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 on Your Second Dose of Mounjaro?
Your second dose of Mounjaro marks the start of your first maintenance phase. After four weeks on the 2.5 mg starting dose, you'll move to 5 mg — a significant step in your tirzepatide treatment journey. This dose escalation is planned and expected as part of the standard titration schedule. Understanding what to expect during weeks 5–8 can help you prepare for changes in side effects, appetite suppression, and how your body responds to the increased dose of this dual GIP and GLP-1 receptor agonist medication.
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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.
Understanding Mounjaro Dose Titration and Your Second Injection
Mounjaro is escalated gradually over several weeks to allow your body to adjust and to identify the lowest effective dose for you. Your second dose of 5 mg comes exactly four weeks after your first 2.5 mg injection. This is not optional — the standard UK dosing schedule requires a minimum of four weeks between dose changes.
- Week 1–4: You receive the 2.5 mg starting dose
- Week 5–8: You receive your second dose — 5 mg (a doubling of the first dose)
- Week 9–12: Potential escalation to 7.5 mg if tolerated and needed
- Onwards: Further escalations to 10 mg, 12.5 mg, and maximum 15 mg at four-weekly intervals
Each KwikPen contains four doses (four weeks' supply). Your second KwikPen will contain four 5 mg injections. You must inject on the same day each week — consistency matters for consistent blood levels and predictable side effect patterns.
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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.
What to Expect: Side Effects and Appetite Changes at 5 mg
Most people experience more noticeable appetite suppression at 5 mg compared to the starting dose. Nausea — the most common side effect — may intensify temporarily before settling around week 8. Some people report that initial nausea from the first four weeks improves, while others find it peaks at the higher dose before adapting. Diarrhoea and constipation are common and can alternate. Fatigue, dizziness, and taste changes may also emerge or intensify.
These side effects are temporary and often settle by week 8 as your body adapts. Eating smaller, frequent meals; staying hydrated; and avoiding fatty or sugary foods can help manage gastrointestinal symptoms. If side effects are severe or you cannot tolerate the 5 mg dose, contact your prescriber — dose escalation can be delayed or adjusted.
- 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.
When exactly do I take my second Mounjaro dose?
Your second 5 mg dose is given exactly four weeks (28 days) after your first 2.5 mg injection. Inject on the same day and time each week thereafter. If you're more than three days late, take it as soon as you remember and resume your normal weekly schedule from that day.
Can I skip the second dose or stay on 2.5 mg longer?
The standard titration schedule escalates to 5 mg at week 5. Staying on 2.5 mg longer is not recommended — tirzepatide works best at therapeutic doses. However, if you have severe side effects, contact your prescriber. They may delay escalation but will not typically keep you at the starting dose long-term.
Will my side effects get worse when I move from 2.5 mg to 5 mg?
Possibly, but not always. Some people experience temporary worsening of nausea, diarrhoea, or constipation. Others find side effects improve as their body adapts to the higher dose. Changes are usually manageable within 1–2 weeks. Eating smaller meals, staying hydrated, and resting helps. If you're very unwell, contact your GP or prescriber.
How do I store my second KwikPen before I use it?
Keep unopened KwikPens in the fridge at 2–8°C in their original carton, away from light. Do not freeze. Once you start using a KwikPen (after the first injection), it can be kept at room temperature (below 30°C) for up to 30 days. Dispose of it after 30 days even if doses remain.
What if I miss my second dose — can I take it late?
Yes. If you're within four days of your scheduled injection, take it as soon as you remember. If more than four days have passed, skip that dose and take your next scheduled dose on your usual day. Never double-dose. Minimum time between injections is 72 hours (three days).
Ready to take control of your weight loss journey?
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