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?
Mounjaro (tirzepatide) typically begins to show appetite-reducing effects within the first week or two of starting treatment, though meaningful weight loss usually becomes noticeable after 4–8 weeks on a stable dose. The timeline depends on your dose titration schedule, individual metabolism, and how strictly you follow dietary guidelines alongside the injection. This guide explains what to expect during the first weeks of Mounjaro treatment, how the dose escalation works, and how to access this prescription-only medication in the UK.
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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.
Mounjaro Timeline: When You'll Notice Effects
Weeks 1–4 (Starting dose 2.5mg): Most people experience mild nausea and reduced appetite within 3–7 days. This is the initial signal that the tirzepatide is working. Weight changes are usually minimal at this stage (0–2 lbs). Some users report feeling fuller after smaller meals immediately.
- Nausea may peak around day 3–4 and often settles by day 10
- Stomach discomfort is common but typically improves with time
- You remain on 2.5mg for a full 4 weeks before increasing
Weeks 5–12 (Doses 5mg–7.5mg): Weight loss becomes more visible. Clinical trial data shows average loss of 2–4 kg during this phase. Appetite suppression strengthens as your dose increases. Many users report significantly reduced cravings and food interest.
Weeks 13+: At higher maintenance doses (10–15mg), consistent weight loss of 0.5–1 kg per week is typical alongside a reduced-calorie diet and physical activity. Results vary considerably between individuals.
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.
Accessing Mounjaro in the UK: Cost and Routes
NHS Route: Available through tier 3 specialist weight management services following GP referral. NICE has approved Mounjaro for eligible adults, but regional availability varies significantly. Waiting lists can extend 6–12 months in many areas. Once approved, NHS provision is free at point of use.
Private Route: Available without NHS referral through UK-licensed prescribers via online clinical assessment. Private Mounjaro costs typically range from £140–£300 per month depending on dose (higher doses cost more). You'll complete a brief health questionnaire, have a consultation with a UK-registered prescriber, and receive your prescription to a GPhC-registered pharmacy. This route is faster—often same week—but you cover the full cost.
Try our GLP-1 weight loss calculator to estimate potential outcomes at different doses, and use the BMI calculator to check your starting point before beginning treatment.
How online prescription works: You complete a health form detailing your BMI, medical history, current medications, and weight-related conditions. A UK prescriber reviews this and decides if Mounjaro is suitable. If approved, your prescription is sent to a pharmacy, which posts your Mounjaro KwikPen to you (usually within 2–3 working days). The whole process typically takes 24–48 hours from initial consultation to prescription.
Important: You cannot simply request a specific dose. Your prescriber will recommend a starting point (usually 2.5mg) and manage your dose escalation based on your response and any side effects. Dose adjustments happen every 4 weeks following the standard titration schedule.
- 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 start working immediately?
Mounjaro begins reducing appetite within days (typically 3–7 days), but noticeable weight loss usually takes 4–8 weeks. The medication works from your first injection, but your body needs time at each dose level to show measurable results. Most people see visible changes after 8 weeks at a stable dose.
What is the Mounjaro KwikPen and how does it work?
The KwikPen is Mounjaro's pre-filled injection device—you simply dial your dose and inject once weekly into your stomach, thigh, or upper arm. Each pen contains 4 weekly doses (4 weeks supply). It's easy to use and requires no mixing. Store unopened pens in a fridge at 2–8°C; after first use, keep at room temperature for up to 30 days.
Can I get Mounjaro on the NHS in the UK?
Yes, Mounjaro is approved by NICE for NHS specialist weight management services. However, you need a GP referral to tier 3 services and typically a BMI of 30+ or 27+ with weight-related health conditions. Regional availability varies widely and waiting lists can be very long (6–12 months in many areas).
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs typically £140–£300 per month depending on your dose (2.5mg–15mg). You can compare prices through different UK online prescribing services, but ensure they're GPhC-registered. Monthly cost increases with higher doses as the medication becomes stronger.
Is Mounjaro the same as Wegovy or semaglutide?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are licensed for weight management in the UK, but they work differently. Mounjaro may produce slightly larger weight loss on average, though individual results vary considerably.
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.