BMI and Weight Loss Treatment: What Qualifies You for Medical Help in the UK?

What is BMI and how is it calculated?

Body mass index (BMI) is calculated by dividing a person's weight in kilograms by the square of their height in metres. It is used as a screening tool to classify weight status in adults. The standard UK classifications are:

• Under 18.5 — Underweight
• 18.5 to 24.9 — Healthy weight
• 25 to 29.9 — Overweight
• 30 to 34.9 — Obesity class I
• 35 to 39.9 — Obesity class II
• 40 or above — Obesity class III (severe obesity)

For people from South Asian, Chinese, and other East Asian backgrounds, the NHS uses lower thresholds — overweight from BMI 23, and obesity from BMI 27.5 — because research shows that metabolic complications occur at lower BMIs in these populations.

What BMI qualifies you for weight loss medication in the UK?

The MHRA-licensed indications for the two main injectable weight loss medicines in the UK are:

• Semaglutide (Wegovy) — licensed for adults with a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related health condition such as type 2 diabetes, high blood pressure, high cholesterol, obstructive sleep apnoea, or cardiovascular disease.
• Tirzepatide (Mounjaro) — licensed for adults with a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related health condition.

BMI is not the only factor

While BMI thresholds determine licensing eligibility, a responsible clinician will consider your full health picture before prescribing. Relevant factors include:

• The presence and severity of weight-related health conditions
• Previous attempts at weight loss and their outcomes
• Current medications and potential interactions
• Medical history, including conditions that may contraindicate treatment
• Mental health history, including any history of eating disorders

What if your BMI is below the threshold?

If your BMI is below 27, prescription weight loss medicines are not currently licensed for you in the UK. However, a clinician can still provide evidence-based support through dietary advice, behavioural strategies, and referral to appropriate services. If you are close to the threshold, an accurate measurement at consultation will confirm your eligibility.

Limitations of BMI

BMI is a useful population-level screening tool but has well-recognised limitations as an individual measure. It does not distinguish between fat mass and muscle mass, does not capture fat distribution (visceral versus subcutaneous fat), and does not reflect metabolic health directly. A person with a BMI of 28 and type 2 diabetes, high blood pressure, and sleep apnoea may have a greater clinical need for treatment than someone with a BMI of 35 and no health complications. Good clinical practice involves assessing the whole person, not just the number.

Book a consultation with Weight Loss Wales to find out whether you are eligible for medical weight loss treatment.