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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How to take your second dose of Mounjaro
Taking your second dose of Mounjaro (tirzepatide) follows the same safe injection technique as your first dose. The second injection occurs one week after your first, using a new KwikPen from your prescription. This guide explains the correct administration method, storage requirements, and what to expect. Always follow your prescriber's specific instructions, as individual treatment plans may vary. If you're unsure about any step, contact your pharmacy or GP before injecting.
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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.
Second dose administration: step-by-step technique
Your second Mounjaro dose uses a fresh KwikPen device taken from the fridge approximately 15 minutes before injection, allowing it to reach room temperature. Check the liquid inside is clear and colourless — do not use if cloudy or discoloured.
- Choose a new injection site (rotate between stomach, thigh, or upper arm)
- Pinch the skin and insert the needle at a 90-degree angle
- Press the injection button until you hear a click
- Keep the needle in for 3 seconds after clicking
- Withdraw the needle and release the pinched skin
- Dispose of the used pen in a yellow sharps bin
Common second-dose experiences include mild injection site reactions (redness, bruising) and possible nausea — these often reduce as your body adjusts. Store your remaining three doses in the fridge at 2–8°C until needed.
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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.
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Mounjaro vs Wegovy: second dose timing and dosing schedules
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly GLP-1 or GIP/GLP-1 medications, so second doses follow the same 7-day schedule. The key difference is escalation: Mounjaro's second dose remains 2.5 mg (titration phase), whereas Wegovy's second dose increases to 0.5 mg. UK clinical trials showed different mechanisms — Mounjaro's dual GIP/GLP-1 action versus Wegovy's single GLP-1 pathway — but both require clinical assessment and individual prescriber guidance. Your prescriber decides which medication suits your health profile. Neither offers studied results; individual responses vary 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.
What's the correct angle to inject Mounjaro the second time?
Insert the needle at a 90-degree angle (perpendicular to your skin). Pinch the skin first, then push the needle straight in. This ensures the medication deposits into the subcutaneous tissue correctly. If using your upper arm, ask for assistance to maintain the right angle.
Can I use the same injection site for my second Mounjaro dose?
No — always rotate injection sites. If your first dose was in the stomach, choose your thigh or upper arm for the second dose. Rotating prevents lipohypertrophy (thickened skin) and ensures consistent medication absorption. Mark your sites to track rotation.
How do I know if my second Mounjaro dose is working?
Effects build gradually over weeks. Some users notice reduced appetite within days; others take 2–3 doses to feel changes. Use our GLP-1 weight loss calculator to track projected outcomes, and keep a food diary to monitor appetite patterns. Your prescriber will assess effectiveness at your next review.
Is the second dose of Mounjaro more likely to cause side effects than the first?
Not necessarily — side effects vary individually. Some people experience more nausea on the second dose; others see improvements as their body adjusts. Gastrointestinal side effects (nausea, diarrhoea) are most common and often reduce by week 3–4. Report any concerns to your prescriber.
What should I do if I miss my second Mounjaro dose?
If you miss your second dose by up to 4 days, take it as soon as you remember and resume your normal weekly schedule. If more than 4 days have passed, skip that dose and inject on your regular day. Never give yourself a double dose. Contact your pharmacy if you're uncertain about timing.
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