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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Feeling Lightheaded on Mounjaro — What This Means
Feeling lightheaded or dizzy while taking Mounjaro (tirzepatide) is not uncommon, though it's not listed as a very common side effect. Lightheadedness can happen for several reasons — ranging from dehydration and low blood sugar to changes in how quickly your stomach empties food. This guide explains what might be causing it, when you should contact your GP, and practical steps you can take to manage it safely whilst using this prescription weight management injection.
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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.
Why Lightheadedness Happens on Mounjaro
Lightheadedness on Mounjaro can occur through several mechanisms. Dehydration is one of the most common causes — nausea and reduced appetite often mean people drink less fluid, which lowers blood pressure and causes dizziness. Blood sugar changes may also contribute, especially if you're eating less overall or have type 2 diabetes — Mounjaro affects how your body regulates glucose. Additionally, slowed gastric emptying (how quickly food leaves your stomach) can affect nutrient absorption in the short term, particularly early in treatment.
Other contributing factors include standing up too quickly (orthostatic hypotension), reduced food intake leading to lower calories and nutrients, and the adjustment period as your body adapts to the medication. In rare cases, lightheadedness may signal a more serious issue like pancreatitis or gallbladder problems, which require immediate medical attention.
- Dehydration from reduced fluid intake
- Lower blood sugar levels
- Reduced calorie and nutrient intake
- Slowed stomach emptying (especially weeks 1–4)
- Blood pressure changes when standing quickly
- Individual sensitivity during dose titration
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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.
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When to Contact Your GP and What to Do
Contact your GP immediately if lightheadedness is accompanied by severe stomach pain spreading to your back (possible pancreatitis), chest pain, severe headache, loss of consciousness, or persistent vomiting. These require urgent medical assessment. For mild lightheadedness, practical steps include drinking more water throughout the day, eating small, balanced meals (even if appetite is reduced), standing up slowly from sitting or lying down, and avoiding sudden position changes.
Keep a food and symptom diary to identify patterns — does it happen after certain meals or at particular times? This information helps your GP or prescriber assess whether the symptom is expected and manageable, or whether your Mounjaro dose needs adjustment. Never stop taking Mounjaro without medical advice, even if you feel lightheaded.
If lightheadedness persists beyond the first few weeks or worsens over time, contact your prescriber. They may adjust your dose titration schedule, recommend dietary changes, or suggest blood tests to check for anaemia or thyroid issues. Use our free BMI calculator to track your overall health context, and consider our GLP-1 weight loss calculator to review realistic expectations for your dose — sometimes lightheadedness relates to rapid changes in body composition early in treatment, which typically settle as your body adapts.
- 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 lightheadedness a common side effect of Mounjaro?
Lightheadedness is listed as common but not very common in clinical data — it affects some people but not the majority. It's more likely in the first few weeks as your body adjusts, especially during dose increases. Most people find it settles as they progress through the titration schedule.
Can I take Mounjaro if I'm already taking diabetes medication?
If you take diabetes medication, especially insulin or sulphonylureas, inform your prescriber before starting Mounjaro. These medications may need dose adjustment because Mounjaro affects blood sugar regulation. Your GP or specialist will manage this carefully to prevent low blood sugar.
What's the difference between Mounjaro and Wegovy for lightheadedness risk?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Both can cause lightheadedness, though clinical data suggests similar incidence rates. Individual responses vary — what triggers lightheadedness in one person may not in another.
Should I drink more water if I feel lightheaded on Mounjaro?
Yes, dehydration is a common contributor. Aim for 2–3 litres of water daily, especially if you're experiencing nausea or reduced appetite. Small, frequent sips throughout the day are often easier to manage than large amounts at once.
When will the lightheadedness stop?
Most side effects, including lightheadedness, improve within the first 4–8 weeks as your body adapts. If it persists beyond week 8 or worsens during dose increases, contact your GP. Dose adjustments or pacing your titration schedule can help.
Ready to take control of your weight loss journey?
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