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 and Headaches: How It Compares to Wegovy
Headaches are reported in clinical trials of both Mounjaro (tirzepatide) and Wegovy (semaglutide), though they are not among the most common side effects of either medication. Both are prescription-only GLP-1 receptor agonist injections licensed for weight management in the UK, but they work through different mechanisms. Understanding how these medications differ—and how headaches fit into the broader side effect profile—can help you and your GP decide which option might be more suitable for your individual circumstances.
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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.
Clinical Trial Data: Headaches and Mounjaro vs Wegovy
Headaches are reported less frequently than gastrointestinal side effects in both the SURMOUNT and STEP clinical trials. In the SURMOUNT-1 trial for Mounjaro (tirzepatide), headache occurred in approximately 8–10% of participants across all dose groups—classified as "uncommon" rather than common. In the STEP-1 trial for Wegovy (semaglutide), fatigue and dizziness are more prominent neurological side effects, with headaches reported less frequently than with Mounjaro.
Key mechanism difference: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 only. This dual mechanism may account for slightly different side effect profiles between the two. Headaches associated with either medication are typically mild to moderate, occur during the dose escalation phase, and often resolve as the body adjusts.
Both medications slow gastric emptying (how quickly food leaves the stomach), which can indirectly contribute to headaches through dehydration or electrolyte changes if fluid intake is not maintained adequately.
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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.
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Which Medication Might Be Right for You?
The choice between Mounjaro and Wegovy is not determined by headache risk alone—it depends on your overall health profile, BMI, comorbidities, and how your body responds to treatment. Your GP or prescriber will assess whether you meet UK eligibility criteria: a BMI of 30 kg/m² or above, or 25–29.9 with weight-related health conditions such as type 2 diabetes or high blood pressure.
If you experience headaches on either medication, your prescriber may recommend staying at your current dose longer before escalating, increasing water intake, or managing symptoms with over-the-counter pain relief. Some people tolerate one medication better than the other—individual responses vary significantly. A prescriber can discuss your medical history and any previous side effect patterns to guide the decision.
- 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 serious side effect of Mounjaro?
Headaches reported in clinical trials are typically mild to moderate and often resolve within a few weeks as your body adjusts to the medication. They are classified as "uncommon" rather than common. However, if you experience severe, persistent, or worsening headaches, contact your GP or prescriber for advice.
Do more people get headaches on Mounjaro or Wegovy?
Clinical trial data suggests headaches are slightly more frequently reported with Mounjaro (tirzepatide) than Wegovy (semaglutide), though both rates are relatively low. The difference is not dramatic enough to make this the primary factor in choosing between medications.
Can I prevent headaches when taking Mounjaro?
Staying well hydrated is important—GLP-1 medications can slow stomach emptying, which may affect fluid absorption. Maintain regular meals with balanced nutrition, avoid skipping doses, and do not escalate your dose more quickly than recommended. If headaches develop, discuss them with your prescriber before adjusting anything.
What should I do if I get a headache on Mounjaro?
For mild headaches, paracetamol or ibuprofen are safe to take alongside Mounjaro. Ensure you're drinking enough water and eating regular balanced meals. If headaches are severe, frequent, or accompanied by other symptoms like vision changes, contact your GP immediately—these could indicate a separate issue requiring investigation.
Can I switch from Mounjaro to Wegovy if I get headaches?
Possibly, but this is a decision for your prescriber. Some people tolerate one medication better than the other. Your prescriber will weigh factors including your weight loss progress, other side effects, and overall health. Any switch between GLP-1 medications should be managed by your GP or specialist to ensure proper dose transition and monitoring.
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