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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Does Mounjaro Cause Headaches?
Headaches are not listed as a common or very common side effect of Mounjaro (tirzepatide) in UK clinical data, though some people may experience them. Headaches can occur for various reasons when starting any new medication, including dehydration, changes in eating patterns, or underlying conditions. If you experience headaches whilst taking Mounjaro, it's important to understand whether they're related to the medication itself or other factors. This guide explains what the clinical evidence shows and when to contact your GP.
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Calculate Your BMI Instantly
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.
Is Headache a Known Mounjaro Side Effect?
According to UK clinical trial data and the Mounjaro product information, headache is not listed as a very common or common side effect. The most frequently reported side effects are gastrointestinal — nausea, diarrhoea, vomiting, and constipation. However, headaches can occasionally occur in individual cases, and they may be related to secondary factors rather than the tirzepatide itself.
Common reasons people may experience headaches when starting Mounjaro include:
- Dehydration — GI side effects can lead to reduced fluid intake
- Changes in eating patterns — skipping meals or eating less may trigger headaches
- Blood sugar changes — particularly in people with diabetes also taking other medications
- Caffeine changes — if nausea causes you to reduce caffeine intake
- Stress or anxiety — starting a new treatment can be stressful
If you experience persistent or severe headaches, contact your GP to rule out other causes and discuss whether Mounjaro may be contributing.
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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.
When to Contact Your GP About Headaches
Most mild headaches resolve within a few days of starting Mounjaro, but seek urgent medical attention if your headache is severe, sudden, or accompanied by other symptoms such as vision changes, confusion, stiff neck, or fever — these could indicate a more serious condition unrelated to Mounjaro.
Contact your GP if:
- Headaches persist for more than a few days
- They are severe or worsening
- They occur alongside other concerning symptoms
- They significantly affect your daily life
Your prescriber can advise whether to continue Mounjaro, adjust your dose, or explore other causes. You can also track any symptoms using our free progress tracker to help your GP understand the pattern and timing of your headaches.
- 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.
Can Mounjaro cause headaches directly?
Headache is not listed as a common side effect of tirzepatide in UK clinical trials. However, headaches may occur due to secondary factors like dehydration, reduced food intake, or changes in caffeine consumption. If you develop headaches after starting Mounjaro, contact your GP to discuss timing, severity, and other potential causes.
How long do Mounjaro headaches last?
If headaches are related to starting Mounjaro, they typically improve within a few days as your body adjusts. Most people do not experience persistent headaches. However, if headaches continue beyond a week or are severe, speak to your GP rather than assuming they will resolve.
What should I do if I have a headache whilst taking Mounjaro?
Try increasing water intake to address potential dehydration, eating regular small meals, and ensuring adequate sleep. Over-the-counter painkillers like paracetamol or ibuprofen may help. If the headache persists, worsens, or is accompanied by other symptoms, contact your GP or call NHS 111 for advice.
Can dehydration from Mounjaro side effects cause headaches?
Yes — gastrointestinal side effects like diarrhoea and vomiting can lead to dehydration, which commonly triggers headaches. Ensure you drink enough water throughout the day, especially if experiencing nausea or diarrhoea. If you're struggling to keep fluids down, speak to your GP promptly.
Is a headache a reason to stop taking Mounjaro?
A mild headache alone is unlikely to require stopping Mounjaro, but persistent or severe headaches warrant GP discussion. Your prescriber can assess whether the headache is medication-related, help manage it, or discuss alternative options. Never stop taking Mounjaro without medical guidance — it should be stopped under professional supervision if necessary.
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.