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 Soon Does Mounjaro Start Working?
Mounjaro (tirzepatide) doesn't work instantly — weight loss typically begins within the first 2–4 weeks, but most people see more noticeable results after 8–12 weeks of consistent use. Since Mounjaro is a prescription-only medication, your GP or a licensed prescriber will start you on a low dose and increase it gradually every 4 weeks, which means the full effects take time to build. Individual results vary significantly, and weight loss depends on dose, how your body responds, and lifestyle changes alongside the injection.
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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 1–4 (Starting dose 2.5 mg): Many people report reduced appetite within days to weeks, though weight loss may be minimal at this low dose. Some experience early side effects like nausea, which often settle within the first week or two.
Weeks 5–8 (Dose increase to 5 mg): As your dose increases, appetite suppression typically becomes more noticeable. You may start to see weight loss on the scales, though this varies. Clinical trials showed that at 5 mg, average weight loss was around 15% over 72 weeks when combined with reduced-calorie diet and increased activity.
Weeks 9–20 (Further escalation to 7.5–15 mg): Most significant weight loss occurs during this period. Results from the SURMOUNT-1 trial showed that at the maximum 15 mg dose, participants lost approximately 21% of body weight on average. However, your prescriber may adjust your dose based on tolerance and results. Your GLP-1 weight loss calculator can help you estimate potential outcomes at different doses.
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.
What Affects How Quickly Mounjaro Works
Several factors influence how fast you see results: individual metabolism varies — some people respond faster than others; diet and exercise matter enormously — weight loss is most effective when combined with a reduced-calorie diet and physical activity; dose level — higher doses (10–15 mg) typically produce greater weight loss than lower doses; weight-related health conditions — people with type 2 diabetes or metabolic syndrome may see different timelines; side effects — if nausea or stomach issues are severe, you may need to stay on a lower dose longer, which slows escalation and results.
Use our free BMI calculator to track where you start, then monitor progress monthly. Many people find that consistent tracking motivates continued engagement with diet and exercise changes.
- 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.
Can Mounjaro start working within 1–2 weeks?
Some people notice reduced appetite within days or the first week, but significant weight loss typically takes longer. Early appetite suppression is common, but the scales may not change much in week 1–2 because your dose is still at the starting level (2.5 mg). Be patient — results accelerate as your dose increases every 4 weeks.
How much weight will I lose in the first month on Mounjaro?
Individual results vary widely. In the first 4 weeks at the starting dose, weight loss is usually modest (0–2 kg). Most noticeable progress happens weeks 5–12 as your dose increases. Clinical trials showed average weight loss of 15–21% over 72 weeks, but this is alongside reduced-calorie diet and exercise — not from the injection alone.
Is Mounjaro the same as Wegovy or Ozempic?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are licensed for weight loss in the UK. Ozempic also contains semaglutide but is approved only for type 2 diabetes, not for weight management. Always use the licensed medication your prescriber recommends.
What if I don't see weight loss after 8 weeks on Mounjaro?
If results are slow after 8 weeks, discuss with your prescriber. They may: check you're at the right dose, assess whether diet and exercise are adequate, review any medications that might affect weight loss, or occasionally explore whether a different medication might suit you better. Don't stop suddenly — always follow your prescriber's guidance.
Do I need to follow a specific diet for Mounjaro to work faster?
Yes. Mounjaro is most effective alongside a reduced-calorie diet and increased physical activity — it's not a standalone solution. The NHS recommends a calorie deficit of around 500 kcal per day for sustainable weight loss of 0.5–1 kg per week. Your GP or a dietitian can offer personalised dietary advice.
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.