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
When Does Mounjaro Start Working?
Most people begin to notice the effects of Mounjaro within the first 2–4 weeks, though significant weight loss typically takes 8–12 weeks to become measurable. Mounjaro (tirzepatide) is a prescription-only injection that works by activating GIP and GLP-1 hormone pathways in the body. The timeline for results varies between individuals depending on dose, lifestyle factors, and how your body responds. Understanding what to expect during the early weeks can help you stay consistent with treatment.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Early Changes
Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways simultaneously. This dual mechanism sets it apart from single GLP-1 medications like Wegovy (semaglutide). Within the first 1–2 weeks on your starting dose (2.5 mg), most patients report reduced appetite and decreased food cravings, even though they may not see weight loss on the scales yet. These appetite changes happen because the medication signals to your brain that you feel fuller after eating smaller portions. The slowing of gastric emptying (how quickly food leaves your stomach) contributes to this sensation of prolonged fullness. However, noticeable weight loss on the scales typically appears between weeks 4–8, once your body has adjusted to the dose and you've maintained a calorie deficit consistently. Remember that the first dose (2.5 mg) is a starter dose — you'll increase gradually every 4 weeks, and weight loss may accelerate as your dose increases to 5 mg, 7.5 mg, and higher.
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.
Timeline: What to Expect Week by Week
Weeks 1–2: You may feel reduced appetite, less interest in food, and smaller portion sizes. This is the medication beginning to work — but don't expect scale changes yet. Some people experience nausea, which usually improves by week 2.
Weeks 3–4: Appetite suppression deepens. You're likely eating significantly less. A few people see initial weight loss, but most see minimal scale change.
Weeks 5–8: Your first dose increase (to 5 mg) occurs around week 5. Measurable weight loss becomes more common. Clinical data shows most people lose 1–2 kg during this period, but this varies widely. Use our GLP-1 weight loss calculator to project potential results at your current dose.
Weeks 9–12 and beyond: As your dose increases further and weeks progress, weight loss typically accelerates. The SURMOUNT-1 trial data showed average weight loss of 15–21% over 72 weeks at the highest doses — but remember, individual results vary significantly.
- 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 immediately when I start Mounjaro?
No. Most people don't see measurable weight loss in the first 2–3 weeks. What you will notice is reduced appetite and smaller portion sizes — this is the medication working, but weight loss on the scales takes longer. Significant changes typically appear after 4–8 weeks. Be patient with the process.
Why does Mounjaro take weeks to show results?
Weight loss requires a sustained calorie deficit over time. Mounjaro reduces your appetite, making it easier to eat less, but your body still needs time to use stored fat. Additionally, you're on the starter dose (2.5 mg) for the first 4 weeks — doses increase gradually, and weight loss often accelerates as your dose increases.
Is Mounjaro the same as Wegovy, or does it work faster?
No — Mounjaro (tirzepatide) is different from Wegovy (semaglutide). Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1-only medication. In clinical trials, Mounjaro showed slightly greater average weight loss (15–21% vs. ~15% for Wegovy), but both work on similar timelines. Individual results vary regardless of which medication is prescribed.
What if I don't see weight loss after 8 weeks on Mounjaro?
Check that you're maintaining a calorie deficit through diet and activity. Mounjaro helps reduce appetite, but you still need to eat fewer calories than you burn. If appetite is well-controlled but weight isn't changing, discuss this with your prescriber — your dose may need adjustment, or other factors (medication interactions, thyroid issues) may be involved.
Can I speed up Mounjaro results by increasing my dose faster?
No. NHS guidance and prescriber protocols recommend increasing your dose every 4 weeks, even if you want faster results. Rapid dose escalation increases side effect risk without providing faster weight loss. Stick to the recommended schedule. Your prescriber can discuss your progress at each stage.
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.