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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Mounjaro Before and After 1 Month in the UK
If you've just started Mounjaro or are considering it, you may wonder what realistic changes to expect after 1 month. Most people in the first month are still in the dose titration phase — typically at 2.5 mg or 5 mg — rather than at the full therapeutic dose. Clinical trial data and UK experience show that meaningful weight loss usually takes 8–12 weeks to become apparent. This guide compares what to expect with Mounjaro versus Wegovy, and explains the timeline for results. You can use our free GLP-1 weight loss calculator to project potential outcomes based on different doses and timelines.
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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.
Clinical Trial Data: SURMOUNT vs STEP
Mounjaro (tirzepatide) was studied in the SURMOUNT-1 trial, which ran for 72 weeks. The trial measured weight loss in adults with a BMI of 27 or above. At the 1-month mark specifically, participants were still titrating doses — most were at 2.5 mg or 5 mg, not yet at full maintenance doses (10–15 mg). Meaningful weight loss in the first month is modest; the real changes emerge after 8–12 weeks when participants reach stable doses.
Wegovy (semaglutide) was studied in the STEP-1 trial over 68 weeks. Similarly, in the first month, users are at the starting dose of 0.25 mg. Both medications work by the same general mechanism (appetite regulation) but Mounjaro uses a dual GIP+GLP-1 approach, whereas Wegovy activates GLP-1 only. This dual mechanism may contribute to Mounjaro's slightly higher average weight loss (approximately 20.9% vs 14.9% at maximum dose) in head-to-head comparisons, though individual results vary significantly.
- SURMOUNT-1: average 15–clinically studied weight loss over 72 weeks with Mounjaro
- STEP-1: average clinically studied weight loss over 68 weeks with Wegovy
- Month 1: both medications are in titration phase; weight loss may be minimal or modest
Clinically BackedWeight Loss
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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 in Month 1: Timeline and Reality Check
In your first month on either Mounjaro or Wegovy, most people experience side effects before weight loss. Nausea, mild appetite suppression, and digestive changes are common in weeks 1–4. Some people lose a small amount of weight (1–2 kg) due to reduced appetite and food intake, but this is not the dramatic change seen in "before and after" images circulating online — those typically represent 8–16 weeks of use at therapeutic doses.
Your prescriber will assess your progress at 4 weeks and decide whether to increase your dose. Your weight loss journey is unique; factors including your current diet, activity level, metabolic rate, and how your body responds to the medication all influence results. Using our free progress tracker can help you monitor your actual progress week-by-week, rather than comparing yourself to others. Both medications work best alongside sustained dietary changes and increased physical activity.
- 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.
Will I lose weight in the first month on Mounjaro?
Many people experience modest weight loss (1–2 kg) in the first month due to reduced appetite. However, meaningful weight loss typically occurs after 8–12 weeks at a stable therapeutic dose. The first month is often dominated by side effects like nausea, not dramatic weight loss. Results vary individually.
How is Mounjaro different from Wegovy in terms of results?
Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1-only. Clinical trials showed Mounjaro users lost approximately 20.9% body weight on average, versus 14.9% for Wegovy — but this is over 68–72 weeks, not 1 month. Individual responses differ; your prescriber will recommend the best option for you.
Why do "before and after" Mounjaro images show such big changes?
Those images typically represent 3–6 months or longer of consistent use at therapeutic doses (10–15 mg), combined with diet and exercise changes. They do not reflect 1-month results. Be cautious of online comparisons; everyone's timeline is different.
When will my prescriber increase my Mounjaro dose?
Your dose typically increases every 4 weeks. Week 1–4: 2.5 mg. Weeks 5–8: 5 mg. Weeks 9–12: 7.5 mg, and so on up to 15 mg. Your prescriber assesses your response and side effects at each step before increasing.
Can I compare my results directly to someone else's after 1 month?
No — weight loss is highly individual and depends on starting weight, metabolism, diet quality, activity level, and dose. Our free GLP-1 weight loss calculator and progress tracker help you monitor your own timeline without comparing to others.
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.