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) doesn't start working immediately — it requires time to reach effective levels in your body and for your appetite regulation to adjust. Most people begin noticing changes around week 2–4, though the full effect builds gradually over weeks and months as your dose is increased. Understanding the timeline helps set realistic expectations for your weight management journey with this prescription 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 vs Wegovy — Timeline Comparison
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways that regulate appetite and food intake. This dual mechanism may contribute to its effects, but like all weight management injections, it takes time to work. During the first week, you receive a 2.5 mg starting dose — deliberately low to allow your body to adjust and minimise side effects. Over the following 4–5 weeks, you remain on this dose while your prescriber monitors your response. By week 5, if tolerated well, you escalate to 5 mg, and the dose titration continues every 4 weeks (5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg maximum). Clinical trial data from SURMOUNT-1 showed that significant weight loss — averaging 15–21% of body weight — was achieved over 72 weeks at higher doses, primarily through sustained calorie deficit alongside reduced appetite. Wegovy (semaglutide), a GLP-1-only medication, follows a similar slow titration pattern: starting at 0.25 mg, escalating to 0.5 mg, then 1 mg, 1.7 mg, and finally 2.4 mg maintenance over approximately 16–17 weeks. STEP-1 trial data showed average weight loss of around 15% over 68 weeks on Wegovy 2.4 mg. The key difference: Mounjaro's dual mechanism may offer stronger appetite suppression at lower cumulative doses, but both medications require patience and consistent use alongside dietary changes.
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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 to Expect in Your First Weeks on Mounjaro
Week 1–2: Many people notice no obvious change. The 2.5 mg dose is sub-therapeutic — designed purely for tolerance-building. You may experience mild nausea, loose stools, or reduced appetite, but weight loss is typically minimal. Week 3–4: Some users report their first real appetite changes — food feels less appealing, portions feel smaller, or cravings reduce. This is when many people begin noticing subtle weight changes on the scale. Week 5–8: After your first dose increase to 5 mg, appetite suppression often strengthens. Weight loss typically becomes more noticeable. However, results vary — some people see rapid changes, others progress more slowly.
Your prescriber will assess your progress and side effects at each milestone. If you're tolerating Mounjaro well and weight loss is progressing, your dose continues to increase every 4 weeks. If you experience troublesome side effects, your prescriber may delay escalation or keep you at a dose longer. Use our free GLP-1 weight loss calculator to estimate your potential weight loss trajectory at different doses, and track actual progress with our progress tracker tool.
- 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 if I don't change my diet?
No. Mounjaro is most effective alongside a reduced-calorie diet and increased physical activity. The medication reduces appetite and supports feeling fuller, but it doesn't burn calories on its own. Clinical trials showed weight loss occurred when participants combined Mounjaro with dietary changes and exercise. Without lifestyle changes, weight loss is minimal or absent.
How is Mounjaro's speed compared to Wegovy?
Both medications show similar average weight loss over 12–18 months (15–21% body weight). Mounjaro's dual mechanism may work slightly faster for some people, but individual variation is huge. Wegovy may suit others better. Your GP or prescriber will assess which is more appropriate based on your health profile — there's no universal "faster" option.
Is the starting Mounjaro dose (2.5 mg) actually doing anything?
The 2.5 mg starting dose is therapeutic but sub-maximal — it helps your body adapt and reduces side effects significantly. Weight loss at this dose is usually minimal, but it's essential for tolerating the medication long-term. Think of it as a necessary foundation before escalating.
What if I don't notice changes after 8 weeks on Mounjaro?
Individual responses vary considerably. Some people see changes within 2–3 weeks; others take 8–12 weeks. If you're not losing weight after 8 weeks, discuss this with your prescriber. They may adjust your dose escalation schedule, review your diet adherence, or assess whether Mounjaro is the right medication for you.
Can I get Mounjaro quickly through the NHS in the UK?
NHS access to tirzepatide (Mounjaro) is through specialist tier 3/4 weight management services, typically requiring a GP referral. Waiting times vary significantly by region — some areas have waits of months to years. Private prescribing offers faster access if you meet clinical eligibility criteria and choose to pay privately.
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.