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 vs Wegovy: Headaches and Side Effects Compared
Headaches are a reported side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), though they're not the most common complaint with either medication. Both are GLP-1-based weight management injections licensed in the UK, but they work differently and have slightly different side effect profiles. Understanding how they compare can help you and your GP or prescriber make an informed decision about which option might suit your circumstances. Use our free BMI calculator above to check where you stand against NHS eligibility criteria.
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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 vs Wegovy: Clinical Trial Data and Side Effect Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1-only agonist — this difference in mechanism may influence side effect frequency. In the SURMOUNT-1 clinical trial, headaches were reported in a minority of participants on Mounjaro; fatigue and dizziness were more commonly reported than headaches specifically. Wegovy's STEP-1 trial similarly showed headaches as an uncommon side effect, with nausea, diarrhoea, and constipation being more prevalent. Neither clinical trial identified headaches as a primary or very common side effect. Individual responses vary significantly — some people experience no headaches at all, whilst others report mild, transient headaches during dose escalation. Both medications can cause dehydration and low blood sugar, which may trigger headaches as a secondary effect. Your prescriber may recommend dose escalation adjustments if headaches develop.
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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.
Which Medication Is Right for You: Individual Suitability and Prescriber Assessment
The choice between Mounjaro and Wegovy depends on your individual health profile, not just side effect profiles. Both are prescription-only medicines requiring clinical assessment and ongoing prescriber support. Eligibility in the UK typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease). Your prescriber will consider your full medical history, current medications, and personal health goals. If you have a history of headaches or migraine, inform your prescriber — they can monitor you closely and adjust your dose if needed. Try our free GLP-1 weight loss calculator to estimate projected outcomes at different doses, helping you and your prescriber discuss realistic expectations. Both medications are available through NHS specialist weight management services (with waiting lists) or private online prescribing following clinical assessment.
- 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.
Is a headache a common side effect of Mounjaro?
Headaches are reported but uncommon with Mounjaro. In clinical trials, nausea, diarrhoea, and fatigue were more frequent. Headaches may occur during dose escalation or if dehydration develops. Most resolve as your body adjusts. Always report persistent headaches to your prescriber.
Do Wegovy and Mounjaro cause headaches at different rates?
Clinical trials don't show a significant difference in headache frequency between the two. Both are uncommon side effects with both medications. Individual experiences vary — some people report none, others mild transient headaches. Your prescriber can help manage this if it occurs.
What should I do if I develop a headache on Mounjaro or Wegovy?
Most medication-related headaches improve with time as your body adjusts. Stay hydrated and consider paracetamol if needed. If headaches are severe, persistent, or accompanied by vision changes, contact your prescriber or GP immediately — these could signal a different issue requiring assessment.
Are headaches a sign that a GLP-1 medication isn't working or isn't right for me?
No. Headaches are a known, usually mild side effect that often settles within days or weeks. They don't indicate treatment failure. Your prescriber may pause dose escalation temporarily or investigate other causes (dehydration, blood sugar changes) before deciding whether to continue.
How do I get assessed for Mounjaro or Wegovy in the UK if I'm concerned about side effects?
Contact your GP to discuss your concerns and health goals. You can be referred to NHS specialist tier 3 weight management services (waiting times vary by region) or seek a private online prescription following clinical assessment with a UK-licensed prescriber. Your prescriber will review your full health picture before recommending a medication.
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.