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
What Are The Side Effects Of Mounjaro Injections?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medications, it can cause side effects. Most are mild and temporary, particularly during the dose titration phase when your body adjusts to the medication. This guide explains what side effects you might experience, how common they are, and when to contact your GP. Always discuss potential side effects with your healthcare provider before starting treatment.
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.
Common Side Effects Of Mounjaro
The most frequently reported side effects of Mounjaro affect the digestive system, especially during the first few weeks of treatment or when increasing your dose. Nausea is the most common, affecting more than 1 in 10 people, often improving as your body adjusts. Diarrhoea and constipation both occur frequently — your digestive system may take time to settle. Vomiting and stomach pain are also common digestive effects.
Beyond digestion, people using Mounjaro may experience fatigue, dizziness, headaches, and hair loss (alopecia). Injection site reactions — redness, bruising, or mild pain at the injection location — are common but typically resolve quickly. Most side effects decrease in frequency and severity as you continue treatment and your body adjusts to the medication. If side effects persist beyond the first few weeks, speak to your GP about whether your dose needs adjustment.
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.
Serious Side Effects And When To Seek Help
Whilst rare, some side effects require urgent medical attention. Severe, persistent stomach pain spreading to your back could indicate pancreatitis — contact your GP or A&E immediately if this occurs. Serious allergic reactions — swelling of the face, lips, tongue, or throat, or difficulty breathing — require emergency care. Changes in vision may signal diabetic retinopathy complications and should be reported to your GP promptly. Gallbladder problems cause pain in the upper right stomach area. If you experience any concerning symptoms, stop injecting and contact your healthcare provider. Your prescriber will assess whether Mounjaro remains appropriate for you. Use our free GLP-1 weight loss calculator to understand how Mounjaro works alongside lifestyle changes, and track your progress with our progress tracker as you adjust to treatment.
- 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.
How long do Mounjaro side effects last?
Most common side effects improve within the first 2–4 weeks as your body adjusts. Nausea and digestive effects tend to be worst in the first few days after injection, then gradually settle. Some people experience side effects during each dose increase phase. If side effects persist beyond 4 weeks, contact your GP — you may need a dose adjustment or additional support.
Is nausea on Mounjaro normal?
Yes, nausea is the most common side effect, affecting more than 1 in 10 people. It's usually mild to moderate and often improves as your body adapts. Eating smaller meals, staying hydrated, and avoiding fatty or spicy foods can help. If nausea is severe or doesn't improve after a few weeks, your GP may suggest ways to manage it or adjust your treatment.
Can Mounjaro cause hair loss?
Hair loss (alopecia) is a reported side effect of Mounjaro, though it's not extremely common. It may occur due to gradual weight management or medication effects. If you notice significant hair loss, discuss it with your GP. Hair loss is often temporary and may improve once your body stabilises. Ensure you're getting adequate protein and nutrients to support hair health.
What's the difference between Mounjaro and Wegovy side effects?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, whilst Wegovy (semaglutide) is a single GLP-1 agonist. Both cause similar digestive side effects, but Mounjaro's dual mechanism may produce slightly different severity patterns in individuals. Clinical trials suggest comparable safety profiles. Your GP can advise which medication's side effect profile suits you better based on your medical history.
Should I stop Mounjaro if I get side effects?
Don't stop without consulting your GP. Most side effects improve with time and don't require stopping treatment. However, if you experience severe symptoms — severe stomach pain, allergic reactions, or vision changes — seek immediate medical help and contact your prescriber. Your GP may adjust your dose, suggest timing adjustments, or explore alternative management strategies rather than stopping entirely.
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.