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 Quickly Does Mounjaro Start Working vs Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) begin showing effects within the first few weeks, but the timeline and mechanism differ. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Clinical trial data shows both support weight loss when combined with lifestyle changes, though individual responses vary. Understanding how each medication works and the typical timeline can help you and your GP or prescriber discuss which might be more suitable for your circumstances.
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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 and Onset: Clinical Trial Evidence
In the SURMOUNT-1 trial for Mounjaro, participants began losing weight from week 1, with continued loss accelerating over 12–16 weeks. By week 4, many reported reduced appetite and food cravings. Maximum weight loss was typically seen by week 52, with an average loss of around 20.9% of body weight at the highest dose (15 mg).
The STEP-1 trial for Wegovy showed a similar pattern: weight loss began within the first 2–4 weeks, with steady loss continuing through 68 weeks. Average weight loss at the maximum dose (2.4 mg) was approximately 14.9% of body weight.
Key timeline differences:
- Mounjaro dose titration: Starts at 2.5 mg, increases every 4 weeks to a maximum of 15 mg over 20 weeks
- Wegovy dose titration: Starts at 0.25 mg, increases over 16 weeks to a maximum of 2.4 mg
- Both require consistent weekly dosing for effects to continue
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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.
Mechanism Differences and What They Mean
Mounjaro's dual mechanism activates both GIP and GLP-1 pathways, potentially leading to faster appetite suppression for some people. Wegovy's single GLP-1 mechanism is well-established and effective, but works through one pathway only. Neither medication works immediately — the dose titration schedule (gradually increasing over weeks) allows your body to adjust and minimises nausea and digestive side effects.
Individual suitability depends on your personal health profile, not onset speed. Your GP or prescriber will assess your medical history, current medications, and weight-related health conditions to recommend the most appropriate option. Use our free BMI calculator to understand your starting point, and discuss individual timeline expectations during your clinical assessment.
- 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.
Does Mounjaro work faster than Wegovy?
Clinical trials show both begin supporting weight loss within weeks 1–4, so the onset is comparable. Mounjaro's dual mechanism (GIP + GLP-1) may work differently for different people, but there's no evidence it works significantly faster. Individual response varies widely, and prescriber choice depends on your health profile, not speed alone.
When will I notice appetite changes with Mounjaro?
Most people report reduced appetite and food cravings within 2–4 weeks of starting Mounjaro, but timing varies. Full effects develop over the dose titration period (20 weeks to reach maximum dose). Changes may be subtle at lower doses and more noticeable at higher doses.
How long does it take to lose weight on Wegovy?
Weight loss typically begins within 2–4 weeks of starting Wegovy, with steady loss continuing over 12–16 weeks. The STEP trials showed most weight loss occurred between weeks 12 and 52. Consistent weekly injections and lifestyle changes are essential for results.
Can I switch from Wegovy to Mounjaro if I'm not seeing results?
Switches between medications should only be made under prescriber guidance. If one medication isn't meeting your needs, your GP or specialist can assess whether a change is appropriate. Allow at least 16–20 weeks on a stable dose before discussing alternatives.
Which injection works better for fast weight loss, Mounjaro or Wegovy?
Neither medication is designed for gradual weight management — the NHS recommends safe, sustainable loss of 0.5–1 kg per week. Both support similar average weight loss percentages (14–21% of body weight) over 12–52 weeks. Your prescriber will recommend the best option for your health, not speed.
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.