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 Happens in Your First Month on Mounjaro?
Many people starting Mounjaro wonder what to expect in their first month. The reality is that month one is primarily about dose adjustment and managing side effects rather than dramatic weight loss. Mounjaro (tirzepatide) is prescribed as a once-weekly injection that starts at a low dose—2.5 mg—and increases gradually over several weeks. During this initial period, your body adapts to the medication, and you'll typically experience changes in appetite and how you feel after eating. Individual experiences vary significantly, and your GP or prescriber will monitor your progress closely.
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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.
First Month Timeline: What to Expect
Your first month on Mounjaro follows a structured approach. Week 1–4: You'll inject 2.5 mg once weekly—this is the starting dose. Many people notice reduced appetite and changes in food preferences within the first 2 weeks, though weight loss is usually minimal at this stage. Weeks 2–4: Gastrointestinal side effects are most common now—nausea, diarrhoea, or constipation. These typically ease as your body adjusts. Most people don't experience significant weight loss in month one; the goal is tolerance and adaptation. At week 4, your prescriber will assess how you're tolerating the medication before moving to the next dose (5 mg at week 5).
- Starting dose: 2.5 mg once weekly (weeks 1–4)
- Expected appetite changes: Within 1–2 weeks for many people
- Expected weight loss month 1: Usually modest (0–2 kg on average, though some see none)
- Most common side effect: Nausea (affects around 2 in 10 people at starting dose)
- Minimum time before dose increase: 4 weeks at each dose level
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.
Realistic First-Month Expectations vs. Hype
It's important to manage expectations. Clinical trials show that significant weight loss happens over several months, not weeks. In the SURMOUNT-1 trial, participants at the maximum dose (15 mg) lost an average of 20.9% of their body weight—but this was measured over 72 weeks, not 4 weeks. During month one, most people experience appetite suppression and lifestyle changes rather than gradual weight management. Some people see 1–2 kg loss; others see none. Weight loss accelerates as doses increase over the following months. Use our free GLP-1 weight loss calculator to project realistic outcomes, and our weight converter to track changes between kg, stone, and lbs. Your prescriber will provide personalised guidance based on your individual response.
- 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 my first month on Mounjaro?
Weight loss in month one is usually modest or absent. Most weight loss occurs after several months at higher doses. Month one is primarily about dose adjustment and managing side effects. Individual results vary—some people lose 1–2 kg, others see no change yet. Clinical trials show meaningful weight loss typically emerges at weeks 8–12 as doses increase.
What's the difference between Mounjaro and Wegovy in the first month?
Both are once-weekly injections, but Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is GLP-1 only. Mounjaro's starting dose is 2.5 mg; Wegovy starts at 0.25 mg. Side effects are similar in month one. Mounjaro may show slightly faster early appetite suppression in some people, but individual variation is significant.
Can I get Mounjaro through the NHS in month one, or do I need to go private?
NHS access is through specialist tier 3 weight management services—you'll need a GP referral. Availability and waiting lists vary widely by region. Private prescribing is faster; you can start within days following an online clinical assessment with a UK-licensed prescriber. Your GP can advise on local NHS options.
What if I feel very sick in my first month—should I stop Mounjaro?
Nausea is very common at the starting dose and usually improves within 2–4 weeks as your body adapts. Small dietary changes (eating smaller, less fatty meals) often help. Contact your prescriber if nausea is severe or you can't manage it. Never stop without medical guidance. Your prescriber may adjust your dose or provide additional support.
Is it normal to have no appetite or feel full very quickly?
Yes. Reduced appetite and early fullness are how Mounjaro works—it signals fullness to your brain. You'll eat less naturally. This is not dangerous if you're still meeting nutritional needs (eating nutrient-dense meals even in smaller portions). If you're struggling to eat enough, discuss this with your prescriber or a dietitian.
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.