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
Most Common Side Effects of Mounjaro
Mounjaro (tirzepatide) is a prescription-only weight management injection available in the UK. Like all medicines, it can cause side effects. The most common ones affect the digestive system and tend to improve as your body adjusts over the first few weeks. Understanding what to expect helps you make an informed decision alongside your GP or prescriber. Most side effects are mild to moderate, though severity varies between individuals.
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 Are The Most Common Mounjaro Side Effects?
According to UK prescribing information, very common side effects (affecting more than 1 in 10 people) include:
- Nausea — the most frequently reported side effect, especially when starting or increasing doses
- Diarrhoea — loose stools, often mild and temporary
- Vomiting — less common than nausea but can occur
- Constipation — paradoxically, some people experience this instead of diarrhoea
- Stomach pain — abdominal discomfort, typically mild
Common side effects (affecting up to 1 in 10 people) include indigestion, heartburn (gastro-oesophageal reflux disease), injection site reactions such as redness or bruising, fatigue, dizziness, and hair loss. Most of these improve within 2–4 weeks as your body adjusts to the medication. Use our GLP-1 weight loss calculator to understand how Mounjaro works alongside lifestyle changes if you're considering this treatment.
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.
How Long Do Mounjaro Side Effects Last?
Most side effects peak during the first 1–2 weeks after starting Mounjaro or increasing your dose, then gradually improve as your body adjusts. Nausea typically resolves within 2–4 weeks for most people. Digestive effects like diarrhoea or constipation often settle within 3–4 weeks. However, individual experiences vary significantly — some people have minimal side effects, whilst others may experience them for longer. If side effects persist beyond 4 weeks or significantly affect your quality of life, contact your prescriber. They may adjust your dose, change the timing of your injection, or discuss alternative weight management options. Never stop taking Mounjaro without medical guidance, as this can affect your treatment outcomes.
- 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.
Are Mounjaro side effects permanent?
No. Most side effects are temporary and improve within 2–4 weeks as your body adjusts. Serious permanent side effects are rare. If you experience persistent effects beyond one month, contact your prescriber for guidance. They can adjust your dose or discuss alternatives.
Can I reduce nausea while taking Mounjaro?
Yes. Eat smaller, more frequent meals; avoid fatty or spicy foods; stay hydrated; and take your injection at a time when you can rest afterwards if needed. Some people find eating ginger biscuits or drinking ginger tea helpful. Your GP can also recommend anti-nausea strategies or, if needed, medication.
Is hair loss a serious side effect of Mounjaro?
Hair loss (alopecia) is reported but usually temporary and relates to gradual weight management rather than tirzepatide itself. It typically stops once weight stabilises. Ensure adequate protein intake and consider a vitamin supplement. If concerned, discuss with your GP or a dermatologist.
When should I seek urgent medical attention with Mounjaro?
Seek immediate medical attention if you experience severe, persistent stomach pain (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue), gallbladder symptoms (pain in upper stomach), or changes in vision. These are rare but require urgent assessment.
How does Mounjaro compare to Wegovy for side effects?
Both are GLP-1-type medications with similar side effect profiles — nausea, diarrhoea, and stomach pain are common. Mounjaro is a dual GIP/GLP-1 agonist whilst Wegovy is GLP-1 only. Clinical trial data suggests similar tolerability between them, but individual responses vary. Your prescriber can advise which suits you better.
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.