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 When Starting Mounjaro
Starting Mounjaro (tirzepatide) is a significant step in your weight management journey. This prescription-only injection works differently than many other weight loss medicines because it's a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help reduce appetite and support feeling fuller. Understanding what to expect in your first weeks, how your body may respond, and how the dose gradually increases will help you prepare and manage the experience confidently. Our GLP-1 weight loss calculator can help you estimate potential outcomes, while the BMI calculator above lets you track your starting point.
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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.
Your First Four Weeks On Mounjaro
You'll start on the lowest dose: 2.5 mg once weekly. Your prescriber will keep you on this starting dose for at least 4 weeks before increasing it. During this time, your body is adjusting to the medication.
What typically happens in week one:
- You may feel slightly nauseous — this is very common and often improves within a few days
- Your appetite may noticeably decrease
- Some people experience mild stomach discomfort or changes in digestion
- Injection site reactions (mild redness or bruising) are normal and temporary
By weeks 2-4, nausea usually settles. Many people report feeling satisfied with smaller meal portions and fewer cravings. Weight loss during this phase is typically modest (1-3 kg), as your body is still adjusting. Keep using our weight converter if you need to track your weight in both kg and stone — many UK adults find it helpful to record both measurements.
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.
What Happens With Dose Increases And Long-Term Expectations
After 4 weeks on 2.5 mg, your dose increases to 5 mg, then 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg — with a minimum of 4 weeks between each increase. Most people see meaningful weight loss between weeks 8-16 as they reach their effective maintenance dose.
Clinical trials (SURMOUNT-1) showed that people taking Mounjaro lost an average of 15-21% of their body weight over 72 weeks when combined with a reduced-calorie diet and physical activity. Results vary significantly between individuals. Some experience side effects during dose escalation (nausea, constipation, or diarrhoea); others have minimal issues. If side effects are severe, your prescriber may delay increasing your dose. Track your progress using our free progress tracker to monitor weight loss alongside how you're feeling.
---COL_J_ALT---After 4 weeks on 2.5 mg, your dose increases to 5 mg, then 7.5 mg, 10 mg, 12.5 mg, and finally 15 mg — with a minimum of 4 weeks between each increase. Most meaningful weight loss happens between weeks 8-16 as you reach your effective maintenance dose.
Clinical trials showed people taking Mounjaro lost an average of 15-21% of their body weight over 72 weeks when combined with a reduced-calorie diet and increased physical activity — though individual results vary. Some people experience side effects during dose increases (nausea, constipation, or diarrhoea); others have minimal issues. If side effects are significant, your prescriber may slow the dose escalation schedule. Our free progress tracker helps you monitor your weight loss journey and how you're feeling over time.
- 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.
What are the most common side effects of Mounjaro in the UK?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain. Most people find nausea improves within the first week or two. Taking smaller meals, staying hydrated, and eating ginger can help. If side effects are severe or persistent, contact your prescriber — they may adjust your dose schedule or offer additional support.
How much weight will I lose on Mounjaro?
Clinical trials showed average weight loss of 15-21% of body weight over 72 weeks — but individual results vary significantly. Some people lose more, some less. Weight loss depends on your starting weight, adherence to a reduced-calorie diet, physical activity level, and how your body responds. Our GLP-1 weight loss calculator can give you a personalised estimate based on your profile.
Can I switch from Wegovy to Mounjaro, or vice versa?
Both are prescription-only weight loss injections, but they work differently. Mounjaro is a dual GIP+GLP-1 agonist; Wegovy (semaglutide) is GLP-1 only. Switching between them must be managed by your prescriber — there is no standard conversion. Your prescriber will assess whether a switch is appropriate for you based on your response and side effects.
Is Mounjaro available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for use within NHS specialist weight management services (tier 3/4). However, NHS availability varies significantly by region, and waiting lists can be long. Private access is available through online prescribing services following a clinical assessment.
What should I do if I miss a weekly injection?
If you remember within 4 days of a missed dose, take it as soon as possible and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next injection on the regular scheduled day. Never double-dose, and ensure at least 72 hours between injections. Contact your prescriber if you're unsure.
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.