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 Happens After 4 Weeks on Mounjaro
If you've been taking Mounjaro for 4 weeks, you're likely on the starting dose of 2.5 mg — the first step in the UK titration schedule. At this stage, you may notice changes in appetite, though significant weight loss typically doesn't occur until higher doses. This guide explains what to expect during your first month on tirzepatide, how the dose progression works through NHS or private prescribing, and when to contact your GP if you have concerns.
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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 After 4 Weeks: Dose Progression and What to Expect
After 4 weeks on Mounjaro 2.5 mg, your prescriber will typically increase your dose to 5 mg for weeks 5–8. This dose titration schedule is standard across both NHS specialist services and private prescribing in the UK. The 2.5 mg starting dose is designed to help your body adjust to the medication and minimise side effects, rather than deliver gradual weight management.
- Week 1–4: You're on 2.5 mg — an adjustment phase focused on tolerability
- Week 5–8: Dose increases to 5 mg — where meaningful weight loss often begins
- Weeks 9–12+: Further escalations possible (7.5 mg, 10 mg, 12.5 mg, up to 15 mg maximum)
At 4 weeks, clinical trial data shows participants have typically lost only a modest amount of weight — the full benefit of tirzepatide emerges at therapeutic doses (10–15 mg). Your GP or prescriber may ask how you're tolerating side effects like nausea or appetite changes at this point, as this informs whether to proceed with the dose increase on schedule.
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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.
Side Effects and Symptoms at the 4-Week Mark
By week 4, common side effects like nausea, diarrhoea, and constipation may have settled or worsened depending on your individual response. Nausea is the most frequently reported side effect of Mounjaro in the UK, affecting more than 1 in 10 people — some find it reduces as their body adapts, whilst others experience it more at higher doses.
After 4 weeks, you should contact your GP or prescriber if you experience severe stomach pain radiating to your back (possible pancreatitis), persistent vomiting preventing food intake, or signs of a serious allergic reaction. Mild to moderate nausea, reduced appetite, or loose stools at this stage are expected. Using our free GLP-1 weight loss calculator can help you understand realistic weight loss expectations across the full dose range — most significant results occur between weeks 12 and 20 when you've reached therapeutic doses.
- 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 lose weight in my first 4 weeks on Mounjaro?
Weight loss varies significantly between individuals. The 2.5 mg starting dose is designed for tolerability, not weight loss. Most people lose minimal weight in the first 4 weeks. Meaningful weight loss typically begins at the 5 mg dose (weeks 5–8) and increases as you progress to higher doses. Clinical trial data shows the most significant results occur at 10–15 mg doses.
What happens to my Mounjaro prescription after 4 weeks?
Your prescriber will typically increase your dose from 2.5 mg to 5 mg to begin week 5. This follows the standard UK titration schedule used in NHS specialist services and private prescribing. You'll remain on 5 mg for at least 4 weeks before any further increase, depending on how well you tolerate it and your response.
Is 4 weeks on Mounjaro long enough to know if it's working?
Four weeks is too early to assess whether Mounjaro is working for weight loss, as you're still on the starting dose. The full therapeutic effect emerges at higher doses (10–15 mg), typically reached around weeks 12–20. This is why prescribers recommend staying on the medication for at least 12 weeks before assessing effectiveness.
What should I do if I'm experiencing severe side effects at week 4?
Contact your GP or prescriber immediately if you experience severe stomach pain, persistent vomiting, signs of allergic reaction, or rapid heart rate changes. Mild to moderate nausea, diarrhoea, or constipation at 4 weeks are common and may improve. Your prescriber can advise whether to proceed with the dose increase or pause escalation.
How does Mounjaro compare to Wegovy at the 4-week stage?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. At 4 weeks, both medications follow similar tolerability-focused starting doses. Clinical trials suggest Mounjaro may produce slightly greater weight loss at equivalent therapeutic doses, though individual results vary significantly and require clinical assessment by your prescriber.
Ready to take control of your weight loss journey?
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