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
Does Mounjaro Start Working Straight Away?
Mounjaro (tirzepatide) does not typically produce immediate weight loss results, even after your first injection. Most people begin to notice changes in appetite and eating patterns within 1–2 weeks, but significant weight loss usually takes 4–8 weeks or longer. This is because Mounjaro uses a gradual dose titration schedule, starting at 2.5 mg and increasing every 4 weeks. Individual responses vary, and results depend on diet, activity level, and how your body responds to the medication.
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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.
How Mounjaro Works and Timeline Expectations
Mounjaro is a dual GIP and GLP-1 receptor agonist — this dual mechanism helps regulate appetite and blood sugar differently than single-action medications like Wegovy (semaglutide). When you receive your first injection, tirzepatide begins activating these hormone pathways, but the dose is intentionally low (2.5 mg) to allow your body to adjust and minimise side effects.
Timeline for effects:
- Week 1–2: Some people notice reduced appetite or changes in food preferences
- Week 4–8: More noticeable appetite suppression and reduced food cravings
- Week 8–12: Early weight loss may become apparent, especially with dietary changes
- Week 12–20: Dose is typically escalated; weight loss often accelerates at higher doses
- Month 6+: Most significant results appear after reaching maintenance dose (often 10–15 mg)
The NHS and clinical guidelines emphasise that Mounjaro works best alongside a reduced-calorie diet and increased physical activity — medication alone is not enough.
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.
What Happens in the First 4 Weeks
Your first 4 weeks on Mounjaro are an adjustment phase, not a weight loss phase. At 2.5 mg, many people experience mild nausea, changes in digestion, or reduced appetite — these are signs the medication is working, but significant weight reduction is unlikely yet. Some people lose no weight in week 1–4; others lose a small amount. This is completely normal. Your prescriber will keep you on 2.5 mg for a full 4 weeks before increasing to 5 mg, allowing your body to adapt. Use our GLP-1 weight loss calculator to estimate potential results at different doses — this helps set realistic expectations based on clinical trial data.
- 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 after my first Mounjaro injection?
Most people do not experience significant weight loss from a single 2.5 mg injection. You may notice reduced appetite within days, but actual weight loss typically takes 4–8 weeks to become apparent. Individual responses vary. Weight loss is most noticeable after dose increases and when combined with dietary changes.
Why is Mounjaro dosed gradually rather than starting at the full dose?
Gradual dose titration (starting at 2.5 mg and increasing every 4 weeks) allows your body to adjust to tirzepatide and reduces side effects like nausea and vomiting. Starting at the maximum dose would cause severe side effects for most people. This approach is safer and more sustainable.
How much weight can I expect to lose with Mounjaro?
Clinical trials (SURMOUNT-1) showed average weight loss of 15–21% of body weight over 72 weeks at maintenance doses. Individual results vary significantly based on dose, diet, activity, and genetics. Your GP or prescriber can discuss realistic expectations for your situation.
Is Mounjaro faster than Wegovy for weight loss?
Mounjaro (tirzepatide) has shown slightly higher average weight loss than Wegovy (semaglutide) in clinical trials, but differences are modest. Individual results vary more than differences between medications. Your prescriber will recommend based on your health profile and eligibility.
When should I contact my GP if Mounjaro isn't working?
Give Mounjaro at least 12–16 weeks (3–4 dose increases) before deciding it is not working. If you experience severe side effects, persistent nausea, or have health concerns at any point, contact your GP or prescriber immediately. They can assess whether to continue, adjust dose, or try an alternative.
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.