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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What to Expect in Your First Month on Mounjaro
When starting Mounjaro (tirzepatide), many people wonder what changes they'll notice in the first month. Realistically, the first four weeks are typically a dose adjustment phase rather than a major weight loss phase. You'll begin on 2.5 mg — a starting dose designed to let your body adapt to the medication whilst minimising side effects. Most weight loss doesn't become noticeable until you've escalated to higher doses and your body has adjusted. This guide explains what typically happens, how the dose titration works, and what results to reasonably expect in month one.
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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.
How the First Month of Mounjaro Works
Mounjaro starts you on a very low dose — 2.5 mg once weekly — regardless of your target dose. This is called dose titration, and it's designed to let your body adapt gradually.
- Week 1–4: You take 2.5 mg. This is your starter dose. At this dose, appetite suppression is mild and weight loss is usually minimal — often just 1–2 kg or even less if there's water weight change.
- Side effects during week 1–4: You may notice mild nausea, changes in appetite, or slight digestive changes — but these often settle within a few days as your body adjusts.
- The UK prescription: Each KwikPen contains four doses (four weeks' supply). After four weeks, you'll move to your second pen with the next dose (5 mg).
- Weight loss expectations month one: Realistic average is 0–2 kg in the first month. Some people see no change; others lose a bit more. This is because the appetite-suppressing effect at 2.5 mg is gentle, and you need higher doses for more noticeable results.
It's important to remember that Mounjaro works alongside a reduced-calorie diet and increased physical activity — the medication doesn't work alone. After month one, as you move to 5 mg, appetite suppression typically increases significantly.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Why Month One Isn't the 'Big Loss' Month
Many people expect dramatic before-and-after results in the first month, but this isn't how Mounjaro works. The clinical trials show that significant weight loss happens over 12–20 weeks as you reach higher maintenance doses (10 mg, 15 mg). Month one is about safety and adjustment, not maximum effect.
The dual mechanism of Mounjaro (it works on both GIP and GLP-1 hormone pathways) does create real appetite suppression — but the 2.5 mg starting dose is deliberately conservative. You can use our free GLP-1 weight loss calculator to estimate potential weight loss at different doses and weeks. As you progress through weeks 5–8 on 5 mg, most people notice appetite changes become more pronounced, and weight loss accelerates.
If you're concerned about progress after month one, your prescriber can advise on whether your dose needs reviewing or whether lifestyle factors need adjusting. Weight loss is individual — genetics, metabolism, diet adherence, and activity level all play a role.
- 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.
How much weight should I lose in the first month on Mounjaro?
Realistic expectations for month one are 0–2 kg loss. The 2.5 mg starting dose is mild, and significant weight loss typically begins after you escalate to 5 mg in weeks 5–8. Clinical trials showed major weight loss over 12–20 weeks at higher doses, not in the first month.
Will I feel different in the first week of Mounjaro?
Some people notice mild nausea, slight appetite reduction, or mild digestive changes in week one. These often settle within a few days. Others feel minimal difference. Individual responses vary, and these early changes don't predict your eventual weight loss.
Do I need to diet during the first month of Mounjaro?
Yes. Mounjaro is most effective when combined with a reduced-calorie diet and increased physical activity. The medication supports appetite control, but you still need to create a calorie deficit. Month one is a good time to establish healthy eating habits alongside the medication.
How does the Mounjaro dose schedule work in month one?
You take one 2.5 mg injection per week for four weeks (weeks 1–4). Each KwikPen has four doses. After four weeks, you move to your second pen and escalate to 5 mg weekly. The escalation continues every four weeks until you reach your target dose.
Is Mounjaro NHS or private prescription in the UK?
Mounjaro is available through NHS specialist weight management services (tier 3) or private online prescribing following a clinical assessment. NHS access varies by region and may involve waiting lists. Private prescriptions are available without NHS referral through GPhC-registered pharmacies.
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.