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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Why Do You Lose Weight When You Have the Flu?
It's common to lose weight during or after the flu, but understanding why helps you recover safely. Weight loss during illness happens due to reduced appetite, dehydration, and your body's increased energy demands fighting infection. This article explains the mechanisms behind flu-related weight loss, how much is normal, and when you should contact your GP. Most weight lost during acute illness returns once you recover and resume normal eating patterns.
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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.
How Illness Causes Weight Loss
When you have the flu, several factors contribute to weight loss:
- Reduced appetite: The flu virus triggers inflammatory responses that suppress hunger signals, making food unappealing even though your body needs nutrition.
- Nausea and vomiting: These symptoms prevent normal food intake and cause fluid loss, contributing to rapid weight reduction.
- Increased metabolism: Fighting infection increases your body's energy expenditure as your immune system works to eliminate the virus.
- Dehydration: Fever, sweating, and reduced fluid intake lead to water weight loss, which can be significant over a few days.
Most weight loss during acute flu is not fat loss — it's primarily water, glycogen depletion, and temporary muscle protein breakdown. This is why weight typically returns quickly once you recover and resume normal eating and hydration.
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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.
When Flu Weight Loss Is Normal vs. Concerning
Losing 2–3 kg (4–7 lbs) during a week-long flu illness is generally within normal range, especially if you've had minimal food intake or significant fever. However, contact your GP if you experience:
- Weight loss exceeding 5% of your body weight during acute illness
- Inability to keep down fluids for more than 24 hours (dehydration risk)
- Continued weight loss beyond two weeks post-flu
- Signs of serious complications: severe chest pain, confusion, or persistent high fever
For managing weight after illness, our free BMI calculator can help you establish your baseline once you've recovered. Focus on gradual return to normal nutrition rather than rapid weight regain.
- 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 weight loss during flu permanent?
No. Most weight lost during flu returns within 1–2 weeks of recovery as you rehydrate and resume normal eating. This rapid regain is primarily water and glycogen, not fat. True fat loss requires sustained calorie deficit, which doesn't occur during acute illness when you're fighting infection.
How much weight is normal to lose with flu?
Losing 1–3 kg (2–7 lbs) during a typical 5–7 day flu is common. Loss rate depends on fever severity, food intake, and initial body weight. If you've eaten very little and had significant fever, higher loss is normal. Beyond 5% of your body weight warrants GP contact.
Should I try to eat if I have no appetite during flu?
Yes, prioritising hydration and nutrition is important for recovery. Eat small, frequent meals of easily digestible foods — soup, toast, porridge, fruit. Fluids are critical. Your GP or NHS 111 can provide specific guidance if you can't keep food or fluids down.
Does flu weight loss count toward my weight management goals?
No. Illness-related weight loss is temporary and reflects dehydration and reduced intake, not sustainable fat loss. For weight management with proper nutrition support, discuss evidence-based approaches with your GP, including prescription weight loss medication if eligible.
When should my weight return after recovering from flu?
Most people regain lost weight within 1–2 weeks of returning to normal eating and activity. If you don't regain weight after two weeks, or if you continue losing weight post-recovery, contact your GP to rule out complications or other health concerns.
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.