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 30 Days
If you're starting Mounjaro or Wegovy, the first 30 days are typically a dose-adjustment period rather than when maximum results appear. Both medications work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy activates GLP-1 only. Understanding these differences helps you know what's normal during your first month on treatment. This guide compares the two injections side-by-side and explains what clinical evidence shows happens in those crucial early weeks. Remember, individual responses vary significantly, and your GP or prescriber will monitor your progress throughout.
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 the First Month
The key difference between Mounjaro and Wegovy lies in their mechanism of action. Mounjaro (tirzepatide) activates two hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 — which is why it may work more powerfully on appetite suppression. Wegovy (semaglutide) activates GLP-1 only, a single but well-established pathway.
During your first 30 days on either medication, you're typically on a starting dose: Mounjaro begins at 2.5 mg, while Wegovy starts at 0.25 mg. Neither dose is the therapeutic maximum. The first month focuses on tolerability and dose titration rather than dramatic weight loss. Clinical trial data from SURMOUNT (Mounjaro) and STEP (Wegovy) trials measured results over 68–72 weeks at therapeutic doses, not after 4 weeks.
Most users experience some appetite reduction within the first few days to one week, but significant weight loss typically becomes more apparent after 8–12 weeks of treatment at stable, therapeutic doses. Both require a UK prescription following clinical assessment and are available through NHS specialist services or private prescribers.
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.
Side Effects and Tolerance in Week 1–4
Nausea and gastrointestinal side effects are most common in the first 2–4 weeks as your body adjusts. Both medications slow gastric emptying (how quickly food leaves your stomach), which can cause nausea, diarrhoea, or constipation — these often settle as your body adapts.
The mounjaro KwikPen and Wegovy pen are both pre-filled auto-injectors; technique and injection site rotation matter for comfort. After 30 days, if side effects are significant, your prescriber may pause dose escalation or adjust timing. Weight loss expectations after 30 days are modest — typically 1–3 kg — because you're still in the dose-titration phase. Weight loss accelerates when you reach and stabilise at your therapeutic dose.
Neither medication is a structured approach; both work best alongside dietary changes and physical activity. Your progress tracker can help you monitor changes beyond just weight — energy levels, appetite, and wellbeing often improve before the scales shift significantly.
- 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.
Will I lose weight in the first 30 days on Mounjaro?
Modest weight loss (1–3 kg) is possible, but the first month is primarily a dose-adjustment period. Both Mounjaro and Wegovy require 8–12 weeks at therapeutic doses before results become significant. Clinical trials measured outcomes over 68–72 weeks, not 4 weeks. Individual responses vary.
Is Mounjaro more powerful than Wegovy after 30 days?
Mounjaro's dual GIP and GLP-1 mechanism may offer stronger appetite suppression in clinical trials, but individual outcomes at 30 days depend on your dose, genetics, and lifestyle. Both require time to reach therapeutic effectiveness. Your GP can advise which suits your situation.
What side effects should I expect in week 1–4 on either injection?
Nausea, diarrhoea, and constipation are common as your body adjusts to slower gastric emptying. These typically settle within 2–4 weeks. Injection site reactions (mild redness or bruising) are normal. Contact your prescriber if side effects are severe or persistent.
Can I switch from Mounjaro to Wegovy after 30 days?
Switching between medications is possible but should only be done under prescriber guidance. There's no official conversion chart, though approximate equivalences exist based on clinical practice. Your GP can advise if a switch is appropriate for your health and goals.
How do I know if Mounjaro or Wegovy is right for me?
Eligibility depends on your BMI, health conditions, and individual circumstances. Both require a UK prescription following clinical assessment. Use our free BMI calculator to check your starting point, then discuss prescription options with your GP or a licensed prescriber who can assess your suitability for either medication.
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.