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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Why Does Mounjaro Make You Thirsty?
Increased thirst is a recognised side effect reported by some people taking Mounjaro (tirzepatide) for weight management in the UK. This occurs because Mounjaro works as a dual GIP and GLP-1 receptor agonist, affecting how your body regulates blood sugar and fluid balance. While increased thirst is not listed as a "very common" side effect in clinical trials, many users report this experience. Understanding why it happens and how to manage it can help you stay comfortable during your weight loss journey. Your GP can provide personalised advice if this becomes problematic.
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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 vs Wegovy: Side Effect Differences and Trial Data
Mounjaro (tirzepatide) activates two hormone pathways—GIP and GLP-1—whereas Wegovy (semaglutide) activates GLP-1 only. This dual mechanism may affect fluid balance differently between the two medications. In the SURMOUNT clinical trials, nausea, diarrhoea, and vomiting were reported more frequently than thirst. However, thirst can develop as your body adjusts to changes in blood sugar regulation and appetite suppression. Both medications slow gastric emptying, which may contribute to dry mouth sensations. Wegovy, based on STEP trial data, shows a similar profile of gastrointestinal side effects but individual experiences vary widely. The KwikPen delivery method and dose titration schedule are identical for Mounjaro—starting at 2.5mg and escalating weekly—but your body's response to the dual mechanism may differ from those taking GLP-1-only medications. Your prescriber will monitor your response during dose escalation phases.
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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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Managing Increased Thirst and When to Seek Medical Advice
If you experience increased thirst on Mounjaro, ensure you're drinking adequate water throughout the day—typically 6 to 8 glasses daily, more if you're active. Thirst often resolves as your body adjusts to the medication over 2 to 4 weeks. However, contact your GP if thirst is extreme, persistent beyond 4 weeks, or accompanied by confusion, rapid heartbeat, or dark urine—these may indicate dehydration or blood sugar issues requiring urgent assessment. Carry a water bottle with you and drink small amounts regularly rather than large quantities at once. Some people find sucking sugar-free sweets or sipping herbal tea helpful. Avoid excessive caffeine and alcohol, which can worsen dehydration. Use our free GLP-1 weight loss calculator to track your progress and discuss any persistent side effects with your prescriber at your next check-in.
- 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 increased thirst a common side effect of Mounjaro in the UK?
Increased thirst is not listed as a "very common" side effect in Mounjaro clinical trials, but many people report experiencing it. Thirst may develop as your body adjusts to changes in appetite regulation and blood sugar control. If it persists beyond a few weeks or becomes severe, contact your GP.
Does Wegovy cause thirst more or less than Mounjaro?
Both medications can cause dry mouth or increased thirst as an indirect effect of how they regulate blood sugar and appetite. Wegovy's single GLP-1 mechanism may affect thirst differently than Mounjaro's dual GIP+GLP-1 approach, but individual responses vary. Your prescriber can advise which is more suitable for you.
How can I manage increased thirst while taking Mounjaro?
Drink water regularly throughout the day (6–8 glasses), avoid caffeine and alcohol, and ensure you're staying hydrated. Most people find thirst improves within 2–4 weeks as their body adapts. If extreme thirst persists, inform your GP.
Could thirst indicate a serious problem with my Mounjaro prescription?
Mild thirst is usually temporary and resolves with hydration. However, seek immediate GP advice if thirst is extreme, accompanied by confusion, dizziness, rapid heartbeat, or dark urine—these may indicate dehydration or blood sugar complications requiring urgent assessment.
Will my GP prescribe Mounjaro if I already have a dry mouth condition?
Yes, but inform your prescriber about any existing dry mouth or thirst conditions during your clinical assessment. Your prescriber will evaluate whether Mounjaro is suitable based on your individual health profile and can advise on managing both conditions alongside the medication.
Ready to take control of your weight loss journey?
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