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
Diet Injections Side Effects and How to Get Them in the UK
Weight loss injections—commonly called slimming injections or fat jabs in the UK—are prescription-only medications that work by managing appetite and supporting sustainable weight loss. If you're considering these injections, understanding potential side effects and how to access them is essential. In the UK, weight loss injections are available through two routes: the NHS specialist services or private online prescribing. This guide explains what to expect from side effects, who qualifies, and how to begin your journey.
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.
Who Qualifies for Weight Loss Injections in the UK?
Eligibility for prescription slimming injections depends on your BMI and health conditions. The standard criteria are:
- BMI of 30 kg/m² or above (obese category), OR
- BMI of 27–29.9 kg/m² with weight-related health conditions (overweight but with comorbidities)
Weight-related health conditions that may qualify include type 2 diabetes, prediabetes, high blood pressure, abnormal cholesterol levels, obstructive sleep apnoea, and history of cardiovascular disease. For adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds, the NHS recommends lower thresholds—eligibility may begin at BMI 27.5 or above, as these populations face higher metabolic risk at lower BMI levels. You cannot access prescription weight loss injections if you're under 18, pregnant, breastfeeding, or have a severe allergy to the medication. Use our free BMI calculator above to check where you fall, then consult your GP or a prescriber to discuss eligibility based on your individual circumstances.
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.
Two Routes to Access Diet Injections: NHS vs Private
NHS Route: Referral starts with your GP, who may refer you to a tier 3 specialist weight management service. Availability varies significantly by region and local ICB (Integrated Care Board) funding. Waiting lists can range from months to years in many areas. Once assessed, prescription and ongoing monitoring are provided free.
Private Route: Faster access through GPhC-registered online pharmacies. You complete an online consultation with a UK-licensed prescriber who assesses your suitability. If approved, a prescription is issued and dispensed by post. This route bypasses NHS waiting lists but involves costs (typically £129–£320 monthly depending on dose). Both routes require clinical assessment—injections are never sold without a valid prescription.
- 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 are the most common side effects of weight loss injections?
The most common side effects include nausea, diarrhoea, constipation, and stomach discomfort—especially during dose escalation. These typically ease as your body adjusts. Less common but important side effects include vomiting, indigestion, headache, fatigue, dizziness, injection site reactions, and hair loss. Most side effects are mild to moderate, though some people experience more severe symptoms. Always report persistent or concerning symptoms to your prescriber.
How long do diet injection side effects last?
Most side effects peak during the first few weeks of starting injections or when increasing doses. For most people, nausea and GI symptoms improve within 2–4 weeks as the body adapts. However, some people experience side effects throughout treatment. The slower you escalate doses, the more time your body has to adjust. Your prescriber can slow dose increases if side effects are bothersome—there's no need to rush through escalation stages.
Can I get weight loss injections through my NHS GP?
Not directly from your GP in most cases. Weight loss injections are typically prescribed through specialist tier 3 weight management services, not general practice. Your GP can refer you to these services, but you'll need assessment by the specialist team. NHS access depends on your local ICB funding and regional availability. Some areas have lengthy waiting lists. Private online prescribing offers faster access if NHS waiting times are a concern.
What happens during a private online consultation for diet injections?
You'll complete a detailed health questionnaire covering your weight, BMI, medical history, medications, allergies, and lifestyle. A UK-licensed prescriber reviews this and may request additional information. If you're suitable, a prescription is issued; if not, the prescriber explains why. The consultation typically takes 24–48 hours. You'll then receive your prescription from a GPhC-registered pharmacy and your first dose by post. Ongoing support is usually available via follow-up consultations or messaging.
Are weight loss injections safe?
Prescription weight loss injections have undergone rigorous clinical trials and are approved by the MHRA for use in the UK. However, like all medicines, they carry potential side effects and are not suitable for everyone. They require a prescription because a healthcare professional must assess your individual suitability. Safety is enhanced when injections are combined with a reduced-calorie diet and increased physical activity. Always use them under medical supervision and report any serious side effects—such as severe abdominal pain or allergic reactions—immediately to your GP.
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.