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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What Are the Common Problems With Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK, but like all medications, it can cause side effects and may not be suitable for everyone. Understanding potential problems — from nausea and digestive issues to injection site reactions and eligibility concerns — helps you make an informed decision about whether this medication is right for you. This guide covers the most common issues patients report and what NHS guidance says about managing them.
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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.
Common Side Effects and Tolerability Issues
Nausea and digestive problems are the most frequently reported issues with Mounjaro. In clinical trials, nausea affected more than 1 in 10 people, particularly in the first weeks of treatment. Other common digestive side effects include diarrhoea, constipation, vomiting, and stomach pain. These typically improve as your body adjusts to the medication, though they can be uncomfortable initially.
Less common but significant problems include injection site reactions (redness, bruising, pain), fatigue, dizziness, and headaches. Some patients report hair loss, though this is less frequent. Serious but rare complications include pancreatitis (severe stomach pain spreading to the back) and gallbladder problems — conditions requiring immediate medical attention. The dose titration schedule (starting at 2.5 mg and increasing gradually) is designed to minimise these tolerability issues, though individual responses vary considerably.
If side effects are severe or persistent, your prescriber may recommend staying on a lower dose or discontinuing treatment entirely.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Eligibility and Access Challenges in the UK
Not everyone is eligible for Mounjaro. You cannot take this medication if you're under 18, pregnant, breastfeeding, or have a history of severe gastroparesis or medullary thyroid cancer. Access through the NHS is limited — specialist weight management services (tier 3/4) control prescribing, and waiting lists can be extremely long in many areas. Private access is available through GPhC-registered online prescribers following a clinical assessment, but cost is a barrier for many — doses typically cost £150–£300 monthly depending on your stage of the dosing schedule.
Another key issue: Mounjaro may reduce the effectiveness of oral contraceptives. Women taking birth control pills need to use additional contraception for 4 weeks after starting and 4 weeks after each dose increase. Your GP or prescriber should discuss these eligibility concerns before treatment begins. Use our free GLP-1 weight loss calculator to understand potential outcomes, and consult your GP about whether Mounjaro is appropriate for your circumstances.
Managing Issues and When to Seek HelpMost side effects improve within days or weeks as your body adapts, but there are steps you can take. Eating smaller, frequent meals and avoiding fatty or spicy foods can help manage nausea. Staying hydrated and maintaining adequate fibre intake supports digestive health. If nausea is severe, your prescriber may recommend anti-nausea medication or suggest pausing dose increases.
Seek urgent medical attention if you experience severe stomach pain, signs of allergic reaction (swelling of face or throat), chest pain, or rapid heart rate. These may indicate pancreatitis or other serious complications. Regular monitoring by your prescriber is essential — inform them of all medications you're taking, as Mounjaro can affect absorption of oral drugs. Weight loss plateaus are also common after several months, and your prescriber can discuss whether continuing treatment remains beneficial for your health goals.
- 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.
What's the difference between Mounjaro side effects and Wegovy side effects?
Both are GLP-1 or dual-action weight management injections, but Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is GLP-1 only. Clinical trials show similar nausea and digestive issues, but Mounjaro may achieve slightly greater average weight loss (~20.9% vs ~15% at maximum doses). Side effect profiles are comparable; individual tolerability varies.
Can I stop taking Mounjaro if I experience problems?
Yes. If side effects are intolerable, discuss stopping with your prescriber. However, weight may return if you discontinue without lifestyle changes. Your prescriber may suggest lowering your dose instead of stopping completely, or trying a different medication. Never stop without medical guidance, as this affects your weight management plan.
Will my GP prescribe Mounjaro if I can't access NHS services?
Your GP cannot prescribe Mounjaro through the NHS outside specialist weight management services due to NICE guidelines. However, private GPs or online prescribers can issue prescriptions following a clinical assessment. Costs are higher privately (typically £150–£300/month) but access is faster than NHS waiting lists.
Is Mounjaro safe for people with type 2 diabetes?
Mounjaro is safe for people with type 2 diabetes, and may help control blood sugar while supporting weight loss. However, if you take insulin or sulphonylureas, your doses may need adjustment — your prescriber will monitor this closely. Always inform your prescriber of all diabetes medications before starting.
What happens if I miss a Mounjaro dose?
If you miss a dose within 4 days, take it as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next one on schedule. Never double-dose. The minimum interval between injections is 72 hours.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.