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
Mounjaro Weight Loss in the First Week
If you've just started Mounjaro or are considering it, you might wonder what weight changes to expect in week one. The truth is: most people don't see significant weight loss in the first week. Mounjaro works gradually, and the initial effects focus on appetite reduction rather than gradual weight management. This guide explains what clinical trials show, how Mounjaro compares to Wegovy, and what realistic expectations look like for a weekly weight loss injection in the UK.
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.
Clinical Trial Data: First Week to Early Weeks
The SURMOUNT-1 clinical trial, which tested Mounjaro (tirzepatide) in the UK and internationally, tracked weight loss over 72 weeks. In the first week, weight changes were typically minimal — most participants lost little to no weight. By week 4, as Mounjaro dosing continued at 2.5 mg (the starting dose), modest changes became apparent. The dual GIP and GLP-1 mechanism of Mounjaro works by reducing appetite and slowing gastric emptying, but these effects take time to accumulate.
- Week 1–4: Primary effects are appetite reduction and nausea; visible weight loss is usually minimal
- Week 5–12: Weight loss typically becomes more noticeable as the body adapts to reduced calorie intake
- Average first month: Participants in clinical trials lost approximately 1–2 kg on average, though this varied significantly
- Key factor: Weight loss depends heavily on calorie deficit — the injection supports appetite control but doesn't cause weight loss without dietary change
By comparison, Wegovy (semaglutide) — a GLP-1 agonist only — showed similar early patterns in the STEP trials: minimal first-week loss, with more noticeable changes from week 4 onwards.
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.
Mounjaro vs Wegovy: First Week Context
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections available in the UK following clinical assessment. The key difference lies in their mechanism: Mounjaro activates both GIP and GLP-1 pathways, whilst Wegovy targets GLP-1 only. In the first week, neither medication produces dramatic weight loss — both work best as part of a reduced-calorie diet and increased physical activity.
Clinical data suggests that after 72 weeks, Mounjaro users lost an average of 15–21% of body weight (depending on final dose), whilst Wegovy users lost approximately 15%. However, individual results vary significantly, and prescriber selection of either medication depends on your personal health profile, not assured outcomes. Use our free GLP-1 weight loss calculator to explore projected results at different doses, and calculate your BMI with our BMI calculator to understand your starting point.
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections available in the UK following clinical assessment. The key difference lies in their mechanism: Mounjaro activates both GIP and GLP-1 pathways, whilst Wegovy targets GLP-1 only. In the first week, neither medication produces dramatic weight loss — both work best as part of a reduced-calorie diet and increased physical activity.
Clinical data suggests that after 72 weeks, Mounjaro users lost an average of 15–21% of body weight (depending on final dose), whilst Wegovy users lost approximately 15%. However, individual results vary significantly, and prescriber selection of either medication depends on your personal health profile, not assured outcomes. Use our free GLP-1 weight loss calculator to explore projected results at different doses, and calculate your BMI with our BMI calculator to understand your starting point.
- 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 my first week on Mounjaro?
Most people do not see significant weight loss in week one. Clinical trials showed minimal weight change in the first week; noticeable loss typically begins from week 4 onwards. Mounjaro works gradually by reducing appetite, allowing you to maintain a calorie deficit alongside diet and exercise changes.
Why is the first week of Mounjaro weight loss so small?
Mounjaro starts at a low dose (2.5 mg) to allow your body to adjust. The medication's appetite-suppressing effects build gradually. Weight loss requires a sustained calorie deficit; the injection supports this by reducing hunger, but visible results take time to appear.
How does first-week weight loss on Mounjaro compare to Wegovy?
Both Mounjaro and Wegovy show minimal first-week weight loss. Wegovy (semaglutide) is a GLP-1 agonist only, whilst Mounjaro is a dual GIP and GLP-1 agonist. By 72 weeks, Mounjaro users lost slightly more on average (15–21% vs 15%), but early weeks are similar for both.
What should I expect in my first month on Mounjaro?
By week 4, many people report reduced appetite and food cravings. Average weight loss in the first month is typically 1–2 kg, though this varies based on your diet and activity. The KwikPen device makes weekly injections straightforward; most people adapt quickly to the routine.
Can I get Mounjaro on the NHS in the UK?
NICE has approved tirzepatide (Mounjaro) for NHS specialist weight management services, though availability varies by region and ICB. Private prescription options are also available through UK-licensed prescribers following an online clinical assessment. Your GP can advise on local NHS availability.
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.