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 vs Wegovy: First Month Weight Loss Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, but they work differently and produce different results in the first month. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. In month one, most users start with their lowest dose to allow their body to adjust, which means substantial weight loss is not typically expected during the initial four weeks. Understanding how each works can help you and your GP make an informed decision about which might be suitable for you.
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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 and Wegovy were tested in separate clinical trials with different populations and timelines, making direct month-one comparisons difficult. The SURMOUNT-1 trial for Mounjaro showed average weight loss of approximately 15–21% body weight over 72 weeks at the maximum 15 mg dose, alongside a reduced-calorie diet and increased physical activity. The STEP-1 trial for Wegovy showed approximately clinically studied weight loss over 68 weeks at the maximum 2.4 mg dose under the same conditions.
In the first month specifically, both medications use dose titration (gradual dose increases). Mounjaro starts at 2.5 mg weekly, whilst Wegovy starts at 0.25 mg weekly. This ramp-up phase means weight loss in month one is typically modest—often only 1–3 kg (2–6 lbs)—as the body adjusts to the medication. Most of the clinical weight loss data comes from weeks 4 onwards when higher therapeutic doses are reached.
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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.
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Mechanism Differences and First-Month Effects
The key difference is that Mounjaro activates both GIP and GLP-1 pathways, whilst Wegovy activates GLP-1 only. The dual mechanism of Mounjaro may theoretically support greater appetite suppression and improved blood sugar control, though individual responses vary significantly. In the first month, both injections commonly cause nausea, reduced appetite, and digestive changes (particularly constipation or diarrhoea) as the body adapts—these side effects often reduce as treatment continues.
Neither medication assures specific weight loss outcomes. Your prescriber will assess your individual health, BMI, and suitability for each option. Use our GLP-1 weight loss calculator to explore estimated weight loss projections at different doses, and our BMI calculator to understand your current health context before discussing prescription weight loss options with your GP.
- 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 month on Mounjaro or Wegovy?
Weight loss in the first month is typically modest—usually 1–3 kg (2–6 lbs)—because both medications use dose titration (gradual increases). Both start at very low doses to allow your body to adjust. Most significant weight loss occurs once you reach higher therapeutic doses, typically from week 4 onwards.
Which is more effective for weight loss—Mounjaro or Wegovy?
Clinical trials show both medications support weight loss when combined with diet and exercise. Mounjaro showed slightly higher average weight loss (15–21% vs 14.9%) in trials, but results vary greatly between individuals. Your GP will assess which is suitable based on your personal health profile and medical history.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 receptor agonist (activates two pathways), whilst Wegovy is a GLP-1 agonist only (single pathway). The dual mechanism of Mounjaro may support greater appetite suppression, but individual responses differ. Both require prescription and clinical assessment.
How quickly can I increase my dose on these injections?
Mounjaro increases from 2.5 mg weekly, with a minimum of 4 weeks at each dose before escalation. Wegovy increases from 0.25 mg, escalating over several weeks to the maximum 2.4 mg dose. Your prescriber controls the titration schedule based on your tolerance and response.
Are Mounjaro and Wegovy available on the NHS?
Both are licensed in the UK. NHS access is through specialist tier 3 weight management services—referral and waiting times vary significantly by region. Private access is available through online UK-licensed prescribers following clinical assessment. Your GP can advise on NHS availability in your area.
Ready to take control of your weight loss journey?
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