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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Headache From Mounjaro vs Wegovy — What You Need to Know
Headaches can occur when starting Mounjaro or Wegovy, though they're not among the most common side effects reported in clinical trials. Both medications work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only — which may influence individual side effect experiences. Understanding how headaches compare between these two prescription weight management injections can help you decide which medication might be more suitable for you, alongside guidance from your GP or prescriber.
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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.
Mounjaro vs Wegovy — Headache Data and Mechanisms
Headaches are reported as a common but not very common side effect in both the SURMOUNT clinical trials (Mounjaro) and STEP trials (Wegovy). In SURMOUNT-1, headaches occurred in approximately 5-8% of participants on tirzepatide. In STEP-1, headaches were reported in around 7-9% of participants on semaglutide 2.4 mg. These rates are significantly lower than nausea or diarrhoea, which affect more than 1 in 10 people on both medications.
The difference in mechanism may affect how your body responds: Mounjaro's dual action (targeting both GIP and GLP-1 pathways) works differently from Wegovy's single GLP-1 pathway. This doesn't necessarily mean one causes more headaches than the other — individual variation is substantial. Headaches during the dose escalation phase (weeks 1-4 when starting, and weeks 4-20 during titration) may be related to changes in blood sugar regulation, dehydration, or dietary changes rather than the medication itself.
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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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Managing Headaches and Choosing Between Medications
If you experience headaches after starting either Mounjaro or Wegovy, ensure you're staying well hydrated — both medications can cause nausea, which may reduce fluid intake. Eating small, regular meals and avoiding very sugary or fatty foods can also help. Most medication-related headaches improve within 2-4 weeks as your body adjusts. Contact your GP if headaches are severe, persistent, or accompanied by vision changes, as these could indicate a more serious condition unrelated to the medication.
Choosing between Mounjaro and Wegovy is a decision made between you and your prescriber based on your individual health profile, not side effect frequency alone. Both are prescription-only medications requiring clinical assessment. If you've experienced headaches with one GLP-1 medication in the past, discuss this with your GP — they can assess whether switching to the alternative medication is advisable for you.
- 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 are reported in clinical trials but are not among the most common Mounjaro side effects. Most are mild to moderate and improve within weeks as your body adjusts. However, if your headache is severe, persistent, or accompanied by vision changes, chest pain, or speech difficulties, contact your GP immediately as these could indicate a more serious condition.
Are headaches more common with Mounjaro or Wegovy?
Clinical trial data shows similar rates — approximately 5-9% of participants experienced headaches on both medications. Individual responses vary considerably. Your GP can discuss which medication might suit you better based on your personal health history and any previous experiences with GLP-1 medications.
What should I do if I get a headache after starting Mounjaro?
Stay well hydrated, eat small regular meals, and rest if possible. Most headaches settle within 2-4 weeks. Paracetamol or ibuprofen are safe to take alongside Mounjaro if needed. If headaches are severe or don't improve after 4 weeks, contact your GP for advice rather than stopping the medication.
Can dehydration cause headaches with GLP-1 injections?
Yes. Both Mounjaro and Wegovy can cause nausea, which may reduce how much you drink. Dehydration is a common cause of headaches. Drinking at least 2 litres of water daily, especially when starting these medications, can help prevent dehydration-related headaches.
Should I switch from Mounjaro to Wegovy if I get headaches?
Not necessarily — most medication-related headaches improve naturally. Switching medications involves a new titration schedule and different side effect patterns. Your prescriber will assess whether switching is appropriate based on the severity of your headaches and your overall tolerance to the current medication.
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