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 Happens at Mounjaro Week 5?
Week 5 is a significant milestone on your Mounjaro journey. By this point, you're stepping up from the starting dose of 2.5mg to the first maintenance dose of 5mg — the dose where most people begin to experience the weight management effects. During weeks 1–4, your body has been adjusting to tirzepatide, the active ingredient. Now the effects typically become more noticeable. This guide explains what to expect at week 5, how your body may respond, and practical tips for managing this dose transition in the UK.
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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 Dose Progression: Week 5 Transition
Week 5 is when you move from the 2.5mg starting dose to 5mg. This is not a new injection — you continue with one KwikPen injection per week, but the pen now contains 5mg instead of 2.5mg. Your prescriber will have provided clear instructions on when to make this change. It's important to follow the timing exactly: if you started on a Monday in week 1, your week 5 injection should also be on a Monday.
At the 5mg dose, clinical trials showed participants began to see more noticeable appetite suppression and weight management support. However, individual responses vary significantly. Some people report stronger effects at week 5; others notice gradual changes over several weeks. Your body has now had four weeks to adjust to how tirzepatide works, which is why this maintenance dose often marks the point where people begin tracking more obvious changes.
- Week 5 = first maintenance dose (not the final dose)
- Continue once-weekly injections on your scheduled day
- Store your new pen in the fridge at 2–8°C before first use
- Effects may feel more pronounced than weeks 1–4
- Side effects may persist or change — this is normal
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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
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Managing Side Effects and Health Changes at Week 5
Many people continue to experience nausea, changes in digestion, or reduced appetite at the 5mg dose — these are expected effects of how tirzepatide works. The good news: side effects often improve as your body adapts over the next 2–3 weeks. If side effects are severe or aren't improving, contact your GP or prescriber immediately.
At week 5, pay attention to how you're eating and drinking. You may naturally eat smaller portions — this is the medication working as intended. However, ensure you're still getting enough nutrients, staying hydrated, and not skipping meals entirely. Our free GLP-1 weight loss calculator can help you understand expected progress, while tracking your weight with our progress tracker lets you see if the transition to 5mg is having an effect on your personal journey. If you have any concerns about how you're responding, your prescriber can assess whether to continue at 5mg or adjust your plan.
- 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 see weight loss by week 5 on Mounjaro?
Weight loss responses vary significantly. Some people notice changes by week 5; others see gradual changes over weeks 5–12. The 5mg dose is when effects typically become more noticeable, but clinical trials showed the most significant weight loss occurred over 12+ weeks. Be patient and consistent with your dose and lifestyle changes.
Is week 5 when you increase the dose?
Yes. You move from 2.5mg (weeks 1–4) to 5mg at week 5. This is your first maintenance dose. You'll stay on 5mg for at least 4 weeks before any potential increase to 7.5mg. Always follow your prescriber's instructions on when to increase.
Can I switch my injection day at week 5?
No — you should keep the same day each week. If you started on a Monday, continue injecting on Mondays. Consistency helps your body maintain steady levels of tirzepatide. If you need to change your day, discuss this with your prescriber first.
Are side effects worse at the 5mg dose?
Some people notice side effects intensify slightly when moving to 5mg; others experience improvement. Nausea, constipation, and reduced appetite are common but often settle over 2–3 weeks as your body adapts. Staying hydrated and eating small, regular meals helps.
What should I do if I feel unwell at week 5?
Contact your GP or prescriber if you experience severe stomach pain, persistent vomiting, signs of allergic reaction, or any concerning changes. These could indicate pancreatitis or other complications requiring urgent assessment. Don't wait — get medical advice promptly.
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.