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
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How Long Do Mounjaro Side Effects Last?
Most people experience Mounjaro side effects temporarily, especially in the first few weeks of treatment. Common issues like nausea and digestive problems typically ease within 2–4 weeks as your body adjusts, though some may persist longer depending on your dose and individual response. Unlike some weight loss treatments, the dual GIP and GLP-1 mechanism of tirzepatide means side effect patterns differ from single-action GLP-1 medications like Wegovy. Understanding what to expect and how this compares to other prescription weight management options can help you make informed decisions about which treatment suits your situation.
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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 vs Wegovy: Side Effect Duration and Mechanism
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work on similar principles but have different onset patterns. Mounjaro uses a dual GIP and GLP-1 receptor agonist mechanism, activating two natural hormone pathways, whereas Wegovy is a GLP-1 agonist only. This dual action in Mounjaro may affect how quickly side effects appear and resolve.
Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) studies show that nausea and digestive issues are most intense during the first 4 weeks of each new dose. In SURMOUNT trials, most participants reported side effects peaked around weeks 1–2 after dose escalation, then gradually improved by week 4. Wegovy trials showed a similar pattern, though individual tolerance varies significantly. Both require careful dose titration—Mounjaro escalates from 2.5mg to 15mg over approximately 20 weeks, whilst Wegovy escalates from 0.25mg to 2.4mg over approximately 16 weeks. Your GP or prescriber will advise on managing side effects during this period and whether your dose should be adjusted if symptoms become problematic.
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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.
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Managing Side Effects: Timeline and When to Expect Improvement
Most Mounjaro side effects—nausea, stomach pain, constipation, and diarrhoea—typically begin within the first few days of injection and peak at days 3–5. By day 7, many people notice improvement as the dose wears off before the next weekly injection. This pattern repeats with each dose increase, meaning you may experience temporary side effects again when escalating to the next strength. Persistent nausea lasting beyond week 4 on the same dose is less common but may indicate your current dose is not well tolerated.
If side effects are affecting your quality of life, speak to your prescriber—they may recommend staying on your current dose longer, slowing your escalation schedule, or trying dietary adjustments like smaller, more frequent meals and reduced fat intake. Some people find side effects resolve completely by week 2–3, while others experience mild symptoms for 4–6 weeks. Tracking your symptoms using a progress tracker can help you and your prescriber identify patterns and make informed decisions about continuing or adjusting your tirzepatide treatment.
- 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 long before Mounjaro side effects go away completely?
Most people experience significant improvement within 2–4 weeks on the same dose. Nausea and digestive issues typically peak in days 1–5 after injection and ease by day 7. However, individual timelines vary—some improve within days, others take 4–6 weeks. If side effects persist beyond week 4, discuss with your prescriber about dose adjustment or escalation timing.
Do Wegovy side effects last longer than Mounjaro?
Clinical trial data shows similar side effect timelines for both medications. Both peak in the first few days and usually improve by week 4. The main difference is their mechanisms—Mounjaro's dual GIP and GLP-1 action versus Wegovy's GLP-1 only—which may affect individual tolerance. Neither has significantly longer side effect duration based on available evidence.
What can I do to reduce nausea whilst taking Mounjaro?
Eat smaller, more frequent meals; avoid fatty or greasy foods; stay hydrated; avoid rapid movements after eating; and consider ginger supplements or peppermint tea. Speak to your GP or pharmacist before taking any supplements. Most importantly, give your body time to adjust—nausea typically improves naturally after 2–3 weeks.
Will my side effects get worse each time my Mounjaro dose increases?
Possibly, but usually less severely than your first dose. Many people experience temporary side effects again when escalating (e.g., from 5mg to 7.5mg), typically peaking around days 2–4 and improving by week 4. Your prescriber can advise on spreading dose increases further apart if needed.
How is Mounjaro prescribed in the UK and who can prescribe it?
Mounjaro is a prescription-only medicine available through NHS specialist tier 3 weight management services (via GP referral, though access and waiting lists vary by region) or private online prescription services following a clinical assessment. Your prescriber will assess eligibility based on your BMI and health conditions before issuing a KwikPen prescription.
Ready to take control of your weight loss journey?
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