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 Long Does It Take Mounjaro To Work?
Mounjaro (tirzepatide) doesn't produce immediate weight loss results — most people begin noticing changes within 2–4 weeks of starting, with more significant effects emerging after 8–12 weeks at a stable dose. The timeline depends on your starting dose, how quickly your dose is increased, your individual metabolism, and how consistently you combine the injection with a reduced-calorie diet and physical activity. Understanding what to expect helps you stay motivated during the early weeks of treatment.
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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.
Understanding Mounjaro's Timeline and Dose Titration
Mounjaro works through a gradual dose escalation process designed to help your body adjust whilst minimising side effects. You start on 2.5 mg once weekly for 4 weeks, then increase to 5 mg, with further increases (7.5 mg, 10 mg, 12.5 mg, 15 mg) every 4 weeks if tolerated. Most weight loss happens once you reach a stable maintenance dose, typically between weeks 12–20 of treatment. Clinical trial data showed participants on the 15 mg dose (the maximum) lost an average of approximately 20.9% of their body weight, but this was measured over 72 weeks alongside diet and exercise changes.
- Weeks 1–4: Starting dose (2.5 mg) — you may notice reduced appetite but weight loss is usually minimal
- Weeks 5–8: First escalation to 5 mg — appetite suppression increases, modest weight loss may begin
- Weeks 9–16: Further dose increases — more noticeable changes in hunger and satiety
- Weeks 17+: Maintenance dose — maximum weight loss effects typically occur here
Individual responses vary significantly. Some people feel the appetite-suppressing effects immediately, whilst others take several weeks to notice changes. Your UK prescriber will monitor your progress and adjust your dose accordingly during tier 3 NHS specialist services or through a private online prescription pathway.
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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 Access and Realistic Expectations
If you're accessing Mounjaro through NHS specialist tier 3 weight management services, the timeline extends beyond just the medication — you'll typically receive support with diet, physical activity, and behaviour change alongside your prescriptions. This integrated approach may improve outcomes compared to injection alone. However, NHS waiting lists for specialist services can be lengthy in many areas, meaning you might wait months before starting treatment. Private online prescribing offers faster access but still requires the same 4-week dose escalation schedule. Weight loss isn't assured at any dose, and your prescriber will continue assessing whether the medication is working for you at each review point.
Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses, and track your progress with our free progress tracker to monitor changes over weeks and months.
- 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 Mounjaro work for me immediately?
No. Mounjaro requires a gradual dose escalation over 4–5 months to reach maximum effect. Most people notice appetite changes within 1–2 weeks, but meaningful weight loss typically begins around weeks 8–12 once you're on a higher, stable dose. Individual results vary based on metabolism, diet, and exercise habits.
How much weight will I lose on Mounjaro in the first month?
Very little in the first month. Clinical trials showed minimal weight loss in the first 4 weeks. Weight loss accelerates once you've reached a maintenance dose (usually weeks 12–20). Expecting dramatic early results can lead to disappointment — focus on appetite changes as your early signal that it's working.
What if Mounjaro isn't working after 12 weeks?
Your UK prescriber will assess your progress during reviews. If you've reached your target dose and haven't lost weight, your prescriber may continue monitoring, increase your dose further (if available), consider alternative medications, or explore whether diet and exercise changes are needed. Never stop Mounjaro suddenly without medical guidance.
Does the mounjaro KwikPen work faster than older tirzepatide formulations?
The KwikPen is simply the injection device — it doesn't change how quickly tirzepatide works in your body. The active ingredient (tirzepatide) enters your system the same way regardless of device. Timeline and effectiveness depend on dose, not the pen type.
How does Mounjaro compare to Wegovy in terms of how fast it works?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggested Mounjaro may produce slightly greater weight loss on average, but timelines are similar — both require 4+ weeks of dose escalation before maximum effects. Individual results vary significantly between people.
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.