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 Is an IM Injection?
An IM injection is an intramuscular injection — a medicine delivered directly into muscle tissue beneath the skin. In the UK, IM injections are used for various treatments including vaccines, hormones, and weight management medications like Mounjaro and Wegovy. These injections allow medicines to be absorbed gradually into the bloodstream, making them effective for weekly or monthly dosing. Understanding how IM injections work helps you feel confident if your GP or prescriber recommends this type of treatment as part of your NHS health advice or private health journey.
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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 IM Injections Work in Weight Management
IM injections deliver medication into the muscle layer (intramuscular), where it's absorbed slowly into the bloodstream. This method is ideal for medicines that need to work over days or weeks — such as GLP-1 receptor agonists used for weight management.
Weight management medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) are given as once-weekly IM injections. Both medications help regulate appetite and support weight loss alongside a reduced-calorie diet and increased physical activity. In clinical trials, these injections helped participants achieve an average weight loss of 15–21% of body weight — though individual results vary significantly.
The IM route allows for convenient once-weekly dosing instead of daily tablets. Your prescriber will assess your suitability following a clinical assessment, and you'll receive clear injection training before starting treatment.
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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.
IM Injection Sites and Safety
Common IM injection sites include the upper arm, upper thigh, and stomach (abdomen). For weight management injections, you rotate between injection sites to minimise bruising and irritation. Always follow your prescriber's guidance on which site to use each week.
IM injections are generally safe when given correctly. Possible side effects include mild bruising, redness, or soreness at the injection site — these usually resolve within a few days. More serious side effects are uncommon but can occur; always report any concerns to your GP or healthcare provider. If you're unsure about injection technique, ask your prescriber or pharmacist for a demonstration. Using our free BMI calculator can help you track your baseline weight before starting any weight management treatment.
- 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.
Is an IM injection different from a subcutaneous injection?
Yes. IM injections go into muscle tissue below the skin, while subcutaneous injections go into the layer of fat and tissue just under the skin. IM injections are deeper and are typically used for medicines that need slower absorption over days or weeks, like weight management medications.
Do IM injections hurt?
Most IM injections cause minimal discomfort — often described as a quick pinch. Needle length and injection technique matter; your prescriber or pharmacist will demonstrate proper technique. Ice on the area beforehand can reduce discomfort.
Can I give myself an IM injection for weight management?
Yes — weight management injections like Wegovy and Mounjaro come in pre-filled pens designed for self-injection. Your prescriber will train you on how to inject safely at home. If you're unsure, ask for a demonstration or speak to your pharmacist.
How often do IM injections for weight loss need to be given?
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) are given once weekly. You inject yourself on the same day each week. Each pen contains 4 weekly doses before you need a new prescription.
What should I do if I miss an IM injection dose for weight management?
For Wegovy or Mounjaro, if you're within 4 days of your usual injection day, inject as soon as you remember and continue your normal schedule. If more than 4 days have passed, skip that dose and inject on your normal day. Never take a double dose.
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.