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
What To Expect After 3 Months On Mounjaro vs Wegovy
After three months on either Mounjaro (tirzepatide) or Wegovy (semaglutide), most people are reaching a stable maintenance dose and beginning to see meaningful weight changes. Both are prescription-only weight loss injections licensed in the UK, but they work differently and produce different results. Understanding the comparison helps you and your prescriber decide which is most suitable for your health needs.
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.
How Mounjaro And Wegovy Compare After 3 Months
The key difference between these medications is their mechanism. Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways simultaneously, whereas Wegovy is a GLP-1 receptor agonist only, working through a single pathway. This dual action in tirzepatide may explain why clinical trials show stronger average weight loss with Mounjaro.
Clinical trial data at 3 months: In the SURMOUNT-1 trial, Mounjaro users at the 5 mg dose (typically reached by week 8–12) showed an average weight loss of around 6–8% of body weight. Wegovy users in the STEP-1 trial, at equivalent escalation points, showed approximately 5–clinically studied weight loss. However, individual variation is significant — some people respond better to one medication than the other. Your GP or prescriber will assess which is more suitable based on your medical history, previous medication responses, and specific health conditions.
By three months, most people have also experienced the peak of initial side effects (nausea, gastrointestinal changes) and many report these settling as the body adjusts. The progression through dose escalation — whether using a Mounjaro KwikPen or Wegovy auto-injector — typically follows a similar timeline, with the first maintenance dose reached by weeks 12–16.
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.
Which Medication Might Be Right For You?
Neither medication is inherently "better" — suitability depends on your individual health profile. Your prescriber will assess: your current BMI and health conditions, any previous medication responses, your ability to tolerate the specific side effect profile, and whether you have any contraindications (such as a family history of thyroid cancer, pregnancy plans, or severe gastroparesis).
After three months, your prescriber may continue your current medication, adjust the dose further, or consider switching if your response has been poor. UK prescriptions for both medications require clinical assessment and ongoing monitoring — neither is available over-the-counter. Use our free GLP-1 weight loss calculator above to estimate projected results at different doses and timepoints, helping you have an informed conversation with your healthcare provider about which option aligns with your weight loss goals.
- 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 should I have lost after 3 months on Mounjaro?
Individual results vary significantly. Clinical trial averages show approximately 6–clinically studied weight loss by 3 months at stable doses, but some people lose more and some less. Factors affecting results include adherence to diet and exercise, starting weight, baseline health conditions, and individual metabolism. Your prescriber can discuss expected outcomes for your specific situation.
Is Mounjaro better than Wegovy for weight loss?
Clinical trials suggest Mounjaro may produce slightly stronger average weight loss due to its dual GIP+GLP-1 mechanism, but the difference is modest and individual responses vary. "Better" depends on your personal health profile, side effect tolerance, and how your body responds. Your GP will assess which is more appropriate for you based on medical factors, not general superiority.
What side effects should I expect at the 3-month mark?
At three months, most people are on or approaching a stable maintenance dose. Common side effects (nausea, constipation, reduced appetite) often peak in weeks 2–8 and frequently improve by month three as your body adjusts. Persistent side effects should be discussed with your prescriber — dose adjustment or medication change may be options.
Can I switch from Mounjaro to Wegovy if Mounjaro isn't working for me?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart between tirzepatide and semaglutide doses, so your prescriber will restart you on an appropriate dose of the new medication based on your tolerance and response history. Do not attempt to switch without professional guidance.
How do I know which medication is prescribed on the NHS vs privately?
Availability varies by region (ICB). NHS access is through tier 3 specialist weight management services (long waiting lists in many areas). Private prescriptions are available through GPhC-registered online pharmacies following clinical assessment. Your GP can advise on NHS options in your area, or you can explore private routes independently.
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.