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 Quickly Will Mounjaro Work in the UK?
Mounjaro (tirzepatide) typically begins to show appetite-suppressing effects within the first few weeks of starting treatment, though weight loss timelines vary significantly between individuals. In NHS and private practice, most people notice early changes within 2–4 weeks, with measurable weight loss usually visible after 8–12 weeks of consistent use. The dose escalation schedule — starting at 2.5 mg and increasing every 4 weeks — means full effects take time. This guide explains the realistic timeline for Mounjaro in UK adults and what NICE-approved data tells us about timeframes.
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.
Mounjaro Timeline: Week-by-Week Expectations
Weeks 1–4 (Starting dose: 2.5 mg) — Most people notice reduced appetite and changes in food preferences within the first 1–2 weeks. Some experience nausea early on, which often settles by week 3–4. Weight loss at this stage is typically modest, around 0.5–1 kg, though some see no change yet.
Weeks 5–8 (Dose: 5 mg) — As the dose increases, appetite suppression typically strengthens. Many people report feeling fuller faster and reduced cravings. Average weight loss increases to approximately 1–2 kg per week, though individual variation is significant.
Weeks 9–16 (Doses: 7.5 mg and 10 mg) — More noticeable weight loss becomes evident for most people. Clinical data shows average cumulative loss of 5–8 kg by week 12. Side effects may peak during escalation but often improve as the body adjusts.
Weeks 17+ (Maintenance doses: 12.5–15 mg) — At maximum dose, weight loss typically plateaus, with most weight lost within the first 16–20 weeks. After this, focus shifts to weight maintenance rather than continued rapid loss.
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.
NHS Prescribing and Realistic Expectations
Through NHS tier 3 specialist weight management services, Mounjaro (tirzepatide) is prescribed following NICE guidance on GLP-1 and dual GIP/GLP-1 agonists. The UK prescription pathway takes longer than private access — referral, assessment, and initiation can take several months depending on your local integrated care board (ICB). Once started, the NHS dosing schedule follows the same 4-week escalation pattern. Clinical trial data (SURMOUNT-1) showed average weight loss of 15–21% body weight over 72 weeks, but this required consistent use alongside reduced-calorie diet and physical activity. Individual results vary — some people lose 5% body weight, others lose over 25%. Use our free GLP-1 weight loss calculator to estimate potential outcomes based on your starting weight.
- 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 see weight loss in the first week of Mounjaro?
Most people do not see measurable weight loss in the first week — Mounjaro typically takes 2–4 weeks to reach appetite-suppressing levels. Early changes are usually appetite reduction and food preference shifts, not weight loss on the scales. This is normal and expected.
How much weight should I lose per week on Mounjaro?
Weight loss rate varies significantly. Early weeks may show 0.5–1.5 kg weekly, but average sustained loss is around 0.5–1 kg per week. Some weeks show no loss — this is normal. NHS recommends not expecting more than 1 kg per week for sustainable, healthy loss.
Why does Mounjaro take 4 weeks between dose increases?
Each dose increase requires a 4-week adjustment period so your body can adapt and your prescriber can assess tolerance and side effects. This staged approach reduces nausea and other adverse effects. Faster escalation increases side effect risk without improving weight loss outcomes.
How long before Mounjaro reaches its maximum effect?
Maximum appetite suppression typically occurs at weeks 17–20 after starting, when doses reach 12.5–15 mg. Most cumulative weight loss happens in the first 16–20 weeks. After this, the focus becomes weight maintenance rather than continued loss.
Can I get Mounjaro faster through the NHS or private route?
Private access via online prescription is faster — you can start within 1–2 weeks of consultation. NHS access through tier 3 specialist services often involves waiting lists of several months depending on your ICB. Both follow the same dose escalation timeline once started.
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.