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
Can't Put Weight On — Why and What to Do
Struggling to put weight on can be just as challenging as losing it, and it's often overlooked in health conversations. Whether you're naturally slim, dealing with a medical condition, or taking medication that affects your appetite, being underweight comes with its own health risks. This guide explains why weight gain might be difficult, when to speak to your GP, and practical strategies to support healthy weight management. Your weight is an important part of your overall health, and there's help available.
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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.
Common Reasons Why You Can't Put Weight On
If you're struggling to gain weight, several factors could be involved. High metabolism means your body burns calories faster than average, making it harder to achieve a calorie surplus needed for weight gain. Underlying medical conditions like hyperthyroidism, coeliac disease, inflammatory bowel disease, or digestive disorders can prevent proper nutrient absorption or increase calorie burn. Mental health conditions such as depression and anxiety often reduce appetite or disrupt eating patterns. Medications can suppress appetite—some diabetes medications and certain antidepressants have this effect. Stress and lifestyle factors including irregular sleep, excessive exercise, or insufficient calorie intake also play a role. Eating disorders require specialist support and should never be self-managed. If weight gain feels impossible despite consistent effort, your GP can investigate underlying causes and refer you to specialist services if needed.
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.
Practical Steps for Healthy Weight Gain
Gaining weight healthily requires a calorie surplus—you need to consume more energy than you burn. Focus on nutrient-dense foods rather than empty calories: nuts, nut butters, avocados, olive oil, whole grains, lean protein, and dairy products are calorie-rich and nutritious. Eat regular, frequent meals including three meals plus two to three snacks daily. Increase portion sizes gradually rather than forcing yourself to eat uncomfortably large amounts at once. Drink calorie-containing beverages like smoothies, whole milk, or oat milk-based drinks between meals. Track your intake using our free weight converter and progress tracker to monitor your weight consistently. Your GP can refer you to a registered dietitian who can create a personalised weight gain plan tailored to your circumstances and any underlying health conditions.
- 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.
When should I see my GP about not being able to put weight on?
Contact your GP if you've lost weight unintentionally, can't gain weight despite eating well, have persistent digestive issues, or feel unusually tired. Weight loss or inability to gain weight can signal underlying health conditions requiring investigation. Your GP can run tests, identify causes, and refer you to appropriate specialist services or a dietitian for support.
What counts as underweight according to NHS guidelines?
The NHS uses BMI (body mass index) to assess weight. A BMI below 18.5 is classified as underweight. Use our free BMI calculator to check your personal BMI instantly. However, BMI alone doesn't tell the whole story—your GP will consider your individual health, muscle mass, and medical history when assessing whether your weight is healthy.
Is it unhealthy to have a high metabolism?
A naturally high metabolism isn't unhealthy—it's a normal variation. However, if you're underweight and experiencing health problems, or if your metabolism suddenly changes, see your GP. The challenge isn't the metabolism itself but ensuring you consume enough calories and nutrients to maintain good health and energy levels.
Can I use a weight converter to track my weight gain progress?
Yes. Our free weight converter helps you switch between kg, stone, and lbs easily. Use it alongside our progress tracker to log your weight consistently and see patterns over time. This helps you and your GP monitor whether your weight gain efforts are working and adjust your plan if needed.
Do certain medications cause weight loss?
Yes. Some medications—including certain diabetes drugs, some antidepressants, and other treatments—can suppress appetite or increase metabolism, making weight gain harder. If you're taking medication and struggling to gain weight, always discuss this with your GP. They may adjust your prescription or refer you to specialist support.
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.