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?
Mounjaro (tirzepatide) typically begins affecting appetite within the first few days, though meaningful weight loss usually becomes noticeable after 4–8 weeks of treatment. Most people see more significant results after 12 weeks on a stable dose. However, individual responses vary considerably, and your specific timeline depends on your starting dose, how quickly it's increased, your lifestyle changes, and your body's response. This NHS-focused guide explains what the clinical evidence shows about Mounjaro's timeline in UK practice.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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 Mounjaro starts working
Week 1–2 (2.5 mg starting dose): Most people notice appetite reduction and changes in food preferences within a few days. Some experience nausea or other gastrointestinal changes. Weight loss may not be measurable yet.
Week 4–8 (dose escalation phase): As your dose increases through 5 mg and towards 7.5 mg, more noticeable weight loss typically becomes apparent. Clinical data suggests average weight loss of 2–3 kg during this phase in UK patients. Energy levels may change.
Week 12–16 (stabilisation phase): By 12 weeks on a consistent dose, most people see clearer results. This is when your GP or prescriber may assess whether your current dose is working effectively. Average weight loss by this point is typically 4–6 kg, though results vary significantly between individuals.
Week 20+ (maintenance): The Mounjaro KwikPen regimen continues with dose optimisation. Weight loss trajectory becomes clearer, allowing realistic assessment of whether your current dose is right for you.
Important: The NICE-approved approach in UK specialist services emphasises that Mounjaro works best alongside a reduced-calorie diet and increased physical activity—medication alone does not produce results.
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 access and realistic expectations
Access to Mounjaro through NHS specialist tier 3 weight management services involves assessment of your BMI and eligibility (BMI ≥30, or ≥27.5 with weight-related health conditions). Once prescribed, expect dose escalation over 16–20 weeks before reaching your optimal dose. Private UK prescribing can be faster, but clinical assessment remains essential.
Mounjaro vs Wegovy: Both are once-weekly injectables licensed in the UK, but Mounjaro (dual GIP/GLP-1 agonist) has shown slightly higher average weight loss in clinical trials compared to Wegovy (GLP-1 agonist only). However, individual timelines and response rates differ—what works quickly for one person may take longer for another.
Use our free GLP-1 weight loss calculator to project realistic timelines based on your starting weight and dose. Combined with our BMI calculator, you can track whether you're moving towards a healthier weight category as treatment progresses.
When to expect plateau: Most people experience an initial gradual weight management phase (weeks 4–16), followed by a slower, steadier decline. A plateau at 3–6 months is common and does not mean the medication is no longer working. Weight stabilisation indicates your body has adjusted to a new equilibrium.
What slows results: Insufficient calorie deficit, inconsistent lifestyle changes, certain medications that interfere with GLP-1 absorption, and dose that is too low for your individual needs. Your prescriber can adjust your dose if progress stalls.
Safety consideration: Very gradual weight management (>2 kg per week) increases risk of muscle loss and gallbladder complications. NHS guidance recommends 0.5–1 kg per week as sustainable. If you lose weight faster than expected, discuss this with your GP.
- 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 Mounjaro start working within the first week?
Appetite suppression and changes in food preferences can occur within days of starting Mounjaro, even at the low 2.5 mg starting dose. However, measurable weight loss on the scales typically takes 2–3 weeks to become noticeable. The initial appetite reduction is the medication working, but scale results lag behind the biological effect.
How much weight should I lose in the first month on Mounjaro?
Individual results vary significantly, but UK clinical practice suggests 1–3 kg in the first month, depending on your starting dose, calorie deficit, and individual metabolism. Some lose more, others less. Progress trackers help identify your personal pattern rather than comparing to others.
What if Mounjaro doesn't seem to be working after 8 weeks?
After 8 weeks, discuss with your prescriber—your dose may need to increase, your calorie deficit may be insufficient, or medication absorption could be affected by other factors. Do not assume it is not working; most people need 12+ weeks at an optimal dose to see substantial results.
Is there a difference in timeline between Mounjaro and Wegovy for UK patients?
Both are once-weekly injectables, but Mounjaro's dual mechanism may produce slightly faster average weight loss. However, individual timelines vary more than medication choice. Some respond better to Mounjaro, others to Wegovy. Your prescriber will assess which is more appropriate.
Do I have to keep taking Mounjaro to maintain weight loss?
Weight typically returns after stopping Mounjaro if lifestyle changes are not sustained. UK NHS guidance emphasises that medication is a tool alongside diet and exercise changes. Discuss long-term plans with your GP—some continue indefinitely, others use it to establish new habits then transition to maintenance alone.
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.