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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Mounjaro Warnings — Important Safety Information for UK Users
Mounjaro (tirzepatide) is a prescription-only weight management injection that has specific warnings and serious risks you should understand before starting treatment. Whilst clinical trials have shown it can help with weight loss, it's not suitable for everyone and carries important safety considerations. This guide covers the key warnings, serious side effects, and contraindications you need to know about. Always discuss these warnings with your GP or prescriber before beginning any Mounjaro treatment.
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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.
Serious Warnings and Contraindications
Mounjaro is not suitable for certain people. Do not use this medication if you are pregnant, planning to become pregnant, or breastfeeding — tirzepatide may harm your unborn baby or nursing infant. You must also avoid Mounjaro if you have a personal or family history of medullary thyroid carcinoma (MTC) or if you have multiple endocrine neoplasia type 2 (MEN 2), as GLP-1 and GIP receptor agonists may increase thyroid cancer risk. If you are allergic to tirzepatide or any ingredients in the Mounjaro KwikPen, you must not use this medication.
Serious side effects requiring urgent medical attention include:
- Severe, persistent stomach pain spreading to your back (possible pancreatitis or gallbladder problems)
- Rapid or unusual changes in your heart rate
- Swelling of your face, lips, tongue, or throat (allergic reaction)
- Severe dehydration or signs of diabetic ketoacidosis in patients with diabetes
- Vision changes, particularly if you have diabetic retinopathy
If you experience any of these symptoms, stop using Mounjaro and seek immediate medical attention from your GP or accident and emergency.
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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.
Important Drug Interactions and Precautions
Mounjaro can interact with other medications. If you take oral contraceptive pills, you should use an additional barrier method of contraception (such as condoms) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase, as Mounjaro may reduce their effectiveness. If you take diabetes medications, particularly sulphonylureas or insulin, your doses may need adjustment — your prescriber must monitor this carefully.
People with certain medical conditions need caution: If you have severe stomach or digestive problems, a history of pancreatitis, gallbladder disease, or vision problems related to diabetes, inform your prescriber. Mounjaro slows gastric emptying, which may worsen existing digestive conditions. Weight loss itself can trigger gallstone formation in some people.
- 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 should I do if I'm pregnant or planning pregnancy and taking Mounjaro?
Mounjaro is not safe in pregnancy. If you become pregnant or plan to become pregnant, stop Mounjaro immediately and contact your GP. You must use effective contraception whilst on this medication. Most prescribers recommend waiting at least 2–3 months after stopping Mounjaro before trying to conceive, though discuss this with your healthcare provider.
Can I take Mounjaro if I have type 2 diabetes?
Some people with type 2 diabetes may be prescribed Mounjaro for weight management, but this requires careful monitoring. If you take insulin or other diabetes medications, your doses may need adjustment as Mounjaro affects blood sugar levels. Your GP must manage this closely to avoid dangerously low blood sugar (hypoglycaemia).
What is the difference between Mounjaro (tirzepatide) and Wegovy (semaglutide)?
Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1-only medication. Both are prescription-only weight management injections used in the UK. Mounjaro showed slightly higher average weight loss in clinical trials (~21% vs ~15%), but both carry similar warnings and side effects. Your prescriber will assess which is more appropriate for you.
What happens if I miss a Mounjaro dose?
If you miss a dose, take it within 4 days of the missed dose and continue your normal schedule. If more than 4 days have passed, skip that dose and take your next dose on the scheduled day. Never take a double dose. Always maintain at least 72 hours between injections.
Are there warning signs I should watch for whilst on Mounjaro?
Yes — watch for severe stomach pain, rapid heartbeat, allergic reactions (swelling, difficulty breathing), unusual thirst or frequent urination, vision changes, or signs of dehydration. Use our GLP-1 weight loss calculator to track expected progress, but also keep a health log to monitor how you feel. Report any unusual symptoms to your GP immediately. ```
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