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
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How Long Does Mounjaro Take To Work — Mounjaro vs Wegovy
When you start Mounjaro (tirzepatide), most people notice appetite changes within the first few days to weeks, though full effects build gradually as your dose increases. Unlike a single-dose medication, Mounjaro works on a titration schedule — you start at 2.5 mg and escalate every 4 weeks up to your target dose. This means the timeline to "kick in" depends on both your individual response and your dose progression. If you're comparing Mounjaro to Wegovy (semaglutide), the onset patterns differ slightly due to their different mechanisms: Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1 agonist only. Understanding these timelines helps set realistic expectations when starting either medication.
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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.
Mounjaro vs Wegovy: How Quickly They Work
Both Mounjaro and Wegovy are once-weekly injections, but their onset timelines differ slightly. Mounjaro (tirzepatide) often shows noticeable appetite suppression within 3–7 days of the first injection for many people, though peak effects at each dose take 4 weeks. The SURMOUNT-1 clinical trial showed meaningful weight loss emerging around weeks 4–8, with average losses of approximately 15% body weight by 72 weeks at the maintenance dose. Wegovy (semaglutide) typically shows initial appetite changes within 1–2 weeks, with gradual weight loss visible from weeks 4–8 onwards, averaging around clinically studied weight loss by week 68 in the STEP-1 trial.
- Weeks 1–4: Mounjaro 2.5 mg — mild appetite changes, nausea common, weight plateau normal
- Weeks 5–8: Mounjaro 5 mg — increased appetite suppression, weight loss may become visible
- Weeks 9–16: Escalation to 7.5–10 mg — more pronounced effects build
- Week 17+: Maintenance dose (10–15 mg) — peak effects typically established
The key difference: Mounjaro's dual GIP+GLP-1 mechanism may produce faster appetite changes in some people, but both require patience through dose escalation. Individual responses vary significantly — some notice effects immediately, others need 8–12 weeks.
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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 To Expect Week By Week — Individual Variation Matters
Everyone's experience differs based on starting weight, metabolism, diet compliance, and individual sensitivity to GLP-1 and GIP. Weeks 1–2: Many people report reduced appetite, changes in food preferences (less interest in sweets or fatty foods), and mild nausea — these are early signs the medication is working. Weeks 3–8: Appetite suppression typically deepens, portion sizes naturally reduce, and weight loss becomes measurable. Weeks 9–16: As your dose increases, effects usually strengthen further. By week 20+: Most people on maintenance doses report stable appetite control and consistent weekly weight loss patterns.
Important: If you're not noticing appetite changes by week 4, or if side effects prevent dose escalation, discuss this with your prescriber — they may adjust your titration schedule or explore whether Mounjaro is the right fit. Your GP or prescriber can assess your personal response and guide you through the process. Try our GLP-1 weight loss calculator above to estimate potential timelines based on your starting weight and dose progression.
- 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.
How long does Mounjaro take to start working?
Most people notice appetite suppression within 3–7 days of their first 2.5 mg injection. However, meaningful weight loss typically becomes visible around weeks 4–8 as your dose increases. Full effects are usually established by week 20+ when you reach your maintenance dose (10–15 mg). Individual timelines vary significantly.
Is Mounjaro faster than Wegovy at suppressing appetite?
Mounjaro's dual GIP+GLP-1 mechanism may produce slightly faster initial appetite changes for some people compared to Wegovy's GLP-1-only approach. However, clinical trial data shows similar average weight loss percentages (around 15% body weight) over 68–72 weeks. Your prescriber will assess which is most suitable for you.
Why does Mounjaro take 4 weeks between dose increases?
The 4-week escalation schedule allows your body to adjust to each dose and reach steady-state medication levels in your bloodstream. This reduces side effects and helps your prescriber monitor your response. Rushing dose escalation can increase nausea and stomach symptoms.
Will I lose weight during the first 4 weeks on Mounjaro 2.5 mg?
The starting 2.5 mg dose is very low and designed for tolerability, not maximum weight loss. Weight loss during weeks 1–4 is possible but often modest. Significant weight loss typically accelerates once you move to 5 mg and higher doses. This is why patience through titration is important.
Should I expect side effects before weight loss kicks in?
Side effects (nausea, diarrhoea, reduced appetite) often appear before substantial weight loss does — sometimes within days of your first injection. This is normal and usually decreases as your body adjusts. If side effects are severe, contact your prescriber; they may adjust your schedule or recommend dietary changes to help manage symptoms.
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.