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
How Long Does Mounjaro Take to Start Working in the UK?
Mounjaro (tirzepatide) typically begins to show effects within the first 2–4 weeks, though most people see more noticeable results after 8–12 weeks of consistent use. The medication works gradually as your body adjusts to each dose increase, and weight loss continues throughout your treatment. This guide explains the expected timeline, how the dose titration process works, and what to expect in terms of UK prescription costs and access routes whether you choose the NHS or private route.
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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.
How Mounjaro Works and When You'll Notice Changes
Mounjaro starts working immediately at a cellular level, but you may not notice appetite suppression or weight changes straight away. Most people report the first effects — reduced hunger and feeling fuller after smaller meals — within 2–4 weeks of their first 2.5 mg injection. However, significant weight loss typically becomes visible after 8–12 weeks on a stable dose.
The reason for this timeline is dose titration. Your prescriber will increase your KwikPen dose every 4 weeks (2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg) to allow your body to adapt and minimise side effects. Each dose increase brings stronger appetite-suppressing effects. Clinical trials show that the majority of weight loss occurs in weeks 12–68 of treatment, meaning patience through the early titration phase is important. Results vary significantly between individuals — some people lose more weight, others lose less, and the timeline can differ based on diet, activity level, and individual metabolism.
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.
Mounjaro Cost in the UK: NHS vs Private Access
NHS access to Mounjaro is free but involves a lengthy referral process. Your GP refers you to a specialist tier 3 weight management service, where a multidisciplinary team assesses your eligibility and prescribes the medication. However, NHS waiting lists for these services vary dramatically by region (ICB) — some areas have waits of 12–24 months or longer, while others may offer faster access. Once prescribed through the NHS, you pay nothing.
Private prescription Mounjaro costs approximately £129–£320 per month depending on your dose and the prescribing provider. A private online consultation with a UK-licensed prescriber typically costs £50–£100 (sometimes included in the overall treatment cost). This route is faster — you can often receive a prescription within 24–48 hours of assessment — and doesn't require an NHS referral. A GPhC-registered pharmacy then dispenses your medication. Try our GLP-1 weight loss calculator to explore potential outcomes and help inform your decision between routes.
- 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.
How quickly will I lose weight on Mounjaro?
Most people see noticeable weight loss after 8–12 weeks of consistent treatment at a stable dose. Clinical trials showed average weight loss of 15–21% of body weight by week 68, but individual results vary significantly. Your GP or prescriber can discuss realistic expectations based on your health profile.
What's the difference between NHS and private Mounjaro in the UK?
NHS access is free but involves waiting lists (often 12–24 months) and requires referral to specialist services. Private access costs £129–£320/month and is faster (24–48 hours) through online prescribers, without NHS referral. Both routes use the same medication; access and cost differ only.
Does Mounjaro side effects timeline match the weight loss timeline?
No. Most people experience side effects (nausea, diarrhoea, constipation) in the first 2–4 weeks of starting or increasing doses. These typically improve as your body adjusts over 4–8 weeks. Weight loss effects may become visible after 8–12 weeks but continue to increase over months.
Can I switch from Mounjaro to Wegovy if Mounjaro isn't working?
Yes, your prescriber can switch you if needed, though this should be managed clinically. Mounjaro (dual GIP/GLP-1) and Wegovy (GLP-1 only) work slightly differently. Some people respond better to one than the other, but any medication switch requires professional guidance.
How do I access Mounjaro privately in the UK without seeing my GP first?
Contact a GPhC-registered online prescriber or clinic. They'll conduct an online consultation to assess your eligibility (BMI, health conditions, medications). If approved, they issue a prescription to a UK pharmacy. No GP referral is needed, though informing your GP is recommended for safety.
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.