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
What to Expect: Mounjaro vs Wegovy After Your First Dose
After taking your first dose of either Mounjaro or Wegovy, you may wonder how these two weight loss injections differ and what happens next. Both are prescription-only GLP-1 based medications licensed in the UK, but they work differently and have distinct dose schedules. Understanding these differences helps you know what to expect as your treatment progresses and why your prescriber chose one over the other based on your individual health needs.
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 and Wegovy Differ in Their First Dose
Mounjaro (tirzepatide) starts at 2.5 mg and uses a dual mechanism — it activates both GIP and GLP-1 hormone pathways. This dual action may be why some people experience stronger appetite suppression earlier. The SURMOUNT clinical trials showed participants on the highest dose lost an average of 20.9% of body weight over 72 weeks, though individual results vary significantly.
Wegovy (semaglutide) begins at a much lower dose of 0.25 mg and works through GLP-1 activation alone. This slower, lower starting point is designed to minimise early nausea. The STEP trials showed participants lost approximately 14.9% of body weight at the maximum dose of 2.4 mg over 68 weeks. Both medications require a UK prescription following clinical assessment, and dose increases happen gradually every 4 weeks.
Your GP or prescriber selected between tirzepatide and semaglutide based on your health profile, weight-related conditions, and medical history — not because one is universally "better". Clinical equivalence cannot be assumed.
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.
What Happens in Weeks 2–4 After Your First Injection
After your first dose, you remain on the same dose for 4 weeks before any increase. This allows your body to adjust and helps your prescriber monitor tolerance and side effects. Common early effects include mild nausea, appetite changes, and possible loose stools — most people find these settle within days or weeks.
Track your symptoms and any weight changes using our free progress tracker to share feedback with your prescriber at follow-up. You may feel noticeably less hungry, or the effect may be subtle — both are normal. The first dose is not the full therapeutic dose; weight loss typically becomes more obvious once you reach maintenance dosing (week 5 onwards for Mounjaro, weeks 17+ for Wegovy). If you experience severe side effects, contact your prescriber immediately.
- 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.
Why does Mounjaro start at 2.5 mg but Wegovy starts at 0.25 mg?
The doses are not directly comparable — they are different chemical compounds with different potencies. Mounjaro's dual GIP+GLP-1 mechanism starts higher; Wegovy's GLP-1-only mechanism uses a gentler titration. Both are evidence-based approaches designed by their manufacturers and approved by the MHRA for safety and efficacy.
Will I see weight loss after just one Mounjaro or Wegovy injection?
Unlikely. One dose is the starting dose for tolerance, not the therapeutic dose. Most weight loss occurs once you reach maintenance dosing (week 5+ for Mounjaro at 5 mg or higher; week 17+ for Wegovy at 2.4 mg). Clinical trials measured results over 68–72 weeks, not 1–4 weeks.
Which causes more nausea after the first dose — Mounjaro or Wegovy?
Individual responses vary widely. Wegovy's lower starting dose (0.25 mg) may cause less initial nausea for some people, while others tolerate Mounjaro's 2.5 mg well. Nausea is common with both and typically improves within days. Your prescriber can advise on managing side effects.
Can I switch from Mounjaro to Wegovy after one dose?
Switching is possible but should only be done under prescriber guidance. Switching between medications involves stopping one and starting another (not a direct switch). There is no official established conversion chart — any change requires a new clinical assessment and prescription.
How do I track which medication is right for me after the first dose?
Use our free progress tracker to log weight, appetite changes, energy levels, and side effects. Share this with your prescriber at follow-up — they will assess tolerability, efficacy, and whether to continue or adjust. Mounjaro and Wegovy eligibility is assessed individually; your prescriber matched you to the one most suitable for your NHS Nice criteria and health profile.
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.