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
Free tracker included with every download this week
Does Mounjaro Have Side Effects?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK, and most people who take it do experience some side effects — particularly during the initial weeks of treatment. However, side effects are often temporary and tend to reduce as your body adjusts to the medication. Understanding what to expect helps you decide whether Mounjaro is right for you. Your GP or NHS specialist can discuss your individual risk factors and help you weigh the benefits against potential effects. Use our GLP-1 weight loss calculator to explore how Mounjaro might help you, and our BMI calculator to understand your current health status.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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.
What Does NHS Guidance Say About Mounjaro Side Effects?
The MHRA (UK Medicines and Healthcare products Regulatory Authority) approved Mounjaro with a Black Triangle ▼ symbol, meaning it requires additional monitoring for safety. NHS specialists prescribing tirzepatide follow strict guidelines based on the SURMOUNT-1 clinical trial data, which showed that most side effects occur early in treatment — typically during dose escalation.
Common side effects reported by NHS patients:
- Nausea (very common, especially weeks 1–4)
- Diarrhoea or constipation
- Vomiting
- Stomach pain or indigestion
- Reduced appetite
- Injection site reactions (redness, bruising)
- Fatigue or dizziness
- Hair loss (alopecia) — less common but reported
Most side effects improve within 4–8 weeks as your body adjusts. NHS tier 3 weight management services monitor patients closely during this adaptation period. Serious side effects are rare but require immediate medical attention — these include severe abdominal pain (possible pancreatitis), gallbladder problems, or serious allergic reactions.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS vs Private Mounjaro Prescribing — What's the Same and What's Different?
Whether you receive Mounjaro through NHS specialist services or a private prescription, the medication is identical — tirzepatide in the same KwikPen device at the same doses. The side effect profile is also the same.
The difference is monitoring and support. NHS tier 3 services provide structured follow-up appointments, blood tests, and management of side effects as part of your care. Private prescribers also conduct clinical assessments, but ongoing monitoring varies by provider — you may need to arrange your own GP follow-up. Both routes require you to use the same dose titration schedule: starting at 2.5 mg and increasing every 4 weeks if tolerated. Your prescriber will discuss how to manage nausea and other early side effects, including dietary adjustments and pacing meals. Use our weight loss calculator to estimate your potential progress and discuss realistic timelines with your prescriber.
- 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.
Can you take Mounjaro without side effects?
Some people experience minimal side effects, but most people do have at least mild nausea or appetite changes in the first 4 weeks. Side effects usually improve significantly within 8 weeks. Individual responses vary considerably — your GP or prescriber can discuss your personal risk factors.
Is nausea from Mounjaro permanent?
No. Nausea is typically very common in weeks 1–4 but usually reduces as your body adjusts. By weeks 8–12, most people find nausea has resolved or become much milder. If severe nausea persists, inform your prescriber — dose adjustment or delay may help.
What should I do if I have serious side effects on Mounjaro?
Seek immediate medical attention if you experience severe abdominal pain (possible pancreatitis), persistent vomiting, signs of gallbladder problems, or difficulty breathing. Contact your GP or NHS 111 (England/Wales) or your local NHS service. Do not stop taking Mounjaro without medical advice.
Does the NHS monitor Mounjaro side effects differently than private prescribers?
NHS tier 3 services typically offer structured follow-up appointments and blood monitoring. Private prescribers assess you before prescribing and may offer follow-up, but support varies. Either way, you should report serious side effects immediately to your prescriber or GP.
Can I reduce Mounjaro side effects by starting at a lower dose?
The standard starting dose is 2.5 mg weekly — this is the lowest approved dose. Your prescriber will not increase your dose until you've been stable for at least 4 weeks, which helps manage side effects. Discuss your concerns with your prescriber before starting treatment.
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.