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 Headaches vs Wegovy — What's the Difference?
Headaches are a reported side effect with both Mounjaro (tirzepatide) and Wegovy (semaglutide), though they are more common with Wegovy according to clinical trial data. Both are prescription-only weight management injections licensed in the UK. While headaches can occur with either medication, the frequency, mechanism, and individual experience differ. Understanding how each works and what the clinical evidence shows can help you and your prescriber decide which might be more suitable for you following a clinical assessment.
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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 in SURMOUNT vs STEP
Headaches were reported differently in the major clinical trials for each medication. In the SURMOUNT-1 trial for Mounjaro (tirzepatide), headaches were reported by approximately 13–15% of participants at the highest dose (15 mg). In the STEP-1 trial for Wegovy (semaglutide 2.4 mg), headaches occurred in approximately 26% of participants — roughly twice as common as with Mounjaro.
The difference may relate to their mechanisms: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 receptors only. This single versus dual mechanism difference may influence how your body responds, though individual variation is significant. Both trials included participants on reduced-calorie diets and increased physical activity, which themselves can cause headaches independently of the medication.
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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.
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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Why Headaches Occur and Individual Suitability
Headaches on weight management injections may result from several factors: rapid fluid shifts as your body adjusts, changes in appetite and eating patterns, dehydration if fluid intake is inadequate, or the medication's direct effect on blood vessels. Some headaches are mild and resolve within weeks; others persist and may prompt a dose adjustment or medication change.
Neither Mounjaro nor Wegovy is "better" for headaches — suitability depends entirely on your individual health profile. Your GP or prescriber will assess your medical history, current medications, and weight management goals to recommend the most appropriate option. If headaches develop on either medication, your prescriber can adjust your dose, suggest management strategies, or consider switching. Use our GLP-1 weight loss calculator to explore projected outcomes with different medications, and discuss specific concerns about side effects with your clinician before starting.
- 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 common with Mounjaro?
Headaches were reported by 13–15% of participants in the SURMOUNT-1 clinical trial at the highest Mounjaro dose. They are a known side effect but not the most common reported. Most reported side effects involve the digestive system (nausea, diarrhoea, constipation) rather than headaches. If you develop persistent headaches, inform your prescriber.
Does Wegovy cause more headaches than Mounjaro?
Yes, according to clinical trial data. Wegovy showed headaches in approximately 26% of participants in STEP-1, roughly twice the rate observed with Mounjaro in SURMOUNT-1. However, individual experiences vary widely. Some people experience no headaches on either medication; others are significantly affected.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways. Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one pathway. This mechanistic difference may contribute to varying side effect profiles, though individual factors matter far more than the medication type.
Can I switch from Mounjaro to Wegovy if I get headaches?
Yes, if headaches become problematic on one medication, your prescriber may suggest switching to the other. A switch requires a new clinical assessment and prescription. Some people find headaches resolve on the alternative medication; others may need dose adjustments or additional support. Always discuss with your prescriber before making changes.
What should I do if I develop headaches on either injection?
Contact your GP or the prescriber who issued your prescription. Do not stop the medication without guidance. Your prescriber can offer several options: continue at current dose (headaches often improve with time), reduce to a lower dose, adjust the dosing schedule, suggest supportive measures like hydration, or consider switching medications. Individual management is personalised to your situation.
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