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
How Fast Does Mounjaro Work in the UK?
Mounjaro (tirzepatide) doesn't work instantly — weight loss with this prescription injection happens gradually over weeks and months. Most people taking Mounjaro notice changes between 4 and 12 weeks, though the timeline varies significantly between individuals. The drug works by activating GLP-1 and GIP hormone pathways, which help regulate appetite and food intake. In the UK, Mounjaro is available through NHS specialist weight management services or private prescription. Understanding realistic timelines and how the medication builds up in your system helps you set achievable health goals and manage expectations.
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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.
When Does Mounjaro Start Working — The Timeline
Mounjaro begins working immediately after injection, but noticeable weight loss typically appears between weeks 4 and 12. The KwikPen device delivers a once-weekly dose that accumulates in your system over time, gradually increasing appetite regulation and satiety signals to your brain.
- Weeks 1–4 (2.5 mg starting dose): You may notice reduced appetite or changes in food cravings, though weight loss on the scales is often minimal during this adjustment phase.
- Weeks 5–8 (5 mg dose): Most people report clearer appetite suppression and may see their first meaningful weight loss — typically 2–4 kg over this 4-week period.
- Weeks 9–16 (7.5–10 mg escalation): Weight loss usually accelerates as the dose increases. Average loss during this phase is 4–8 kg across the 8-week window.
- Week 17+ (12.5–15 mg maintenance): At maximum dose, weight loss continues but at a steadier pace. Most people reach their plateau between 16 and 24 weeks.
Clinical context: The SURMOUNT-1 trial measured results at 72 weeks. Participants on 15 mg tirzepatide lost approximately 20.9% of body weight on average — but this was achieved over a full year with consistent dosing, dietary changes, and increased physical activity. Individual results vary considerably.
Clinically BackedWeight Loss
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
NHS Eligibility and Access Routes in the UK
In the UK, access to Mounjaro through the NHS is limited to specialist tier 3 weight management services, and NICE guidance supports this route for eligible patients. However, waiting lists for NHS specialist services can be extremely long — sometimes 12–24 months in some regions. This is why many people in the UK access Mounjaro via private prescription following an online clinical assessment.
Before starting Mounjaro, your prescriber (whether NHS or private) will assess whether you meet eligibility criteria: BMI ≥30, or BMI 27–29.9 with weight-related conditions such as type 2 diabetes, hypertension, or cardiovascular disease. South Asian and Black African heritage individuals may qualify at lower BMI thresholds (27.5+). Use our free BMI calculator above to check your current BMI, and our GLP-1 weight loss calculator to estimate potential 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.
How long before you see weight loss on Mounjaro?
Most people notice appetite changes within 1–2 weeks, but visible weight loss on the scales typically appears between weeks 4 and 8. The speed depends on your starting weight, how closely you follow a reduced-calorie diet, activity level, and individual metabolism. Expect gradual, steady loss rather than rapid results.
Does Mounjaro work faster at higher doses?
Higher doses don't necessarily work faster — they tend to produce greater total weight loss over time. You start at 2.5 mg and increase gradually every 4 weeks. The dose titration allows your body to adjust and minimises side effects. Maximum dose is 15 mg weekly.
What does the SURMOUNT trial show about Mounjaro speed?
SURMOUNT-1 followed participants for 72 weeks (approximately 18 months). At 15 mg weekly, participants lost an average of 20.9% body weight — roughly 18–25 kg for most people. However, this was alongside a reduced-calorie diet and exercise, not Mounjaro alone.
Can you get Mounjaro faster through the NHS or private prescription in the UK?
Private prescription is faster — you can usually start within 1–2 weeks of assessment. NHS access is through specialist tier 3 services, with waiting lists often extending many months. NICE supports NHS prescribing, but availability depends on your regional ICB and local waiting lists.
What factors affect how quickly Mounjaro works for you personally?
Speed of weight loss depends on starting weight, adherence to a reduced-calorie diet, physical activity level, age, metabolism, and individual response. Consistency matters more than the injection alone — medication supports dietary changes, it doesn't replace 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.