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
Mounjaro 5 mg Side Effects vs Wegovy — What's the Difference?
Both Mounjaro and Wegovy are prescription-only weight loss injections licensed in the UK, but they work through different mechanisms and can cause different side effects. At the 5 mg maintenance dose, Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Understanding how side effects compare can help your prescriber assess which medication might suit you better. This guide compares the side effect profiles based on clinical trial data and UK prescribing experience.
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.
How Mounjaro 5 mg and Wegovy Side Effects Compare
Mounjaro 5 mg (tirzepatide) activates two hormone pathways — GIP and GLP-1 — which may explain why some patients report slightly different side effect patterns compared to Wegovy. In the SURMOUNT-1 clinical trial, the most common side effects at the 5 mg dose were gastrointestinal: nausea (25–30% of patients), diarrhoea (19–23%), vomiting (7–11%), and constipation (16–19%). Injection site reactions occurred in approximately 2–4% of users.
Wegovy (semaglutide) showed similar but subtly different patterns in the STEP-1 trial. Nausea was reported by 25–29% of patients, diarrhoea by 23%, and constipation by 26%. Vomiting occurred less frequently (3–4%) compared to Mounjaro.
- Mounjaro may cause slightly more vomiting at lower doses
- Wegovy users report more constipation relative to vomiting
- Both cause nausea and diarrhoea in similar proportions
- Individual responses vary significantly — side effects are not assured
Side effects typically improve after the first 4 weeks on each dose, particularly if nausea and diarrhoea occur.
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.
Which Medication Might Be Right for You?
There is no universally "better" medication — suitability depends on your individual health profile, medical history, and how your body responds to treatment. Some patients tolerate Mounjaro better; others prefer Wegovy. Your prescriber will assess factors including any existing gastrointestinal conditions, other medications you take (particularly oral contraceptives, which require additional precautions with Mounjaro), and your personal preferences.
Both medications require a clinical assessment before prescription. Your GP or a licensed prescriber will discuss which option is most appropriate for you. If you experience troublesome side effects on one medication, switching to the other is sometimes possible, though this should always be managed by your prescriber.
Use our free BMI calculator to check your current status, and our GLP-1 weight loss calculator to explore potential outcomes with either medication at different doses.
- 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.
Is Mounjaro 5 mg more likely to cause side effects than Wegovy?
Not necessarily. Both medications cause similar side effects (nausea, diarrhoea) at comparable rates in clinical trials. Mounjaro may cause slightly more vomiting at lower doses, whilst Wegovy users report more constipation. Individual tolerance varies — some patients experience minimal side effects, others more pronounced symptoms. Your prescriber can discuss which medication might suit you better based on your health history.
Do side effects from Mounjaro 5 mg improve over time?
Yes, typically. Most gastrointestinal side effects are worst in the first 2–4 weeks on each dose. As your body adjusts, nausea and diarrhoea usually improve or resolve. This is why your prescriber spaces out dose increases by at least 4 weeks, allowing time for adjustment. If side effects persist beyond this period, discuss with your prescriber — they may adjust your dose or consider an alternative.
Can I switch from Mounjaro to Wegovy if I experience bad side effects?
Possibly, but only under medical supervision. Switching between medications requires prescriber guidance because the doses are not directly equivalent and your body needs time to adjust. If you experience severe or intolerable side effects, contact your prescriber immediately rather than stopping treatment abruptly. They may offer dose adjustments, temporary breaks, or a switch to an alternative medication.
What serious side effects should I watch out for with Mounjaro 5 mg?
Seek urgent medical attention if you experience severe stomach pain spreading to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), gallbladder problems, or significant changes in vision. These are rare but require immediate assessment. Most common side effects (nausea, diarrhoea) are uncomfortable but not dangerous and typically resolve with time.
Does Mounjaro's dual mechanism (GIP+GLP-1) cause more side effects than Wegovy's single mechanism?
Not consistently. Whilst Mounjaro activates two hormone pathways, clinical trial data shows side effect rates are comparable to Wegovy overall. Mounjaro may cause slightly more vomiting at lower maintenance doses, but both medications are well tolerated by most patients. The dual mechanism may actually improve outcomes for some individuals, though prescriber assessment is needed to determine this.
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.