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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Headaches and Mounjaro — What the Evidence Shows
Headaches are a reported side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), though they are not among the most commonly experienced effects. Understanding how headaches compare between these two prescription weight management injections can help you and your GP make an informed decision about which medication might suit you best. This guide explains what clinical trials showed, how headaches fit into the broader side effect profile, and when to seek medical advice.
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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.
Headaches in Mounjaro vs Wegovy — Clinical Trial Data
In the SURMOUNT clinical trials for Mounjaro (tirzepatide), headaches were reported but occurred at a relatively low frequency compared to gastrointestinal side effects like nausea and vomiting. The exact percentage of participants reporting headaches varied by trial phase and dose, but headaches were generally listed as an uncommon to common side effect rather than very common.
For Wegovy (semaglutide), clinical data from the STEP trials showed a similar pattern — headaches were reported by some participants but were not among the most frequently reported adverse effects. Both medications affect appetite regulation through GLP-1 (and in Mounjaro's case, dual GIP+GLP-1) mechanisms, yet neither is specifically noted for causing headaches as a primary concern.
- Mounjaro: headaches reported but not very common
- Wegovy: similar frequency to Mounjaro
- Both: gastrointestinal effects (nausea, constipation) are far more common
- Both: headaches often resolve within the first few weeks as the body adjusts
Individual experiences vary significantly — some UK users report no headaches at all, whilst others experience mild, temporary headaches during dose titration.
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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.
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Individual Suitability and Prescriber Guidance
Because both Mounjaro and Wegovy require a prescription and individual clinical assessment, your prescriber will consider your full medical history, current medications, and any existing headache conditions when deciding which medication is most suitable. If you have a history of migraines or frequent headaches, this is important information to share during your consultation.
Headaches reported in clinical trials may be related to dehydration (common with gastrointestinal side effects), rapid blood sugar changes, or medication adjustment rather than the medication itself. Use our free BMI calculator to check your eligibility first, then discuss any side effect concerns with your GP or prescriber. Both medications are equally prescription-only — neither is available without clinical assessment and a valid UK prescription.
- 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.
Are headaches a common side effect of Mounjaro?
Headaches are reported with Mounjaro but are not among the very common side effects. Nausea, diarrhoea, and constipation are far more frequently reported. Clinical trials showed headaches occur but less commonly than gastrointestinal effects. Most reported headaches are mild and often resolve within the first few weeks of treatment.
Do Wegovy and Mounjaro cause headaches at the same rate?
Clinical trial data suggests headaches occur at similar frequencies with both medications. Neither Wegovy nor Mounjaro is specifically associated with headaches as a primary side effect. Individual experience varies — some users report no headaches at all, whilst others may experience mild headaches during dose titration.
Should I tell my prescriber if I have frequent headaches?
Yes, absolutely. Inform your prescriber about any existing headache conditions, migraines, or medications you take for headaches before starting either Mounjaro or Wegovy. This information helps them assess suitability and monitor your health safely throughout treatment.
What should I do if I develop headaches while taking Mounjaro or Wegovy?
Mild headaches are often temporary and may improve with increased water intake — dehydration can occur with these medications. If headaches are severe, persistent, or accompanied by other concerning symptoms, contact your GP. Most headaches resolve naturally as your body adjusts to the medication.
Can I use our weight loss calculator with either medication?
Yes — our free GLP-1 weight loss calculator can help you estimate projected weight loss at different doses of either Mounjaro or Wegovy. This tool is for informational purposes alongside your prescriber's guidance and can be useful when discussing realistic outcomes during your clinical assessment.
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.