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 Happens If You Start Mounjaro at 5mg?
Starting Mounjaro at 5mg is the standard first escalation dose after the initial 2.5mg adjustment phase. This dose is reached after four weeks of the starting dose, and many people experience noticeable effects at this level. Understanding what to expect — from appetite changes to potential side effects — helps you manage your weight loss journey effectively. Our GLP-1 weight loss calculator can help you estimate how this dose might affect your weight over time, based on clinical trial data.
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.
How 5mg Mounjaro Compares to Wegovy
At 5mg, Mounjaro has a different mechanism and dose trajectory compared to Wegovy. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously — this dual action is its key difference from Wegovy, which is a GLP-1 agonist only. In the SURMOUNT-1 clinical trial, participants on 5mg Mounjaro achieved an average weight loss of approximately 15% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Wegovy, by comparison, typically achieves around clinically studied weight loss at its maximum dose of 2.4mg. However, individual results vary significantly — some people respond better to one medication than the other. Both require a UK prescription and clinical assessment to determine suitability based on your personal health profile and medical history.
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.
Side Effects and What to Expect at 5mg
At 5mg Mounjaro, side effects are common but typically manageable. Very common effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — these affect more than 1 in 10 people. Nausea is most common in the first few weeks and often improves as your body adjusts. Taking smaller meals, staying hydrated, and eating slowly can help. Less common but possible effects include indigestion, acid reflux, injection site reactions, fatigue, and hair loss. Most side effects are temporary and reduce over time. If you experience severe, persistent stomach pain spreading to your back — a possible sign of pancreatitis — seek immediate medical attention. Your prescriber can help manage side effects and adjust your treatment if needed. Track your symptoms using our free progress tracker alongside your weight loss journey.
- 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.
Is 5mg the starting dose for Mounjaro in the UK?
No. The starting dose is 2.5mg for the first four weeks. After four weeks, the dose escalates to 5mg, which is the first maintenance dose. Most people stay on 5mg for at least another four weeks before considering further escalation to 7.5mg or higher, depending on clinical response and tolerance.
How does a 5mg Mounjaro KwikPen differ from Wegovy at equivalent doses?
Mounjaro's 5mg dose contains tirzepatide, which activates both GIP and GLP-1 receptors. Wegovy only activates GLP-1 receptors. This dual mechanism means Mounjaro may work differently in your body. Both are once-weekly injections, but Mounjaro's escalation path (2.5mg to 15mg) differs from Wegovy's (0.25mg to 2.4mg). Your prescriber will determine which is more suitable after a clinical assessment.
Will I lose weight immediately when I start 5mg Mounjaro?
Weight loss at 5mg is gradual, not immediate. Clinical trials show average weight loss of around 15% over 72 weeks at this dose. Most people notice appetite reduction and fullness signals within the first week or two, but significant weight loss takes several weeks. Use our free BMI calculator to track your progress and monitor whether the dose is working for you.
Can I switch from Wegovy to Mounjaro at an equivalent dose?
Switching between medications requires prescriber guidance — there's no official established conversion chart. Approximate equivalence suggests Wegovy 0.5mg is roughly equivalent to Mounjaro 5mg, but this varies by individual. Any switch must be managed by your prescriber to ensure safety and monitor your response to the new medication.
What UK prescription documentation do I need for Mounjaro at 5mg?
You need a valid UK prescription from a licensed prescriber following a clinical assessment. This can be obtained through NHS specialist tier 3 weight management services (with potentially longer waiting lists) or private online prescribing services. All prescriptions must be issued by a UK-registered doctor or independent prescriber and dispensed by a GPhC-registered pharmacy.
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.