a pre-filled KwikPen device that you inject once weekly under the skin (subcutaneously). You can inject into your stomach, upper thigh, or upper arm. Rotate injection sites each week to prevent irritation. Your prescriber or nurse will demonstrate the injection technique at your first appointment—it takes just a few seconds and most people find it straightforward.
What happens if I miss my first dose or injection?
If you miss a dose within 4 days of when you should have taken it, inject as soon as you remember and continue with your normal schedule. If more than 4 days have passed, skip that dose and take your next dose on the normal scheduled day. Never double up on doses. Contact your prescriber if you're unsure.
Should I start Mounjaro with diet and exercise changes?
Yes. Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Clinical trials showed weight loss results when Mounjaro was combined with lifestyle changes. Your prescriber or a weight management dietitian can help you plan sustainable dietary and exercise adjustments. Mounjaro supports these efforts—it's not a replacement for them.
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.