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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When Does Mounjaro Kick In — Timeline & UK Access
Mounjaro (tirzepatide) typically begins affecting appetite and food intake within the first 2–3 weeks of starting treatment, though meaningful weight loss usually becomes noticeable after 4–8 weeks. However, the timeline varies between individuals, and how quickly Mounjaro "kicks in" depends on several factors including your dose, metabolism, diet, and activity level. This guide explains the expected timeline, UK prescription options, and what to expect when accessing Mounjaro privately or through NHS specialist services.
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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 Long Before You Notice Mounjaro Working
Week 1–2: Most people begin on the 2.5 mg starting dose. Within the first few days, many users report reduced appetite and less interest in foods they normally crave. Nausea is common during this initial phase and typically settles within the first week.
Week 4–8: After escalating to 5 mg (week 5), weight loss becomes more noticeable for most people. Clinical trials showed average weight loss of 15% body weight by 72 weeks on Mounjaro, though individual results vary significantly. Some people see changes on the scale within 2–3 weeks; others take 8–12 weeks.
Weeks 9+: Doses continue escalating every 4 weeks (7.5 mg, 10 mg, 12.5 mg, maximum 15 mg) until your prescriber finds your effective maintenance dose. Weight loss typically accelerates as you reach your therapeutic dose.
Important: Mounjaro works best alongside a reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator above to estimate potential weight loss at different doses based on your personal starting weight.
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 & Access in the UK
NHS access: NICE has approved tirzepatide (Mounjaro) for NHS use, but access is through specialist tier 3 weight management services following a GP referral. Waiting lists vary significantly by region (ICB), ranging from weeks to many months. Once prescribed, Mounjaro is free on the NHS.
Private UK prescription: A GPhC-registered online pharmacy typically charges £129–£320 per month depending on your dose, with higher doses (10–15 mg) costing more than lower doses (2.5–5 mg). Each prescription requires a clinical assessment by a UK-licensed prescriber. Most private consultations cost £20–£50. No referral needed — you can access private prescription without waiting for NHS services. Check your BMI and eligibility using our free BMI calculator to understand your starting position before consulting a prescriber.
- 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.
Does Mounjaro work immediately?
No. Most people begin to notice reduced appetite within 2–3 weeks on the 2.5 mg starting dose, but meaningful weight loss typically takes 4–8 weeks to become visible. Results depend on your dose level, diet adherence, and individual metabolism. Clinical trials showed weight loss accelerates as you reach your therapeutic maintenance dose.
How much does Mounjaro cost on the NHS?
Mounjaro is free on the NHS once prescribed by a specialist weight management service. However, NHS access requires a GP referral and specialist assessment, with waiting times varying from weeks to many months depending on your local ICB. Private prescription costs are typically £129–£320 per month depending on your dose.
Can I get Mounjaro on NHS prescription without waiting?
NHS waiting times vary by region. If your local NHS weight management service has a long waiting list, you may choose to access Mounjaro privately through an online prescription service while you wait for NHS assessment. Some people use private Mounjaro initially, then switch to NHS once approved.
What is the difference between tirzepatide and Mounjaro?
Tirzepatide is the active ingredient name; Mounjaro is the brand name made by Eli Lilly. They are the same medication. You might also encounter tirzepatide under other brand names in future, but Mounjaro is currently the only UK-licensed tirzepatide formulation for weight management.
Is Mounjaro vs Wegovy a cost difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) cost similar amounts privately (roughly £129–£320/month depending on dose). NHS costs are zero if prescribed through specialist services. Mounjaro and Wegovy use different mechanisms — Mounjaro is a dual GIP/GLP-1 agonist while Wegovy is GLP-1 only — but private cost is comparable between the two.
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.