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 Fast Does Mounjaro Work — What to Expect
Mounjaro (tirzepatide) is a prescription-only weight management injection, but it's not an overnight solution. Weight loss typically begins within the first few weeks, though meaningful results usually emerge after 8–12 weeks of consistent use. The medication works gradually as your body adjusts to each dose. Most people don't experience dramatic changes immediately — instead, you'll likely notice reduced appetite and feeling fuller after smaller meals. How fast Mounjaro works depends on your starting dose, how quickly you escalate, your diet, physical activity, and individual metabolism. Results vary significantly between people, so comparing your progress to others' isn't helpful.
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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.
When Does Mounjaro Start to Work?
Week 1–4 (Starting dose, 2.5 mg): Many people notice early appetite suppression or mild nausea within days. Some experience reduced food cravings almost immediately. Weight loss may begin but is typically modest — often 0.5–2 kg in the first month. Gastrointestinal side effects like nausea or constipation are most common at this stage.
Week 5–12 (After dose increases to 5 mg and beyond): More noticeable weight loss usually appears here. Clinical data shows that meaningful weight reduction accelerates as doses increase. Most people report significantly reduced appetite and better portion control by week 8–12. Our GLP-1 weight loss calculator can help you estimate potential weight loss at different Mounjaro doses.
Week 13–20 (Maintenance doses): Weight loss continues steadily. The medication works by activating dual GIP and GLP-1 hormone pathways, which regulate hunger and satiety. Combined with a reduced-calorie diet and increased physical activity, this is when most people see the results they expected.
Beyond week 20: Continued weight loss is possible, though the rate may plateau. Some people reach a natural plateau; others continue losing weight as they maintain their dose and lifestyle changes.
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.
How Mounjaro Dose Timing Affects Speed of Weight Loss
Mounjaro uses a structured dose titration schedule — you stay on each dose for a minimum of 4 weeks before increasing. This titration matters because jumping doses too quickly increases nausea risk; staying on a dose allows your body to adapt. The standard escalation is 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg (maximum). Most people reach meaningful weight loss at 5 mg or 10 mg. Clinical trials (SURMOUNT-1) showed that at 15 mg, participants lost an average of 20.9% of body weight over 72 weeks — but individual results vary significantly. Your prescriber will advise on the best escalation pace based on how you tolerate each dose. If you experience intolerable side effects, dose increases can be paused or extended, which naturally slows the overall timeline. Check your progress using our free progress tracker to monitor your weight loss journey alongside Mounjaro use.
- 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.
Can I speed up how fast Mounjaro works?
No — increasing doses faster than the prescribed schedule (minimum 4 weeks per dose) risks severe nausea and gastrointestinal side effects without improving outcomes. The titration schedule exists for safety. The fastest safe approach is to follow your prescriber's dosing plan exactly, maintain a consistent reduced-calorie diet, and increase physical activity. Weight loss is a gradual process; there's no safe way to accelerate it beyond what the medication and lifestyle changes provide.
Is Mounjaro faster than Wegovy for weight loss?
Clinical trials suggest Mounjaro may produce slightly greater average weight loss than Wegovy (semaglutide) — Mounjaro averaged clinically studied weight loss versus Wegovy's approximately 15% in separate trials. However, individual results vary dramatically. Some people lose more weight on Wegovy; others on Mounjaro. Speed also depends on your starting dose, tolerance, and lifestyle adherence. Your prescriber will advise which is more suitable for you based on your health profile and preferences.
Why isn't my weight dropping after 4 weeks on Mounjaro?
Four weeks is very early — most people see noticeable weight loss after 8–12 weeks, not four weeks. Some people do lose weight initially, but others need time for the medication to fully take effect. Weight naturally fluctuates week-to-week due to water retention, hormones, and digestion. If you're maintaining or gaining weight after 12 weeks, discuss this with your prescriber — you may need a dose increase, dietary adjustment, or a different medication may be more suitable. Use our free BMI calculator and progress tracker to monitor changes over time rather than daily weigh-ins.
Does Mounjaro work immediately or gradually?
Mounjaro works gradually. Appetite suppression can begin within days for some people, but significant weight loss typically takes 8–12 weeks to become obvious. The dual GIP and GLP-1 mechanism works by regulating hunger hormones — this isn't an instant process. Your body needs time to adapt to each dose, which is why the 4-week escalation schedule exists. Patience and consistency are key; sustainable weight loss isn't fast, but Mounjaro can support steady, meaningful results over 3–6 months.
What if I've been on Mounjaro for 3 months and haven't lost much weight?
At 3 months, if weight loss is minimal, discuss with your prescriber. Possible reasons include: suboptimal dosing, calorie intake still too high, medical conditions affecting weight (thyroid, PCOS), or medication interactions. Your prescriber may increase your dose, refer you to a dietitian, or review whether Mounjaro is the right choice. MyWeightCheck's GLP-1 weight loss calculator can help estimate whether your rate of loss is in the expected range given your starting weight, dose, and effort level.
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.