BMI (body mass index) is the primary measure used to determine eligibility for weight loss medications in the UK. But the threshold varies depending on whether you're accessing treatment through the NHS or privately, which medication you're seeking, and your ethnic background. Here's a clear breakdown.
BMI thresholds at a glance
NHS — Mounjaro (Cohort 1)
Plus 4 of 5 specific comorbidities. The strictest threshold currently. Cohort 2 (BMI ≥35) expected June 2026.
NHS — Wegovy (Tier 3)
With at least one weight-related comorbidity. Or BMI 30-34.9 with comorbidities in some areas.
Private — most providers
Or BMI ≥27 with a weight-related health condition. Some providers may consider from BMI 25 off-label.
OTC — Alli (orlistat 60mg)
Available from pharmacies without a prescription after a consultation with the pharmacist.
Ethnicity-adjusted BMI thresholds
This is an important detail that many guides overlook. Clinical evidence shows that people from certain ethnic backgrounds experience obesity-related health risks at lower BMI levels than people from White ethnic backgrounds. To account for this, NICE and NHS England apply reduced BMI thresholds for the following groups:1
- South Asian
- Chinese
- Other Asian
- Middle Eastern
- Black African
- African-Caribbean
The adjustment is typically a reduction of 2.5 BMI points. So where the standard threshold is BMI 35, the adjusted threshold becomes 32.5. Where it's BMI 40, it becomes 37.5. This applies to both NHS and many private eligibility criteria.
What is BMI and how do I calculate it?
BMI is calculated by dividing your weight in kilograms by your height in metres squared:
BMI = weight (kg) ÷ height (m)²
For example, if you weigh 95kg and are 1.72m tall: 95 ÷ (1.72 × 1.72) = 95 ÷ 2.96 = 32.1
Standard BMI categories:
- Below 18.5 — underweight
- 18.5 to 24.9 — healthy weight
- 25 to 29.9 — overweight
- 30 to 34.9 — obese (Class I)
- 35 to 39.9 — obese (Class II)
- 40 and above — obese (Class III)
Our eligibility checker includes a built-in BMI calculator that supports both metric and imperial measurements and applies the ethnicity adjustment automatically.
BMI isn't perfect
BMI is a useful screening tool but has well-known limitations. It doesn't distinguish between muscle and fat (so very muscular people may have a "high" BMI without being overweight), it doesn't account for where fat is distributed (central/abdominal fat is more metabolically harmful), and it's a population-level measure being applied to individuals.
Despite these limitations, BMI remains the standard measure used in NICE guidelines, NHS commissioning criteria, and most private prescribing protocols. Your prescriber may also consider waist circumference, overall health profile, and other factors alongside BMI when assessing your eligibility.
What if my BMI is just below the threshold?
If your BMI is close to but below the threshold for the route you're interested in, a few things are worth considering:
- Check whether the ethnicity-adjusted threshold applies to you (this could bring you into range)
- Some private providers have lower thresholds (BMI 27+ with comorbidities, or even BMI 25+ on an off-label basis with full clinical justification)
- Remember that BMI fluctuates — a measurement taken at different times of day, with different clothing, or on different scales may vary by a point or more
- Discuss your situation with your GP or prescriber — they can consider the full clinical picture, not just a single number