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 Give You Headaches?
Headaches are not listed as a common side effect of Mounjaro (tirzepatide) in UK prescribing information, though individual responses to the medication vary. Most reported side effects relate to the gastrointestinal system. If you experience headaches whilst taking Mounjaro, it's important to discuss this with your GP or the prescriber managing your treatment, as headaches can sometimes be related to dehydration or other factors rather than the medication itself. Understanding what to expect helps you use your Mounjaro KwikPen safely and confidently.
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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 Side Effects — What the Evidence Shows
Mounjaro (tirzepatide) side effects are primarily gastrointestinal, according to NHS and MHRA guidance. The most common side effects reported in clinical trials include nausea, diarrhoea, vomiting, and constipation. Headaches are not listed as a common or very common side effect. However, they may occur in a small percentage of users, as with most medications. Key points:
- Very common side effects (affecting more than 1 in 10 people): nausea, diarrhoea, vomiting, stomach pain
- Common side effects (up to 1 in 10): indigestion, fatigue, dizziness, injection site reactions
- Headaches may occasionally occur but are not prominently reported in UK prescribing data
- If you experience persistent headaches, inform your prescriber — they may be unrelated to tirzepatide
The SURMOUNT clinical trials, which led to UK approval, documented side effects in detail. Headaches were reported at similar rates in both treatment and control groups, suggesting they may not be directly caused by the medication.
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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 on Mounjaro
If you develop headaches whilst taking Mounjaro, contact your GP or prescriber if:
- The headaches are severe or persistent (lasting more than a few days)
- They are different in nature or intensity from headaches you normally experience
- They are accompanied by visual changes, neck stiffness, or confusion (rare but seek urgent help)
- They interfere with your daily activities or the medication's tolerability
Your prescriber can assess whether the headaches are medication-related or due to other factors such as dehydration (which can occur with GLP-1 medications), caffeine changes, or stress. Always stay hydrated and monitor your fluid intake when taking Mounjaro, as dehydration is a known trigger for 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.
Are headaches a known side effect of Mounjaro in the UK?
Headaches are not listed as a common or very common side effect of Mounjaro (tirzepatide) in MHRA and NHS prescribing information. The most frequently reported side effects are gastrointestinal — nausea, diarrhoea, vomiting, and constipation. If you experience headaches, they may be unrelated to the medication and should be discussed with your prescriber.
What are the most common Mounjaro side effects?
The most common side effects of Mounjaro are gastrointestinal: nausea (very common), diarrhoea, vomiting, constipation, and stomach pain. These typically occur when starting the medication or during dose increases and often improve over time. Dizziness and fatigue are also commonly reported.
Could Mounjaro headaches be caused by dehydration?
Yes, dehydration can trigger headaches when taking GLP-1 and GIP/GLP-1 medications like Mounjaro. These medications slow gastric emptying and may affect fluid intake. Ensure you drink adequate water throughout the day and stay hydrated, especially if experiencing nausea or vomiting.
Should I stop taking Mounjaro if I get headaches?
Do not stop taking Mounjaro without consulting your prescriber. If headaches develop, contact your GP or the service managing your tirzepatide treatment. They can assess the cause and advise whether to continue, adjust your dose, or investigate other factors. Never discontinue a prescription medication independently.
How do Mounjaro headaches compare to Wegovy headaches?
Headaches are reported at similar low rates with both Mounjaro (tirzepatide) and Wegovy (semaglutide). Neither medication lists headaches as a common side effect in UK prescribing data. Most reported side effects for both are gastrointestinal. Individual responses vary, and any concerning symptoms should be discussed with your prescriber.
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