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 Do Mounjaro Side Effects Last?
Side effects from Mounjaro (tirzepatide) typically decrease over time as your body adjusts to the medication. Most people experience the most noticeable side effects in the first few weeks of starting treatment or after a dose increase, with many reporting improvement within 2–4 weeks. However, side effect duration varies significantly between individuals, and some may persist longer. Understanding what to expect—and the costs involved in accessing Mounjaro in the UK—can help you make an informed decision about this prescription weight management option.
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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.
Mounjaro Side Effects Timeline & Duration
Most Mounjaro side effects occur during the first 1–2 weeks after starting or increasing your dose. Common early side effects include nausea, diarrhoea, and stomach discomfort. Many people report these symptoms settle significantly by week 3–4 as their body adapts.
- Nausea: Often peaks in the first week, typically improves by weeks 2–4 in most users
- Diarrhoea or constipation: May persist longer (4–8 weeks) but usually becomes more manageable
- Reduced appetite: Generally continues (this is the intended effect) but becomes less extreme
- Stomach pain: Usually mild and temporary, settling within 1–2 weeks
- Injection site reactions: Minor redness or bruising typically resolve within a few hours to days
During dose titration—when your Mounjaro KwikPen dose increases every 4 weeks—you may experience a brief return of side effects as your body adjusts to the new dose. This is normal and temporary.
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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.
UK Cost, Access & Private Prescription Options
In the UK, Mounjaro is available through two routes: NHS specialist weight management services and private prescription. NHS access requires referral to a tier 3 or tier 4 specialist service, which varies significantly by region and typically involves waiting lists of several months. Private prescription costs generally range from £180–£320 per month depending on your dose, with each Mounjaro KwikPen containing 4 weekly injections. To access Mounjaro privately, you'll need an online clinical assessment with a UK-licensed prescriber who will evaluate your Mounjaro eligibility (BMI of 30+ or 27+ with weight-related health conditions). Once prescribed, your medication is dispensed by a GPhC-registered pharmacy. Use our free GLP-1 weight loss calculator to estimate potential outcomes, and discuss which access route suits your situation with a healthcare professional.
- 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 do Mounjaro side effects improve?
Most people notice improvement within 2–4 weeks as their body adjusts. Nausea typically improves first, while digestive effects may take 4–8 weeks. Side effects may briefly return when your dose increases, then settle again as you adapt. If side effects persist beyond 8 weeks or worsen, contact your prescriber.
What is the difference between Mounjaro and Wegovy side effects duration?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) have similar side effect timelines, typically improving within 2–4 weeks. Mounjaro's dual GIP/GLP-1 mechanism may produce slightly different symptom profiles, but duration is comparable. Individual experiences vary significantly regardless of which medication you're taking.
Can I reduce Mounjaro side effects while on treatment?
Yes. Eating smaller meals, staying hydrated, avoiding fatty or spicy foods, and eating slowly can help manage nausea and digestive symptoms. Your prescriber may also adjust your dose schedule if side effects are severe. Always discuss management strategies with your healthcare provider before making changes.
Is Mounjaro available on the NHS in the UK?
Yes, Mounjaro (tirzepatide) was approved by NICE for NHS use in specialist weight management services. However, NHS availability varies by region and waiting times can be long. Private prescription is available more immediately through online clinical assessments, typically costing £180–£320 per month depending on your dose.
What is the typical cost of Mounjaro in the UK?
Private Mounjaro typically costs £180–£320 per month (approximately £45–£80 per weekly injection). Costs vary by dose—starting doses (2.5–5 mg) are usually cheaper than maintenance doses (10–15 mg). NHS access is free but requires a specialist service referral and may involve significant waiting times.
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.