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 Negatives of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK, but it's not right for everyone and comes with real limitations worth understanding. While clinical trials showed meaningful weight loss results on average, individual outcomes vary significantly, and the medication carries potential side effects, access challenges through the NHS, and substantial costs if using private services. This guide explores the genuine drawbacks of Mounjaro in a UK context to help you make an informed decision alongside your GP.
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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.
Common Side Effects and Limitations
Mounjaro's most frequently reported side effects include nausea, diarrhoea, vomiting, and constipation — affecting more than 1 in 10 users. Many people experience stomach pain, indigestion, and reduced appetite, particularly during dose titration when moving from 2.5 mg up to the maintenance dose of 15 mg. These effects often ease after 4–8 weeks, but some people find them persistent enough to discontinue treatment. Additional drawbacks include injection site reactions (bruising, redness, pain), hair loss reported by some users, and fatigue. For people taking oral contraceptives, Mounjaro reduces their absorption — requiring backup contraception for 4 weeks after each dose increase. Weight regain is common after stopping treatment, as the medication doesn't create permanent metabolic change.
- Gastrointestinal side effects can last weeks during dose escalation
- Hair loss and fatigue reported by some users
- Requires continuous use — weight often returns if stopped
- Contraception interference requires additional precautions
Clinically BackedWeight Loss
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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.
NHS Access Challenges and Cost Reality
NHS access to Mounjaro in the UK remains extremely limited despite NICE approval. Prescribing is restricted to specialist tier 3 and tier 4 weight management services, not standard GP prescription, meaning you must be referred through your GP to a specialist clinic — with waiting lists often stretching 12–24 months in many regions. Private access costs £150–£300+ per month depending on dose, representing significant ongoing expense. MHRA Black Triangle status (additional monitoring) means adverse event reporting is ongoing. The medication also interacts with diabetes medications, potentially requiring dose adjustments, and is unsuitable if you have a history of pancreatitis, gallbladder disease, or diabetic retinopathy complications. Using our GLP-1 weight loss calculator can help you explore realistic projections, but understand that results vary — some people lose far less than clinical trial averages, and realistic expectations are crucial before starting 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.
What are the most serious side effects of Mounjaro to watch for?
Seek urgent medical attention for severe stomach pain radiating to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, throat), gallbladder pain, or sudden changes in vision. These are rare but require immediate hospital assessment. Most side effects are gastrointestinal and resolve within weeks.
Can you get Mounjaro through the NHS in the UK?
Yes, but access is highly limited. NICE has approved tirzepatide for NHS specialist weight management services (tier 3/4), but prescribing is not available through standard GP practices. You must be referred to a specialist clinic, which typically has long waiting lists — sometimes 12–24 months depending on your region and ICB (Integrated Care Board) resources.
How long can you stay on Mounjaro?
There is no specified maximum duration, but it's considered a long-term treatment. Most people remain on Mounjaro as long as it's helping and side effects are tolerable. Weight regain commonly occurs after stopping, so many people remain on maintenance doses indefinitely. Your prescriber will review this regularly.
Does Mounjaro work for everyone?
No. Clinical trials showed average weight loss of 15–21% of body weight, but individual responses vary widely. Some people lose significantly less, and others lose minimal weight despite treatment. Age, genetics, metabolism, and adherence to diet and exercise all affect outcomes.
Is Mounjaro better than diet and exercise alone?
For some people, yes — clinical evidence shows Mounjaro supports greater weight loss when combined with lifestyle changes. However, diet and exercise alone remain effective and don't carry medication risks or costs. Mounjaro is not a replacement for healthy habits; it works alongside them. Your GP can discuss whether it's appropriate for your situation.
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.