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 Quickly Does Mounjaro Take To Work?
Mounjaro (tirzepatide) doesn't work instantly — most people begin noticing effects within the first 2–4 weeks, though significant weight loss typically takes 8–12 weeks or longer. Because Mounjaro requires gradual dose titration (starting at 2.5 mg and increasing every 4 weeks), the timeline for noticeable results varies considerably between individuals. Understanding what to expect week-by-week can help you set realistic goals and stay committed to your treatment plan.
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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.
Timeline: When Does Mounjaro Start Working?
Weeks 1–4 (2.5 mg starting dose): Most people experience reduced appetite within 3–5 days, though weight loss in week one is typically minimal (often just 0.5–1 kg due to water loss). Nausea is common but usually improves as your body adjusts.
Weeks 5–8 (5 mg dose): Noticeable appetite suppression intensifies. Many people report feeling fuller after smaller meals. Weight loss acceleration begins — expect approximately 1–2 kg loss during this period.
Weeks 9–16 (7.5–10 mg escalation): Cumulative weight loss becomes more visible. Clinical trials show meaningful results emerging around 8–12 weeks. The dual GIP and GLP-1 mechanism of tirzepatide becomes more pronounced at higher doses.
Week 17+ (15 mg maintenance): Peak effect typically achieved. Most significant weight loss occurs from weeks 12–24. In clinical trials, the SURMOUNT-1 study showed participants lost an average of 15–21% body weight over 72 weeks.
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.
What Affects How Quickly Mounjaro Works
Individual response varies significantly based on several factors: starting weight (higher starting weights often see faster percentage loss), diet adherence (Mounjaro works alongside reduced-calorie diet — results are not medication alone), physical activity level, age and metabolism, and underlying health conditions like type 2 diabetes or PCOS.
Side effects like nausea and constipation can initially slow progress if they reduce food intake unsustainably or cause dehydration. Your prescriber may adjust your dose titration schedule if side effects are severe. It's important to use our free GLP-1 weight loss calculator to set personalised expectations and track progress realistically alongside our progress tracker tool.
- 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 soon will I lose weight on Mounjaro?
Most people see the first 1–2 kg loss in the first 4 weeks, accelerating significantly from weeks 8–12 onwards. However, the rate varies. Some people lose weight more slowly initially due to side effects or dose titration, while others see faster results. Clinical trials showed an average loss of 15–21% body weight over 72 weeks at the maximum 15 mg dose.
Why does Mounjaro take 4 weeks between doses?
The 4-week intervals allow your body to adjust to each dose safely and let your prescriber assess tolerability before increasing. This reduces side effects and lets the medication's full effect at each level become apparent. Faster titration isn't recommended because it increases nausea and gastrointestinal side effects without improving long-term weight loss outcomes.
Can I skip doses to speed up weight loss?
No — skipping doses or increasing doses yourself is dangerous and ineffective. Mounjaro works best with steady, consistent once-weekly injections. Skipping doses can cause blood sugar instability, increased side effects on resumption, and loss of appetite suppression benefits. Always follow your prescriber's dose schedule exactly.
Is Mounjaro available on the NHS and how long does NHS access take?
Yes, NICE has approved tirzepatide for NHS weight management services, but access is through specialist tier 3 services, not routine GP prescription. NHS waiting lists vary by region — some areas have waits of several months to years. Private online prescribing offers faster access but involves costs. Your GP can discuss both NHS and private routes.
Does Mounjaro work better than Wegovy?
Mounjaro (tirzepatide) and Wegovy (semaglutide) are different medications. Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1 only. In trials, Mounjaro showed slightly higher average weight loss (15–21% vs ~15% for Wegovy), but individual results vary widely. Effectiveness depends on your personal metabolism, not which is objectively "better" — your prescriber chooses based on your health profile.
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.