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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Monjaro Side Effects: UK Cost and Access Guide
Monjaro (tirzepatide) is a weekly weight loss injection licensed in the UK that works as a GIP and GLP-1 receptor agonist. Before considering this prescription-only medicine, it's important to understand the common side effects, how much it costs in the UK, and whether it's available through the NHS or private prescribing. This guide covers everything you need to know about accessing Monjaro and managing its effects.
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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 Monjaro Side Effects and What They Mean
The most frequently reported side effects of Monjaro affect the digestive system. Nausea is very common, particularly during the first weeks or after dose increases, though it typically improves over time. Diarrhoea and constipation both occur in more than one in ten people—constipation is especially common as your body adjusts to the medication.
- Stomach pain or indigestion (dyspepsia)
- Vomiting (less common than nausea)
- Acid reflux or GORD
- Injection site reactions: redness, bruising, or mild pain at the injection site
- Fatigue and dizziness
- Hair loss (alopecia) reported in some users
Most side effects are mild to moderate and reduce after the first month. Serious side effects—including severe stomach pain suggesting pancreatitis, gallbladder problems, or allergic reactions—require urgent medical attention. Your prescriber will assess whether Monjaro is safe for you and discuss managing these effects.
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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.
UK Cost and How to Access Monjaro
Monjaro costs vary depending on the dose and whether you access it through the NHS or privately. NHS access is through specialist tier 3 weight management services—your GP can refer you, though waiting lists vary significantly by region and can be lengthy. Private access is faster and available without NHS referral through UK-licensed online prescribers and GPhC-registered pharmacies.
Private Monjaro typically costs between £150–£300 per month depending on your dose, with the starting 2.5mg dose being less expensive than higher maintenance doses. Once you've had an online clinical assessment with a UK-licensed prescriber, your prescription is dispensed by a regulated pharmacy. You'll inject once weekly at home using the KwikPen device. Track your progress with our free progress tracker to monitor weight changes alongside managing side effects.
Before starting Monjaro, use our free BMI calculator to understand your baseline, and our GLP-1 weight loss calculator to project potential outcomes at different doses. A prescriber will assess your individual health during consultation—disclosing all medications is essential as Mounjaro may interact with oral contraceptives and diabetes medications. Starting with the lowest dose (2.5mg) minimises initial side effects, with gradual increases every four weeks. Keeping a symptom diary helps you and your prescriber manage effects effectively.
- 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 are the most common side effects of Monjaro in the UK?
Nausea, diarrhoea, and constipation are the most frequently reported side effects, affecting more than one in ten people. Most improve within the first month as your body adjusts. Stomach pain, vomiting, and injection site reactions are also common but usually mild.
How much does Monjaro cost in the UK privately?
Private Monjaro typically costs £150–£300 per month depending on your dose. The starting dose (2.5mg) is less expensive than higher doses. Each KwikPen contains four weekly injections. Always check with your prescriber or pharmacy for exact current pricing.
Can I get Monjaro through the NHS in the UK?
Yes, but through specialist tier 3 weight management services, not standard GP prescription. You'll need a GP referral. NHS availability and waiting times vary significantly by region—some areas have long waiting lists. Private access is typically faster if you prefer not to wait.
What's the difference between Monjaro and Wegovy?
Monjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a GLP-1 receptor agonist only. Clinical trials showed Monjaro produced slightly higher average weight loss (~21% vs ~15%), but individual results vary. Both are prescription-only and available via NHS or private routes.
Do side effects get better over time?
Yes, most side effects significantly improve after the first four weeks as your body adjusts. Nausea and digestive issues typically reduce with each dose adjustment. Starting with the lowest dose (2.5mg) and increasing gradually helps minimise initial effects. If serious effects occur, contact your GP or prescriber immediately.
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.