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If you are considering weight loss medication, one of the first things a prescriber will look at is your BMI. But what is my BMI exactly, how is it calculated, and why does it carry so much weight in clinical decisions? Understanding the answer can help you know where you stand before you begin. Get in touch with our prescribing team if you have questions, or complete our online consultation to find out whether you are eligible for weight-loss medication.
Quick Answer
BMI (Body Mass Index) is a number calculated from your height and weight. It is used as a screening tool to categorise weight status and assess health risk. For weight loss medication in the UK, most treatments require a minimum BMI of 30, with some requiring 35 or above alongside a weight-related health condition. A lower BMI threshold may apply for certain ethnic groups. You can calculate your BMI by dividing your weight in kilograms by your height in metres squared (kg/m²).
What Is BMI and How Is It Calculated?
BMI stands for Body Mass Index. It is a simple numerical value derived from two measurements: your weight and your height. The formula is:
BMI = weight (kg) divided by height (m²)
For example, if you weigh 90 kg and are 1.70 m tall, your BMI is 90 divided by (1.70 x 1.70), which equals 31.1.
BMI does not measure body fat directly, but it provides a useful and widely accepted indicator of whether a person’s weight falls within a healthy range for their height. It is used by clinicians, NHS services, and private prescribers across the UK as a starting point for assessing weight-related health risk.
What Do the BMI Categories Mean?
In the UK, BMI is categorised as follows for adults:
It is worth noting that BMI thresholds are adjusted downward for people of South Asian, Chinese, or Black African and Caribbean heritage. For these groups, a BMI of 27.5 or above may be used to define overweight with increased health risk, and eligibility for treatment may begin at a lower threshold.
Why Does BMI Matter for Weight Loss Medication?
BMI is used by prescribers and regulatory bodies to determine whether a patient is likely to benefit from medical intervention for weight management. It is not the only factor considered, but it is typically the primary eligibility criterion for weight loss medications.
The reasoning is straightforward: weight loss medications carry potential side effects and are intended for people whose weight poses a meaningful health risk. Using BMI as a threshold helps ensure that treatment is targeted at those most likely to benefit clinically, rather than those who may want to lose a modest amount of weight for cosmetic reasons.
In addition to BMI, prescribers will consider:
- Whether you have any weight-related health conditions such as type 2 diabetes, high blood pressure, high cholesterol, or obstructive sleep apnoea
- Your overall medical history and any contraindications to specific medications
- Previous attempts at weight loss through diet and lifestyle changes
- Your current medications and whether there are any interactions to consider
BMI Eligibility Compared Across Weight Loss Medications
Different medications have different eligibility thresholds. Here is how the main options available privately in the UK compare:
Please note: Eligibility is determined on an individual clinical basis during your consultation. The above reflects general prescribing guidance and does not guarantee eligibility. A qualified prescriber will assess your full medical history before making any prescribing decision.
Does a Higher BMI Mean Better Results?
Not necessarily. People with a higher starting BMI often see larger absolute weight loss in kilograms, simply because there is more weight to lose. However, the percentage of body weight lost tends to be broadly similar across different starting weights when the same medication and dose are used.
What matters more than your starting BMI is your commitment to combining medication with consistent dietary and lifestyle changes. Medication reduces biological hunger signals and makes it easier to eat less, but it works best when supported by:
- A diet rich in protein and whole foods
- Reduced intake of ultra-processed, calorie-dense foods
- Regular physical activity, including resistance training where possible
- Good sleep habits and stress management
What If My BMI Is Just Below the Threshold?
If your BMI falls slightly below the standard threshold for a particular medication, it does not automatically mean you are ineligible. Your prescriber may still consider treatment appropriate depending on your individual clinical circumstances, particularly if you have a weight-related health condition or fall into an ethnic group where lower BMI thresholds apply.
The most reliable way to find out whether you qualify is to complete a clinical consultation. Our prescribing team will review your full health profile and advise on which, if any, treatment options are suitable for you.
Frequently Asked Questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in metres squared. For example, if you weigh 85 kg and are 1.68 m tall: 85 divided by (1.68 x 1.68) = 85 divided by 2.82 = a BMI of approximately 30.1. Many online BMI calculators will do this automatically if you enter your height and weight.
What BMI do I need for Mounjaro?
For private prescribing, Mounjaro is generally available to adults with a BMI of 35 or above, or a BMI of 30 or above alongside at least one weight-related health condition such as type 2 diabetes, high blood pressure, or high cholesterol. Lower thresholds may apply for people of South Asian, Chinese, or Black African and Caribbean heritage.
Can I get weight loss medication with a BMI under 30?
In most cases, the standard minimum BMI for prescription weight loss medication in the UK is 30. However, for certain ethnic groups where health risks occur at lower body weights, a BMI of 27.5 or above may be sufficient. Your prescriber will advise based on your individual circumstances during your consultation.
Is BMI the only factor that determines eligibility?
No. BMI is the primary criterion, but prescribers also consider your medical history, any existing health conditions, current medications, and previous attempts at weight management. Some medications require the presence of a weight-related comorbidity even if your BMI meets the threshold.
Does BMI affect how much weight I will lose on medication?
Your starting BMI influences the absolute amount of weight you can lose, but the percentage of body weight lost tends to be similar across different starting weights. Clinical trials for Mounjaro showed average weight loss of around 20 to 21% of total body weight at the highest dose across the trial population, provided patients followed the recommended treatment and lifestyle guidance.
Will I still be eligible for treatment if my BMI changes during the consultation process?
Eligibility is assessed at the time of your consultation based on the information you provide. If your BMI has recently changed, provide your current accurate measurements. Your prescriber will assess your suitability based on your health at the time of prescribing, not historical data.
Ready to Find Out If You Are Eligible?
Understanding your BMI is the first step, but it is only one part of the picture. Our prescribing team reviews each patient individually and takes the time to recommend the most appropriate treatment based on your full health profile. Whether you are a good candidate for Mounjaro, Wegovy, or another option, the process begins with a simple online consultation. Reach out to our team with any questions, or complete our online consultation to take the next step towards clinically supported weight-loss treatment.
Work out how much weight you could lose on Mounjaro or Wegovy here:
This article was written by Pharmacy Mentor and clinically reviewed by Mohammed Ismail Lakhi, MPharm, MRPharm, Superintendent Pharmacist at The Care Pharmacy. Mohammed is registered with the General Pharmaceutical Council (GPhC registration number 2072815) and leads clinical governance across The Care Pharmacy’s weight management services.Last reviewed: May 2026
Disclaimer: This article is for general information only and is not a substitute for individual medical advice. Always consult a qualified prescriber before starting any prescription weight loss treatment.
Medically reviewed by
Mohammed Lakhi
Superintendent Pharmacist
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