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 Work?
Most people don't see significant weight loss results from Mounjaro immediately — it typically takes several weeks for the medication to start working, and the full effects can take 12–16 weeks or longer. The timeframe depends on your starting dose, how your body responds, and whether you're following a reduced-calorie diet alongside the injection. Understanding what to expect can help you stay committed to treatment whilst your body adjusts to this tirzepatide-based 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 The Timeline Works With Mounjaro
Mounjaro starts working on appetite and blood sugar regulation from your first injection, but noticeable weight loss typically appears after 4–8 weeks of consistent use. The medication follows a dose titration schedule — you begin at 2.5 mg and increase every 4 weeks to allow your body to adapt. Weight loss usually becomes more visible as you reach maintenance doses (around 10–15 mg).
- Weeks 1–4 (2.5 mg dose): Appetite may start to reduce; most people report early nausea and digestion changes
- Weeks 5–8 (5 mg dose): First measurable weight loss often appears; most people lose 2–4 kg at this stage
- Weeks 9–12 (7.5 mg dose): Weight loss accelerates as your body adjusts to higher doses
- Weeks 13+ (10 mg and above): Maximum weight loss effects typically achieved; this is where clinical trial results (15–clinically studied weight loss) were measured
Your GP or prescriber should review your progress every 4 weeks during dose escalation.
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.
Factors That Affect How Quickly Mounjaro Works
Individual responses vary significantly — some people see results in 3–4 weeks, whilst others take 12+ weeks to notice meaningful change. Several factors influence the timeline: your starting weight and metabolic rate, how strictly you follow a reduced-calorie diet, your exercise level, other medications you're taking, and your baseline health conditions like type 2 diabetes.
If you're also tracking progress with our free progress tracker, you can log weekly measurements and spot trends even when the scales aren't moving quickly. Consistency matters — missing doses or not maintaining a calorie deficit will delay results. If you haven't seen any progress after 16 weeks on a maintenance dose, discuss this with your prescriber.
- 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 in the first week of Mounjaro?
Unlikely. First-week weight changes are usually water loss or digestion changes, not fat loss. Mounjaro begins regulating appetite from day one, but measurable fat loss typically takes 3–8 weeks. Stay consistent with reduced-calorie diet and the medication will work cumulatively.
Why do I need to wait 4 weeks between dose increases?
Dose titration allows your body to adjust to the medication and manage side effects. Each 4-week period gives your system time to adapt before increasing. This slow escalation also helps identify the lowest effective dose for you, maximising results whilst minimising nausea and stomach upset.
Does Mounjaro work faster than Wegovy?
Clinical trials suggest Mounjaro (tirzepatide) may work slightly faster than Wegovy (semaglutide) because it's a dual GIP and GLP-1 agonist versus GLP-1 alone. However, individual variation is significant — some people respond better to one than the other. Your prescriber can discuss which is more suitable for you.
Can I speed up Mounjaro results with a very low-calorie diet?
No — extremely restrictive diets won't speed up Mounjaro and increase muscle loss risk. The medication works best with a moderate, sustainable calorie deficit (500 kcal/day). NHS guidance recommends losing 0.5–1 kg per week for healthy, sustainable results.
What if I don't see results after 12 weeks?
If you're on a maintenance dose (10 mg or higher) and haven't lost weight after 12 weeks despite following a reduced-calorie diet, contact your prescriber. They may adjust your dose, review your overall approach, or discuss whether this medication is right for you. It's not effective for everyone.
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.