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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Tips When Taking Mounjaro
Getting the most from Mounjaro (tirzepatide) means understanding how to use it correctly, manage side effects, and maintain realistic expectations. This guide covers practical tips for UK adults taking this prescription-only weight management injection, including dosing schedules, storage, injection technique, and what to expect during treatment. Always follow your prescriber's guidance and contact your GP if you have concerns.
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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.
Practical Administration and Side Effect Management
Mounjaro is a once-weekly KwikPen injection that requires proper storage and technique to work effectively. Store unopened pens in the fridge at 2°C to 8°C, and after first use you can keep them at room temperature below 30°C for up to 30 days. Never freeze your pens. When injecting, rotate your injection site each week between your stomach (abdomen), upper thigh, or upper arm to prevent lipohypertrophy (thickened skin). Allow the pen to warm to room temperature for 30 minutes before injecting if it's been refrigerated.
- Inject at the same time each week for consistency
- Use a new needle for every injection
- Dispose of used pens in a sharps container — ask your pharmacist how to safely dispose of them
- Nausea is very common, especially at the start — try eating smaller, bland meals and staying hydrated
- If constipation occurs, increase fibre gradually and drink plenty of water
- Start with anti-nausea strategies before symptoms become severe — ginger tea or anti-nausea medication from your GP can help
If side effects persist beyond the first 1–2 weeks on a dose, speak to your prescriber about whether staying at your current dose longer before increasing is appropriate.
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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.
Dosing, Expectations, and What to Monitor
Mounjaro starts at 2.5 mg and increases every 4 weeks: 5 mg, 7.5 mg, 10 mg, 12.5 mg, then 15 mg maximum. Never skip doses or double up — if you miss a dose within 4 days, take it as soon as you remember. Weight loss isn't immediate: clinical trials showed average loss of 15–21% body weight over 72 weeks alongside a reduced-calorie diet and increased physical activity. Individual results vary considerably.
- Keep a food and mood diary to track how tirzepatide affects your appetite and cravings
- Stay active — injections work best with lifestyle changes, not instead of them
- Monitor your blood sugar if you have diabetes — your prescriber may adjust other medications
- Check your waist circumference monthly using our free weight converter and progress tracker to track changes even if the scales don't move weekly
- 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 should I do if I feel very sick when taking Mounjaro?
Mild to moderate nausea is common, especially when starting or increasing doses. Eat smaller, blander meals, stay hydrated, and avoid fatty foods. Anti-nausea medication from your GP can help. If nausea is severe or persistent after 2 weeks on a dose, contact your prescriber — they may recommend staying at your current dose longer before increasing.
Can I switch between Mounjaro and Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is GLP-1 only. Any switch must be managed by your prescriber. They are not directly interchangeable — approximate equivalences exist (5 mg Mounjaro ≈ 0.5 mg Wegovy) but only your prescriber can advise on switching safely.
What happens if I miss a Mounjaro injection?
If you miss your dose within 4 days, take it as soon as you remember, then resume your normal weekly schedule. If more than 4 days have passed, skip that dose and take your next dose on your normal scheduled day. Never take a double dose. Ensure at least 72 hours between injections.
Will Mounjaro affect my other medications?
Yes — Mounjaro may reduce how quickly your gut absorbs oral medications, including oral contraceptives. If you take the pill, add a barrier method or switch contraception for 4 weeks after starting and for 4 weeks after each dose increase. If you take diabetes medications or other prescriptions, tell your prescriber so they can monitor or adjust doses as needed.
How long does weight loss take on Mounjaro?
Weight loss is gradual. Clinical trials showed measurable changes over several weeks, with more significant losses visible after 12+ weeks. Use our free GLP-1 weight loss calculator to estimate potential results at different doses. Remember that results vary — some people lose more or less than average — and weight loss is most effective alongside reduced-calorie diet and increased physical activity.
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.