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?
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection used for weight management in the UK. Most people begin noticing initial effects within the first 2–4 weeks, though the full impact develops over several weeks as your dose increases. The timeline varies significantly between individuals depending on your starting dose, metabolism, and how your body responds to the medication. Understanding what to expect can help you set realistic goals and track your progress with confidence.
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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 Mounjaro Effects Typically Begin
Weeks 1–2: You're on the starting dose (2.5 mg). Most people don't notice significant appetite suppression yet, though some experience mild nausea or digestive changes. This is your body adjusting to the medication.
Weeks 3–4: As you continue the 2.5 mg dose, appetite reduction may begin. Some users report feeling fuller faster or less interested in food. Nausea often improves as your body adapts.
Weeks 5–8: You move to the 5 mg dose. This is where many people notice clearer appetite suppression and weight changes becoming measurable. Energy levels may fluctuate.
Weeks 9–12 onwards: At 7.5 mg and higher doses, weight loss acceleration typically becomes more apparent. The full effects build progressively as you titrate upward.
Individual timelines vary — some people feel effects by week 2, others by week 6. Consistency with diet and exercise amplifies results.
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.
Factors That Affect How Quickly Mounjaro Works
Your personal response depends on several factors. Baseline weight affects speed — people with higher starting weights often see faster initial loss. Dose escalation matters; the starting 2.5 mg is intentionally mild, so effects are modest until you reach 5 mg and beyond. Diet and activity level are crucial — Mounjaro works best alongside a reduced-calorie diet and regular physical activity, not instead of them. Metabolism and genetics influence individual variation — some people are naturally more responsive to tirzepatide. Other medications and medical conditions can also affect response speed. Use our free GLP-1 weight loss calculator to project potential results based on your starting point and dose.
- 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. The starting 2.5 mg dose is designed to be gentle and allow your body to adjust. Weight loss typically becomes noticeable by weeks 3–4 when appetite suppression develops, or more clearly by weeks 5–8 when you've escalated to 5 mg. Initial changes may be bloating or mild fluid shifts rather than fat loss.
What does Mounjaro feel like when it "kicks in"?
Most people describe a reduced appetite, feeling fuller after smaller meals, and less food cravings—especially for sweet or fatty foods. Some notice reduced interest in snacking. Nausea, if present, often improves after the first 1–2 weeks. The sensation is subtle, not dramatic.
How much weight can I expect to lose, and when?
Clinical trials showed average weight loss of 15–21% body weight over 72 weeks at maximum doses. Individual results vary significantly. Realistic expectation: steady loss of 0.5–1 kg per week when combining Mounjaro with diet and exercise, though results vary. Use our BMI calculator and weight converter to track progress in kg or stone.
Is there a difference between Mounjaro and Wegovy in terms of speed?
Mounjaro (tirzepatide, dual GIP+GLP-1) typically shows effects slightly faster than Wegovy (semaglutide, GLP-1 only) because of its dual mechanism, though individual variation is high. Clinical data shows Mounjaro may achieve slightly greater average weight loss, but both require consistent use over weeks to show full effects.
What if I don't feel Mounjaro working after 4 weeks?
This is normal at the 2.5 mg starting dose. The escalation schedule is intentional—effects typically become clearer at 5 mg (weeks 5–8) and more pronounced at higher doses. If you have concerns, discuss with your prescriber. Never adjust doses yourself; dose titration is managed clinically.
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.