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
When Does Mounjaro Start Working?
Mounjaro (tirzepatide) typically begins to have noticeable effects within the first few weeks of starting treatment, though individual timelines vary significantly. Most people experience early changes in appetite and food cravings within days to weeks, whilst measurable weight loss usually becomes apparent after 4–8 weeks of consistent use. However, Mounjaro is not a quick-fix medication — meaningful, sustainable results develop gradually over months as your body adjusts to the dose and your lifestyle habits become embedded. Understanding what to expect at each stage helps you stay motivated and assess whether this prescription-only medication is right for your weight management journey.
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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 Mounjaro Works and When Effects Begin
Mounjaro works by activating two natural hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism signals your brain that you feel fuller with smaller portions, reduces cravings, and slows gastric emptying — how quickly food leaves your stomach. Early changes typically appear within 3–7 days for some people: reduced appetite, less interest in foods you normally enjoy, and earlier satiety at meals. However, the UK KwikPen uses a dose titration schedule, starting at 2.5 mg, meaning your dose increases every 4 weeks up to the maximum of 15 mg. Weight loss acceleration often occurs as doses increase. Clinical trial data shows that meaningful weight loss — typically 5–10 kg — becomes noticeable after 8–12 weeks on a stable dose, with progressive loss continuing over 12–24 months. The timeline depends on your starting weight, which dose you reach, how strictly you follow a reduced-calorie diet, and your physical activity levels. Calculate your potential weight loss trajectory using our free GLP-1 weight loss calculator above.
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.
Timeline: What to Expect Week by Week
Weeks 1–4 (2.5 mg dose): Mild appetite reduction, possible nausea or stomach changes as your body adjusts, minimal weight change (typically none to 1 kg). Weeks 5–8 (5 mg dose): More noticeable appetite suppression, improved food choices, 1–3 kg weight loss for many users. Weeks 9–16 (7.5–10 mg doses): Accelerating weight loss, sustained appetite control, 4–8 kg cumulative loss. Week 17+ (12.5–15 mg): Maximum effect potential, average 15–clinically studied weight loss over 12 months alongside diet and exercise. Important: this timeline is typical but not assured. Some people experience rapid early changes; others progress more gradually. If you have concerns about your progress after 12 weeks, discuss adjustments with your 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.
How quickly will I lose weight on Mounjaro?
Typical weight loss begins between weeks 4–8, with average loss of 0.5–1 kg per week on effective doses. Clinical trials showed average loss of 15–20% body weight over 68 weeks at the maximum 15 mg dose, alongside a reduced-calorie diet and increased physical activity. Individual results vary significantly based on starting weight, dose, diet adherence, and exercise.
Why does Mounjaro take so long to start working?
Mounjaro uses a gradual dose titration schedule — you start at 2.5 mg and increase every 4 weeks up to 15 mg. This approach minimises side effects and allows your body to adapt. Lower doses have milder effects, so meaningful weight loss typically requires reaching a maintenance dose of 5 mg or higher, which takes 4–8 weeks.
Will I feel Mounjaro working immediately?
Many people notice appetite reduction, reduced food cravings, or changes in food preferences within days to 1 week. However, measurable weight loss on a scale typically takes 2–4 weeks to become apparent. These early appetite changes are signs the medication is working, even if the scales don't shift immediately.
What if Mounjaro isn't working after 12 weeks?
After 12 weeks on a stable dose (usually 5 mg or higher), discuss progress with your prescriber. Some people may benefit from reaching the next dose level (e.g., 7.5 mg), whilst others may need to reassess diet, physical activity, or whether this medication is right for them. Consistency matters — Mounjaro works best alongside a reduced-calorie diet.
How does Mounjaro's timeline compare to Wegovy?
Both medications are once-weekly injections licensed for weight management in the UK. Wegovy (semaglutide) typically shows similar timelines: appetite changes within days to weeks, measurable weight loss after 4–8 weeks. Mounjaro is a dual GIP/GLP-1 agonist versus Wegovy's single GLP-1 action. Clinical trials suggest Mounjaro may support slightly greater average weight loss (15–21% vs 15%), though individual response varies.
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.