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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Mounjaro Pen After 4 Doses vs Wegovy — What's the Difference?
After 4 weeks (one KwikPen), you're completing the starting dose of tirzepatide (Mounjaro) — 2.5 mg. At this point, your prescriber will assess whether to continue, hold, or escalate. Understanding how Mounjaro compares to Wegovy at this stage helps you understand what to expect next. Both are prescription-only GLP-1 medications (Mounjaro is technically a dual GIP + GLP-1 agonist), but they work differently and follow different dose titration schedules. Here's what you need to know.
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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.
How Mounjaro and Wegovy Differ in the First Month
Mounjaro (tirzepatide) starts at 2.5 mg weekly — the lowest available dose. After 4 weeks, you've received one complete KwikPen (4 doses) and are ready to escalate to 5 mg in week 5 if tolerated. Mounjaro uses a dual GIP + GLP-1 mechanism, meaning it activates two hormone pathways simultaneously, which may explain stronger appetite suppression for some users.
Wegovy (semaglutide) also starts low (0.25 mg) but escalates faster — after 4 weeks you move to 0.5 mg in week 5. Wegovy uses a single GLP-1 mechanism only. Clinical STEP trial data showed average weight loss of approximately 15% body weight at the 2.4 mg maintenance dose (weeks 17+), whilst SURMOUNT trial data showed Mounjaro achieved approximately 15–21% at higher doses.
Both medications slow gastric emptying (how quickly food leaves your stomach), reducing hunger and supporting smaller meal portions. Individual responses vary significantly — some people tolerate escalation quickly, others need to pause at a dose longer.
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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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Which Medication After 4 Doses — Prescriber Decision
After your first KwikPen of Mounjaro, your prescriber will review your tolerability — specifically nausea, vomiting, diarrhoea, and constipation, which are common initially. If you're managing well, escalation to 5 mg follows. If side effects are significant, your prescriber may hold at 2.5 mg for additional weeks or switch medications entirely.
The choice between Mounjaro and Wegovy is not about one being "better" — it's about individual suitability. Factors include: your medical history, concurrent medications (especially oral contraceptives, which Mounjaro affects), tolerability to gastrointestinal side effects, and prescriber experience. Only a UK-licensed prescriber can assess your personal circumstances. Use our free GLP-1 weight loss calculator to understand potential outcomes at different dose levels, and our BMI calculator to track baseline health context.
- 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.
After 4 weeks on Mounjaro, what happens next?
Your prescriber will assess tolerability. If well tolerated, you escalate to 5 mg in week 5. If side effects are significant, you may hold at 2.5 mg longer or discuss alternative medications like Wegovy with your prescriber.
Is Mounjaro's starting dose lower than Wegovy's?
Yes. Mounjaro starts at 2.5 mg weekly; Wegovy starts at 0.25 mg but has faster escalation. After 4 weeks, Mounjaro is still at starting dose whilst Wegovy users have already moved to 0.5 mg (week 5). Maximum doses differ too — 15 mg for Mounjaro vs 2.4 mg for Wegovy.
Why does Mounjaro use a dual mechanism?
Mounjaro activates both GIP and GLP-1 receptors. GLP-1 reduces appetite; GIP supports glucose control and fat utilisation. This dual action may offer stronger satiety for some users, though clinical efficacy depends on individual biology and lifestyle.
Can I switch from Mounjaro to Wegovy after 4 doses?
Yes, but only with prescriber guidance. There is no official established conversion — your prescriber would assess your tolerability on Mounjaro and discuss alternatives if appropriate. Any switch requires a new clinical assessment.
Do both medications require a prescription in the UK?
Yes. Both Mounjaro and Wegovy are prescription-only in the UK. Access is through NHS specialist services (tier 3) or private online prescribing following a clinical assessment. Neither is available over-the-counter.
Ready to take control of your weight loss journey?
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