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 Work After Your First Injection?
Mounjaro (tirzepatide) typically begins working within the first week of your initial injection, though most people notice stronger appetite reduction effects around weeks 2–4 as the medication accumulates in your system. Weight loss itself usually becomes measurable after 4–8 weeks, but significant results typically emerge over 12 weeks and beyond. According to NHS and NICE guidance, patience is key—Mounjaro works gradually alongside reduced-calorie diet and increased physical activity. Individual timelines vary considerably based on your starting weight, metabolism, and lifestyle changes. Our free weight loss calculator can help you track realistic expectations as you begin treatment through NHS specialist services or private prescription routes.
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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.
NHS Eligibility and Specialist Service Access
NICE has approved tirzepatide (Mounjaro) for use within NHS weight management services, but access differs significantly from private routes. If you're eligible through your GP—BMI 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes or hypertension—you'll be referred to your local tier 3 specialist weight management service.
Once referred, the NHS specialist team will assess your suitability and initiate the weekly weight loss injection programme. However, NHS waiting lists vary by region (ICB), and some areas have waiting times of several months. When treatment begins, you'll start on the lowest dose (2.5 mg) for weeks 1–4, then escalate gradually—this is the dose titration process outlined in NICE guidance. Each dose increase takes a minimum of 4 weeks, meaning it may take 4–5 months to reach the maximum 15 mg dose where most weight loss occurs.
Throughout your NHS specialist service care, your progress is monitored regularly by the multidisciplinary team.
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.
NHS vs Private Access: Speed and Timeline Differences
NHS access is free but slower due to specialist service waiting lists and structured dose escalation protocols. Private access via a GPhC-registered online pharmacy with a UK-licensed prescriber is faster—you can receive your KwikPen within days—but costs £129–£320 monthly depending on dose.
The critical difference: both routes follow the same dose titration safety profile (starting 2.5 mg, increasing weekly injections gradually), so the biological onset timeline is identical. What differs is how quickly you access treatment. Private prescription routes suit those who cannot wait for NHS specialist referral; NHS routes suit those eligible and willing to wait. Regardless of route, realistic weight loss injection outcomes emerge after 8–12 weeks, not immediately after your first injection. MyWeightCheck's free GLP-1 weight loss calculator helps you model both timelines and compare mounjaro vs wegovy expectations based on your starting BMI and health profile.
- 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.
When will I see weight loss after my first Mounjaro injection?
Measurable weight loss typically appears after 4–8 weeks on the starting dose, with more significant results after 12+ weeks. This is because Mounjaro starts at a low dose (2.5 mg) and requires 4-week intervals before increasing. Appetite suppression may be noticeable within days, but the cumulative weight loss effect takes time alongside diet and activity changes.
Does Mounjaro work differently in NHS vs private prescription routes?
No. The medication itself works identically regardless of whether you access it through NHS specialist services or privately. The difference is in access speed—NHS has waiting lists but is free; private is faster (days) but costs £129–£320 monthly. Both follow the same safe dose escalation protocol.
Why does NICE recommend a slow dose titration schedule?
Dose titration (gradually increasing from 2.5 mg to 15 mg over weeks 1–20+) minimises side effects and allows your body to adjust. NICE guidance prioritises safety and tolerability, ensuring you remain on treatment long-term rather than discontinuing due to nausea or other early effects.
What should I expect in weeks 1–4 after my first Mounjaro injection?
In weeks 1–4, most patients experience initial side effects like nausea, diarrhoea, or constipation. Appetite reduction may become noticeable. Significant weight loss is unlikely yet. At week 4, your dose is reviewed; if tolerated, you'll progress to 5 mg for weeks 5–8, continuing the gradual escalation pathway outlined by NHS specialists or your private prescriber.
How does Mounjaro onset compare to other weekly weight loss injections?
Mounjaro's onset is similar to Wegovy (semaglutide), another weekly weight loss injection. Both show measurable effects after 4–8 weeks and require dose titration over months. Individual variation is significant. MyWeightCheck's comparison tools help you understand both options if exploring alternatives with your GP or prescriber.
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.