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 for Weight Loss?
Mounjaro (tirzepatide) is a prescription-only weekly injection that may help with weight loss, but results don't happen overnight. In clinical trials, most people began noticing weight loss effects within the first 4 weeks, though significant results typically take 8–12 weeks to become apparent. The timeline depends on several factors including your starting dose, how quickly it's increased, your diet and exercise habits, and your individual metabolism. This guide explains what to expect during your Mounjaro journey in the UK.
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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 Weight Loss Typically Begins
Mounjaro works through dose titration — meaning your dose increases gradually over several weeks. You start on 2.5 mg, the lowest dose, which gives your body time to adjust. Most people don't see dramatic weight loss at this stage because the dose is intentionally mild.
- Weeks 1–4: Starting 2.5 mg dose. Many people experience reduced appetite and early appetite control, but weight loss is typically minimal.
- Weeks 5–8: Increased to 5 mg. This is when most people begin noticing meaningful weight loss — averaging around 3–5 kg at this dose level.
- Weeks 9–16: Further escalation to 7.5 mg and 10 mg. Weight loss typically accelerates as the dose increases.
- Week 17 onwards: Maintenance doses (up to 15 mg maximum). By this stage, many people have lost 10–15% of their starting body weight.
According to clinical trial data, average weight loss at 72 weeks was approximately 15–21% of body weight across different doses — but individual results vary significantly.
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.
What Affects How Quickly You Lose Weight
Your weight loss timeline isn't determined by Mounjaro alone. Several factors influence how fast results appear:
- Diet quality: Mounjaro works best alongside a reduced-calorie diet. The medication reduces appetite, but you still need to make healthy food choices.
- Physical activity: Regular exercise accelerates weight loss and preserves muscle mass during weight loss.
- Starting weight: People with higher starting weights often see faster initial results.
- Metabolism: Individual metabolic differences mean some people lose weight faster than others at the same dose.
- Adherence: Missing injections delays progress. Mounjaro is most effective when taken exactly as prescribed.
Use our free GLP-1 weight loss calculator above to see projected results at different doses based on clinical trial data.
- 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 I see weight loss results in the first week of Mounjaro?
Unlikely. The first dose (2.5 mg) is intentionally low to let your body adjust. Most people experience reduced appetite in week one, but weight loss is minimal. Meaningful weight loss typically becomes apparent after 4–8 weeks as your dose increases.
How much weight should I expect to lose on Mounjaro per week?
Safe weight loss is 0.5–1 kg per week. On Mounjaro, average weight loss depends on your dose and individual factors. At the maximum 15 mg dose, clinical trials showed approximately clinically studied weight loss over 72 weeks — but results vary significantly between individuals.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials suggest Mounjaro may produce slightly faster weight loss on average (tirzepatide is a dual GIP+GLP-1 agonist, versus Wegovy's GLP-1 only). However, individual results vary. A prescriber can help you choose which is most suitable based on your health profile.
What if I'm not losing weight after 8 weeks on Mounjaro?
Contact your prescriber. They may increase your dose or assess whether Mounjaro is working for you. Weight loss depends on diet, activity, and individual response — if you're not seeing results, your prescriber might adjust your approach or medication.
Will I keep losing weight after reaching my target on Mounjaro?
Mounjaro is used to reach a healthier weight, then typically continued at maintenance dose to help prevent weight regain. Some people stop after reaching their target; others continue long-term. This is a decision to discuss with your prescriber based on your individual goals.
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.