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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Mounjaro vs Wegovy: Long-Term Side Effects Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work through different mechanisms and may have different long-term side effect profiles. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates GLP-1 alone. Understanding the differences between these medications can help you and your GP or prescriber make an informed decision about which might be suitable for your individual health situation. Clinical trial data and real-world experience provide important safety context for both options.
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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.
Clinical Trial Data: SURMOUNT vs STEP Long-Term Outcomes
Mounjaro (tirzepatide) was studied in the SURMOUNT trials, which tracked participants for up to 72 weeks on doses ranging from 5mg to 15mg. The most common long-term side effects reported were gastrointestinal: nausea, diarrhoea, constipation, and vomiting. These symptoms typically emerged during the dose titration phase and often improved over time as the body adjusted to the medication.
Wegovy (semaglutide) was evaluated in the STEP trials, with participants observed for up to 68 weeks on doses up to 2.4mg. Side effect patterns were similar—predominantly nausea, diarrhoea, and constipation—though the severity and duration varied between individuals. Wegovy's slower dose escalation (starting at 0.25mg) may result in a gentler adjustment period for some users.
- Mounjaro: gastrointestinal effects most common in weeks 1–8, often resolve by week 12–16
- Wegovy: similar timeline, though some users report continued mild nausea throughout treatment
- Both medications: injection site reactions (redness, bruising) are common but typically mild
- Serious adverse events in trials: rare, but pancreatitis and gallbladder complications reported in both cohorts
Individual responses vary significantly—some participants experienced minimal side effects whilst others required dose adjustments or paused treatment.
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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
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Long-Term Safety: What Prescribers and Patients Should Know
Long-term data (beyond 72 weeks) for both medications is still emerging, as both are relatively newer treatments in the UK market. Mounjaro received MHRA approval more recently than Wegovy, so extended real-world safety data is still accumulating. What we know from clinical trials is that discontinuation due to side effects occurred in approximately 5–10% of participants on both medications.
Hair loss (alopecia) has been reported by some users in real-world experience, particularly after extended use, though this was not a prominent finding in formal trials. Gallbladder problems and pancreatitis, whilst rare, remain important warning signs requiring immediate GP attention. Neither medication should be used during pregnancy or breastfeeding. Your GP or prescriber will assess your individual risk factors, medical history, and comorbidities to determine which medication—if either—is most appropriate for you, based on your personal long-term safety profile.
- 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.
Are long-term side effects worse with Mounjaro or Wegovy?
Clinical trials show similar side effect patterns for both—predominantly nausea, diarrhoea, and constipation. Neither has a clearly "worse" long-term profile. Individual tolerance varies significantly. Your prescriber will assess which is safer for your specific health situation based on your medical history and comorbidities.
Do gastrointestinal side effects get better over time with these medications?
Yes, for most users. In SURMOUNT and STEP trials, nausea and stomach upset typically improved by weeks 12–16 as the body adjusted. However, some users experience persistent mild symptoms. Staying hydrated, eating smaller meals, and avoiding fatty foods often helps. Always discuss persistent symptoms with your GP.
What serious long-term side effects should I watch for with Mounjaro or Wegovy?
Seek urgent medical attention if you experience severe, persistent stomach pain (possible pancreatitis), upper abdominal pain (gallbladder problems), or signs of a serious allergic reaction (facial swelling, difficulty breathing). These are rare but require immediate GP evaluation. Both medications carry a Black Triangle warning in the UK.
How does the dose titration schedule affect side effects between these medications?
Mounjaro increases by 2.5mg weekly (starting at 2.5mg, reaching 15mg maximum), whilst Wegovy increases more gradually (0.25mg to 2.4mg over 16+ weeks). Wegovy's slower escalation may result in fewer initial side effects for some users, though long-term outcomes are similar. Your prescriber will recommend the schedule best suited to you.
Can I switch from Wegovy to Mounjaro if I experience side effects?
Potentially, yes—but this requires a new clinical assessment and prescription from your GP or a licensed prescriber. The two medications are not interchangeable. Switching should only be considered under medical guidance. The approximate equivalence is Mounjaro 5mg ≈ Wegovy 0.5mg, but individual tolerance differs significantly, so your prescriber may adjust dosing accordingly.
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