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?
Mounjaro (tirzepatide) is a prescription-only weight management injection that works as a dual GIP and GLP-1 receptor agonist. Like all medicines, it can cause side effects. Most are mild and temporary, especially when starting or increasing your dose, but it's important to know what to expect. This guide covers common side effects, serious reactions, and when to seek medical help — all aligned with NHS guidance for UK patients.
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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.
Most Common Mounjaro Side Effects
The most frequently reported side effects of Mounjaro occur in the digestive system. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, and constipation — sometimes alternating between the two. Stomach pain and indigestion are also typical, especially when first starting the Mounjaro KwikPen or moving to a higher dose.
- Nausea often peaks in the first few weeks and tends to improve with time
- Gastrointestinal side effects usually ease within 2–4 weeks as your body adjusts
- Fatigue, dizziness, and reduced appetite are common but typically mild
- Injection site reactions (redness, bruising, pain) are generally short-lived
- Hair loss has been reported but is usually temporary
These side effects are not dangerous but can affect daily life. Dose titration — starting at 2.5 mg and increasing gradually every 4 weeks — helps your body adapt and reduces severity. Your prescriber will monitor your response throughout your tirzepatide treatment.
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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.
Serious Side Effects and When to Seek Help
Serious side effects are rare but require immediate medical attention. Seek urgent care if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), or changes in vision. Gallbladder problems, rapid heart rate, or severe dehydration also warrant contacting your GP immediately.
- Do not ignore warning signs — contact NHS 111 or your GP at once if you suspect a serious reaction
- If you have a history of medullary thyroid cancer or thyroid tumours in your family, inform your prescriber before starting
- Monitor your blood sugar closely if you also take diabetes medication — your doses may need adjusting
Use our free BMI calculator and GLP-1 weight loss calculator to track your health progress alongside Mounjaro treatment, and discuss any concerning symptoms with your prescriber or GP without delay.
- 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 Mounjaro side effects are temporary and improve within 2–4 weeks as your body adjusts. Nausea and digestive issues are most common initially. If side effects persist beyond a few weeks or worsen, inform your GP — your dose or timing may need adjustment.
Does Mounjaro cause weight loss side effects?
Mounjaro causes reduced appetite as part of its mechanism, which is intentional and therapeutic — not a side effect. However, this combined with nausea or diarrhoea may reduce food intake. Eating smaller, frequent meals and staying hydrated helps manage this.
Can Mounjaro cause thyroid problems?
Mounjaro carries a theoretical risk related to GLP-1 medications and thyroid safety. If you have a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia, you are not eligible for Mounjaro. Your prescriber will assess this before starting treatment.
What should I do if I experience severe stomach pain on Mounjaro?
Severe stomach pain, especially if it radiates to your back, could indicate pancreatitis — a medical emergency. Contact NHS 111 or attend A&E immediately. Do not ignore this symptom. Always report stomach issues to your GP for proper evaluation.
Does Mounjaro cause hair loss?
Hair loss (alopecia) has been reported in clinical use but is uncommon and usually temporary. It is thought to be linked to gradual weight management or nutritional changes. If you notice significant hair loss, inform your GP — ensuring adequate protein and micronutrient intake may help.
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.