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 Mounjaro Take To Start Working?
Mounjaro (tirzepatide) typically begins affecting appetite and food intake within the first few weeks, though noticeable weight loss usually takes 4–8 weeks to become measurable. The timeline depends on your starting dose, how your body responds, and whether you're following a reduced-calorie diet alongside the injection. Because Mounjaro is prescription-only and requires individualised dose titration, your GP or prescriber will monitor your progress and adjust your dose based on your response. Results vary significantly between individuals.
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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.
Timeline: When Mounjaro Effects Appear
Weeks 1–2 (2.5 mg starting dose): You may notice appetite changes or mild nausea as your body adjusts. Weight loss is not yet typically visible. This is the adjustment phase.
Weeks 3–4: Many people report reduced food cravings and feeling fuller after smaller meals. Subtle changes in hunger patterns may emerge. Nausea often improves as your body adapts.
Weeks 5–8 (5 mg dose if escalated): Noticeable weight loss typically becomes measurable on scales. The appetite-suppressing effects become more pronounced. Clinical trials showed measurable weight loss usually appears in this window.
Weeks 9–16+ (continued dose titration to 7.5 mg, 10 mg, or higher): Weight loss continues to accumulate as your dose increases. Each dose increase may cause temporary nausea that typically settles within days. Your GP or prescriber adjusts based on your tolerance and response.
Week 21+ (maintenance at target dose, typically 15 mg maximum): Maximum weight loss effects are usually reached. The kwikpen delivery system makes weekly injections straightforward. Long-term results depend on sustained adherence and lifestyle changes.
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.
Key Factors Affecting Timeline
Dose titration schedule: Mounjaro requires a 4-week minimum on each dose before escalating. Faster timelines are not recommended — gradual escalation reduces side effects and allows your body to adapt.
Diet and activity: Weight loss is slower if you're not maintaining a reduced-calorie diet or increasing physical activity. Mounjaro works best alongside lifestyle changes, not instead of them.
Individual variation: Some people see measurable loss by week 3–4; others take 8–10 weeks. Metabolism, starting weight, age, and existing health conditions all influence the speed of response.
Consistency: Missing injections delays results. The once-weekly schedule is designed for consistent weekly dosing — adherence is crucial for effectiveness.
Use our GLP-1 weight loss calculator to estimate potential weight loss at different doses based on clinical trial data. Your BMI and starting weight affect projected timelines.
- 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.
Why does Mounjaro take weeks to show weight loss if I feel full immediately?
Feeling full is an appetite signal — your brain receives it within days. But weight loss requires sustained calorie deficit over time. You may eat less immediately, but your body takes weeks to mobilise stored fat and show scale changes. Clinical trials measured weight loss at 4+ weeks because this is when consistent deficits show measurable results.
Can I escalate my Mounjaro dose faster to lose weight quicker?
No. Mounjaro requires a minimum 4-week period on each dose before increasing. Faster escalation increases nausea and side effects without improving weight loss speed. Your prescriber will manage dose titration carefully. Faster weight loss is not safer — sustainable loss of 0.5–1 kg per week (NHS guideline) is recommended.
What if I don't see weight loss after 8 weeks on Mounjaro?
If you're not losing weight by week 8–12, speak to your GP or prescriber. Possible reasons include: insufficient calorie deficit, very low starting weight, certain medications interfering, or medical conditions affecting metabolism. Your prescriber may adjust your dose, review your diet, or assess for other factors.
Is Mounjaro the same speed as Wegovy for weight loss?
Mounjaro (tirzepatide, dual GIP+GLP-1 agonist) and Wegovy (semaglutide, GLP-1 agonist only) work through different mechanisms. Mounjaro showed slightly higher average weight loss in trials (~20.9% vs ~14.9%), but individual timelines vary. Both require weeks 4–8 for measurable results. Your prescriber will advise which suits you.
Does Mounjaro work faster at higher doses?
Slightly, but not dramatically. Clinical trials show marginal additional weight loss moving from 10 mg to 15 mg. The main benefit of dose escalation is reaching your individual optimal response point — not necessarily faster overall loss. Gradual titration also reduces side effects, making adherence easier long-term.
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.