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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8 Week Weight Loss: Mounjaro vs Wegovy Clinical Comparison
When considering a weekly weight loss injection in the UK, many people ask what realistic weight loss looks like in the first 8 weeks. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only medicines approved for weight management, but they work differently and produce varying individual results. Clinical trial data shows what happened in structured research settings — but your personal outcome depends on your starting weight, dose, diet, and activity level. This guide compares what the evidence actually shows about early weight loss timelines with these medications.
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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.
STEP vs SURMOUNT Trial Data: What 8 Weeks Showed
Clinical trials testing Mounjaro (SURMOUNT-1) and Wegovy (STEP-1) measured weight loss over longer periods, but early data gives insight into the 8-week phase. In STEP-1, participants on Wegovy typically saw initial weight loss within the first 4 weeks as the body adjusted to the medication. By week 8, most were on doses between 0.5mg and 1mg, with average losses ranging from 2–4kg reported in early trial phases — though this varied significantly. SURMOUNT-1 data showed Mounjaro, a dual GIP and GLP-1 receptor agonist, produced similar early patterns at equivalent doses. The key difference: Mounjaro's dual mechanism may support slightly faster initial appetite suppression for some users, but individual responses differ widely. Both medications require a mounjaro dose titration schedule (starting at 2.5mg for Mounjaro, 0.25mg for Wegovy), increasing every 4 weeks, so 8 weeks typically covers the first two dose escalations only.
- Week 4: typically on first maintenance dose (5mg Mounjaro or 0.5mg Wegovy)
- Week 8: often escalating toward second dose (7.5mg or 10mg Mounjaro; 1mg Wegovy)
- Early loss heavily influenced by dietary adherence and reduced food intake, not medication alone
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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.
Why "Before and After" Frames Don't Tell the Full Story
The internet is full of "before and after" images suggesting dramatic 8-week transformations, but these don't reflect clinical reality for most people. Clinical trial data shows average early weight loss — meaning some lose more, many lose less. At 8 weeks, you're still in dose escalation phase, not at maximum therapeutic dose. The medication works best alongside a reduced-calorie diet and increased physical activity, not instead of these changes. Individual factors that affect 8-week results include: starting weight (heavier individuals often see larger absolute losses); baseline metabolic health; medication tolerance and side effects (nausea and constipation can reduce food intake, supporting loss); sleep quality; and stress levels. Your GP or prescriber will assess whether mounjaro uk prescription or Wegovy is suitable for you individually — this is not a decision based on promised 8-week outcomes, but on your health profile and eligibility. Realistic expectations: many UK adults see 3–6kg loss in the first 8 weeks, though this is not assured.
- 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 much weight can you lose in 8 weeks on Mounjaro or Wegovy?
Average early weight loss (weeks 1–8) ranges from 3–6kg for most people, though individual results vary widely. You're still in dose escalation phase, not at maximum dose. Weight loss depends on your starting weight, diet adherence, activity level, and how your body responds. Clinical trials showed varying results — some people lost more, others less.
Is 8 weeks long enough to see real results on a weight loss injection?
Yes, most people notice appetite suppression and some weight loss by week 4–6. However, 8 weeks is relatively early — maximum benefit typically occurs after 12–16 weeks once you've reached your target dose. Patience with the dose escalation process is important.
What's the difference between Mounjaro and Wegovy for early weight loss?
Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1-only. Some evidence suggests Mounjaro may support slightly faster appetite suppression, but clinical differences at 8 weeks are modest. Your prescriber will choose based on your individual health profile, not expected speed of loss.
Do you need to diet and exercise while taking these medications?
Yes, absolutely. These medications work best alongside a reduced-calorie diet and increased physical activity. The injection supports appetite control, but behaviour change is essential. Without dietary changes, weight loss will be minimal regardless of the medication.
Can I get Mounjaro or Wegovy on the NHS for 8-week weight loss?
Both are available through NHS tier 3 specialist weight management services if you meet eligibility criteria (BMI ≥30, or BMI ≥27 with weight-related health conditions). However, NHS access varies by region and waiting times can be significant. Private prescription through online assessment is also available in the UK.
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.