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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What to Expect From Mounjaro After 1 Month
If you've just started Mounjaro (tirzepatide), the first month is crucial for understanding how your body responds to this prescription weight loss injection. After 4 weeks on your starting dose of 2.5 mg, you may notice changes in appetite, energy levels, and how you feel after eating. This guide explains what typically happens during your first month on Mounjaro, how it compares to Wegovy, and what to expect as you progress through the dose titration schedule. Remember, individual experiences vary significantly — your GP or prescriber will monitor your response at each stage.
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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.
Mounjaro vs Wegovy: First Month Differences
During your first month, both Mounjaro and Wegovy work through similar appetite-suppressing mechanisms, but they differ in strength and dosing. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously, whereas Wegovy (semaglutide) is a GLP-1 agonist only. In the SURMOUNT-1 clinical trial, participants on Mounjaro 2.5 mg reported reduced hunger and earlier fullness at meals within the first few weeks. Wegovy's STEP trials showed similar appetite reduction but with a lower starting dose of 0.25 mg. After one month, many users report nausea is most intense, then gradually improves. Mounjaro's dual mechanism may produce slightly stronger appetite suppression earlier, but individual responses vary considerably. Your prescriber will assess your tolerance and may adjust your dose at week 4 based on how well you're managing side effects and tolerating the injection.
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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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Side Effects and Individual Response at Week 4
After 1 month on Mounjaro, nausea is typically the most common side effect, though severity varies widely. Some people report it eases after the first 2–3 weeks; others find it persists longer. Other frequent effects include constipation, diarrhoea, and reduced appetite (which is the desired therapeutic effect). Stomach pain, indigestion, and fatigue are also reported. By week 4, you should have a clearer picture of how well you tolerate Mounjaro — this informs whether your prescriber increases your dose at the standard 4-week interval. Weight loss in month one is often modest (typically 1–3 kg) because you're on the lowest dose (2.5 mg). The Mounjaro KwikPen is injected once weekly on the same day; ensure you're rotating injection sites (abdomen, thigh, upper arm) to minimise bruising and irritation. If side effects are severe, contact your prescriber before your next dose.
- 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 lose in my first month on Mounjaro?
Weight loss in month one is typically modest because 2.5 mg is a starting dose designed for tolerability, not maximum effect. Most people lose 1–3 kg in the first 4 weeks. Clinical trials show significant weight loss occurs over months 2–6 as doses increase. Individual results vary based on diet, activity, and metabolism. Your GP can discuss realistic expectations for your situation.
Is Mounjaro or Wegovy more effective after 1 month?
After 1 month, both are on their lowest doses, so direct comparison is limited. SURMOUNT trials (Mounjaro) and STEP trials (Wegovy) show both reduce appetite effectively. Mounjaro's dual GIP+GLP-1 mechanism may produce stronger appetite suppression, but individual tolerance and side effects differ. Your prescriber will recommend based on your health profile and suitability.
Can I use the BMI calculator and weight converter to track my first month?
Yes. Use our free BMI calculator to establish your baseline BMI before starting, then check it monthly as you progress. The weight converter helps you track your weight in your preferred units (kg, stone, lbs) — logging weekly with our progress tracker gives you a clear picture of your first month's changes.
What should I do if nausea is severe after 1 month on Mounjaro?
Contact your prescriber immediately. Severe nausea may indicate the dose is too high for you at present, or you may need dietary adjustments. Never skip doses or stop without guidance. Your prescriber might delay your dose increase or recommend anti-nausea strategies. Nausea often improves over time, but severe cases warrant professional review.
Will my dose increase after my first month on Mounjaro?
Yes, the standard titration schedule increases your dose every 4 weeks if tolerating well. After month one at 2.5 mg, you'll typically move to 5 mg in week 5. Your prescriber confirms this is appropriate based on your side effect profile and progress. If you're struggling, dose increases can be delayed until you adapt.
Ready to take control of your weight loss journey?
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