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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Most Common Side Effects Of Mounjaro — And How To Access It In The UK
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that works as a dual GIP and GLP-1 receptor agonist. Like all medications, it can cause side effects — most commonly nausea, diarrhoea, vomiting, and constipation, especially during the initial dose titration phase. Understanding what to expect, how to access Mounjaro in the UK, and what it costs will help you make an informed decision about whether this treatment is right for you.
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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.
What Are The Most Common Mounjaro Side Effects?
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are typically mild to moderate and often improve as your body adjusts during the dose titration process. Nausea is the most frequently reported side effect, particularly in the first four weeks after starting or increasing your dose.
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss. Most gastrointestinal side effects are temporary and reduce as your body adapts to the medication.
The KwikPen delivery device itself rarely causes problems, though some users report mild injection site discomfort that typically subsides quickly. Your prescriber may suggest dietary adjustments — eating smaller, frequent meals and avoiding fatty foods — to help minimise nausea 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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Accessing Mounjaro In The UK: NHS Vs Private Routes
NHS access: Mounjaro is available through NHS specialist (tier 3) weight management services, though availability varies significantly by region and waiting lists can extend many months. You'll need a GP referral to access NHS specialist services, and eligibility is assessed by the specialist team.
Private access: You can access Mounjaro through a GPhC-registered online pharmacy following a clinical assessment with a UK-licensed prescriber. This route does not require NHS referral and prescriptions are typically issued within 24–48 hours if approved. Private prescription costs range from approximately £129–£320 per month depending on your dose and provider. Compare our GLP-1 weight loss calculator to estimate potential costs and outcomes at different doses.
The online consultation process is straightforward: you complete a confidential health questionnaire, a prescriber reviews it, and if appropriate, issues a prescription. Most private providers deliver to your door within 2–3 working days. NHS prescriptions are free once issued by a specialist service. Both routes require a BMI of 30 or above (or 27+ with weight-related health conditions), and individual eligibility is assessed based on your personal health circumstances. Your GP or online prescriber can advise whether Mounjaro is suitable 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.
What is the most common side effect of Mounjaro?
Nausea is the most frequently reported side effect, especially during the first 4 weeks of starting or increasing your dose. It typically improves as your body adjusts. Eating smaller, frequent meals and avoiding fatty foods can help. Most people find nausea manageable and temporary.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs typically range from £129–£320 per month depending on your dose (2.5 mg to 15 mg). Prices vary between providers. NHS prescriptions are free once issued by a specialist weight management service, though NHS access involves referral and may involve waiting lists.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is available through NHS specialist tier 3 weight management services following a GP referral. Availability and waiting times vary significantly by region (ICB). You'll need a BMI of 30+ (or 27+ with comorbidities) and must be assessed by the specialist team.
Do Mounjaro side effects go away?
Most common side effects — nausea, diarrhoea, constipation — improve within 1–2 weeks as your body adjusts. Dose titration (gradually increasing your dose every 4 weeks) helps minimise side effects. If side effects persist or worsen, inform your prescriber, as dose adjustment may be needed.
How does Mounjaro compare to Wegovy for side effects?
Mounjaro and Wegovy share similar common side effects (nausea, diarrhoea, constipation), as both are GLP-1 receptor agonists. Mounjaro is a dual GIP and GLP-1 agonist, which may result in slightly different tolerability for individual users. Individual responses vary; your prescriber can advise which may suit you better.
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