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
Free tracker included with every download this week
What to Expect in Your First Week on Mounjaro
Starting Mounjaro for weight loss raises questions about what happens immediately. The reality is that week one is typically about adjustment rather than dramatic weight loss. Mounjaro (tirzepatide) is a prescription weight loss injection licensed in the UK that works by regulating appetite and blood sugar. Most people are prescribed the starting dose of 2.5mg in week one — a dose designed to allow your body to adapt gradually. Our GLP-1 weight loss calculator can help you understand potential outcomes across your full treatment plan, but week one is usually about tolerating the injection and observing early appetite changes rather than seeing the scale move significantly.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
NHS Eligibility and Week 1 Reality
To access Mounjaro through the NHS in the UK, you must be referred to a tier 3 specialist weight management service by your GP. Eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. NICE has approved tirzepatide for NHS use, though regional availability varies significantly — waiting lists can extend several months in many areas. In week one, you'll receive your first Mounjaro KwikPen pre-filled with 2.5mg. This is your starting dose, designed as a dose titration phase rather than a therapeutic dose. Most weight loss occurs during weeks 5–16 as your dose increases gradually.
- Starting dose: 2.5mg (weeks 1–4)
- Minimum 4 weeks at each dose before escalation
- Maximum dose reached at week 21+ (15mg)
- NHS or private prescription routes both available
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.
What Happens in Week 1: Physical Effects and Timeline
During your first week on Mounjaro, you may experience nausea, which is very common — affecting more than 1 in 10 people. This typically settles within days as your body adjusts. Some people report early appetite reduction, whilst others notice little change in week one. Weight loss in week 1 is rarely significant — many people see no change or lose only 0.5–1kg (1–2lbs), which is largely water weight rather than fat loss. The dose is so low at 2.5mg that it functions more as a tolerance assessment than a treatment dose. Continue your reduced-calorie diet and physical activity throughout week one. Use our free progress tracker to log how you feel, any side effects, and your starting weight — this baseline helps you see the full picture over weeks 5–16 when weight loss typically accelerates as your dose increases.
- 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.
Will I lose weight in my first week on Mounjaro?
Unlikely significantly. Week one uses the 2.5mg starting dose, a tolerance dose designed to let your body adjust. Most weight loss occurs from week 5 onwards as your dose increases. Any loss in week one is usually water weight, not fat loss. Real results emerge over the full 16–20 week dose escalation period.
What side effects should I expect in week 1?
Nausea is very common in week one but typically improves within days. Other early side effects include constipation, diarrhoea, or mild stomach discomfort. These often settle as your body adapts. If side effects are severe, contact your prescriber — they may adjust your plan.
Is the NHS or a private prescription better for Mounjaro?
Both routes require a prescription and clinical assessment. NHS access is through tier 3 specialist services and may involve longer waits but costs nothing. Private online prescribing is faster but costs £129–£320+ per month depending on dose. Choose based on timeline and budget.
How should I use my first Mounjaro KwikPen?
Your prescriber will show you how to inject. Most people inject into the stomach, thigh, or upper arm once weekly, rotating injection sites. Store your unused KwikPen in the fridge at 2–8°C. After first use, store at room temperature — it lasts 30 days.
What makes Mounjaro different from other weekly weight loss injections?
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways rather than one. This may help it deliver slightly stronger appetite regulation than single GLP-1 injections like semaglutide (Wegovy). Clinical trials showed average weight loss of 15–21% body weight over 72 weeks.
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.