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 vs Wegovy?
Mounjaro (tirzepatide) typically begins to show weight loss effects within 4 to 8 weeks of starting treatment, though individual results vary significantly. Unlike some medications, Mounjaro uses a gradual dose titration approach — you start at 2.5 mg weekly and increase your dose every 4 weeks. This structured approach allows your body to adjust while building towards the therapeutic dose. Many people notice initial appetite reduction within the first few weeks, but meaningful weight loss usually becomes evident after 8 to 12 weeks of consistent use. Our free GLP-1 weight loss calculator can help you explore potential outcomes based on clinical trial data.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
Mounjaro vs Wegovy: Timeline and Trial Data
Clinical trial evidence shows important differences in how quickly these medications work. Mounjaro's SURMOUNT-1 trial measured weight loss over 72 weeks, with participants losing an average of 20.9% of body weight at the maximum 15 mg dose. Weight loss became noticeable around weeks 4–8 and continued steadily throughout the trial. Wegovy's STEP-1 trial showed similar timelines, with average loss of 14.9% body weight over 68 weeks at the maximum 2.4 mg dose.
The key difference: Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1-only agonist. This dual mechanism may contribute to Mounjaro's slightly higher average weight loss in trials, though results vary considerably between individuals. Both use once-weekly injections via KwikPen (Mounjaro) or pre-filled pen (Wegovy), but the dose escalation schedules differ.
- Mounjaro: starts 2.5 mg, increases every 4 weeks up to 15 mg maximum
- Wegovy: starts 0.25 mg, increases every 4 weeks up to 2.4 mg maximum
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.
What Affects How Quickly You'll See Results?
Weight loss speed depends on multiple factors beyond the medication itself. Diet and physical activity are essential — both Mounjaro and Wegovy work alongside a reduced-calorie diet and increased movement. Your starting weight, metabolism, adherence to lifestyle changes, and whether you have type 2 diabetes all influence outcomes. Some people experience appetite reduction within days; others take 2–3 weeks to notice changes. Use our free BMI calculator to establish your baseline, then track progress consistently over 8–12 weeks before assessing effectiveness.
Your prescriber will assess whether either medication suits you based on UK eligibility criteria (BMI ≥30, or ≥27 with weight-related health conditions). Neither Mounjaro nor Wegovy is a structured approach — sustainable weight loss requires patience, typically 6 months to a year for full benefits.
- 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 feel Mounjaro working within the first week?
Some people experience reduced appetite or nausea within days of starting. However, meaningful weight loss usually takes 4–8 weeks to become apparent. The first dose (2.5 mg) is a starting dose — your prescriber will increase it gradually over weeks to reach a therapeutic level.
Is Wegovy faster than Mounjaro?
Clinical trials show similar timelines for both. Wegovy begins at a very low dose (0.25 mg), while Mounjaro starts at 2.5 mg. Both reach their maximum dose after 16–20 weeks. Mounjaro showed slightly higher average weight loss in SURMOUNT trials, but individual results vary significantly. Your prescriber will recommend the most suitable option based on your health profile.
When should I expect to see weight loss on Mounjaro?
Most people see noticeable weight loss between 8–12 weeks of consistent use, alongside diet and exercise changes. Some people experience results earlier (4–6 weeks); others take longer. Keeping a progress tracker helps you monitor changes that may not be visible on the scales immediately.
What if Mounjaro isn't working after 8 weeks?
If you're on a low starting dose (2.5 mg or 5 mg), your prescriber may increase your dose. Effectiveness also depends on adherence to a reduced-calorie diet and physical activity. If you have concerns, discuss them with your prescriber or GP — they may adjust your dose or explore alternative options.
Does the Mounjaro KwikPen work differently than Wegovy?
Both are once-weekly injections using pre-filled pens. Mounjaro uses a KwikPen device; Wegovy uses a standard prefilled pen. The mechanism is different (dual vs single hormone pathway), but injection technique and frequency are identical. Both require a prescription following clinical assessment from a UK-licensed prescriber.
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.