Background
What is BMI and why does it matter?
A simple measure of body size
Body Mass Index (BMI) is a number worked out from your height and your weight. It gives a quick estimate of whether your weight sits within a healthy range for someone of your size. BMI was first developed in the nineteenth century and is now used by doctors, nurses, and pharmacists all over the world as a first-line screening tool.
Why BMI is important
BMI does not measure body fat directly, but it gives a reliable indication, across a population, of how much body fat a person is likely to be carrying. As BMI rises above the healthy range, so does the risk of a number of long-term health conditions, including type 2 diabetes, high blood pressure, heart disease, stroke, certain cancers, and joint problems such as arthritis. For this reason, clinicians use BMI as the first step in deciding whether weight-management treatment is likely to be helpful — and whether a prescription medicine such as Mounjaro (tirzepatide) or Wegovy (semaglutide) may be appropriate.
How to calculate your BMI
The formula is:
A word on the limits of BMI
BMI is a useful starting point but it is not perfect. It does not tell the difference between muscle and fat, so a very muscular person may have a high BMI without carrying excess body fat. It also does not show where fat is stored on the body, and the standard thresholds may need to be adjusted for some ethnic backgrounds. For this reason, your BMI should always be looked at alongside the rest of your medical picture.
BMI = weight (kg) ÷ height (m)²
For example, an adult who weighs 80 kg and is 1.70 m tall would have a BMI of:
80 ÷ (1.70 × 1.70) = 27.7 kg/m²
If you prefer imperial units, you can multiply your weight in pounds by 703 and then divide by the square of your height in inches.
BMI categories
< 18.5
Underweight
18.5 – 24.9
Healthy Weight
25 – 29.9
Overweight
30 +
Obesity
Three Eligibility Pathways
Which patient group do you fall into?
Our prescribing framework is structured around three distinct clinical pathways, each with its own minimum BMI threshold. This reflects evidence-based guidance from NICE and adapts it for safe, continuous private weight-management care.
Pathway 1 — New Starter
Beginning prescription weight-management treatment for the first time
≥ 30BMI KG/M²
If you are starting treatment with a GLP-1 receptor agonist such as tirzepatide (Mounjaro) or semaglutide (Wegovy) for the very first time, you must meet one of the eligibility criteria below.
- BMI of 30 kg/m² or above — the standard clinical definition of obesity, and the principal entry point for treatment under the licensed indication.
- BMI of 27 kg/m² or above in the presence of at least one weight-related comorbidity (see the checklist below).
- BMI of 24.5 kg/m² or above if you are of South Asian, Chinese, Black African, African-Caribbean, or Middle Eastern family background.
Pathway 2 — Switching Provider
Already established on treatment with another UK provider
≥ 21BMI KG/M²
If you are already receiving a licensed GLP-1 weight-management medicine through another UK-regulated prescriber and wish to transfer your care to us, we may continue your treatment at a current BMI as low as 21 kg/m².
- Documented prescription history from a CQC-registered clinic or GPhC-registered pharmacy in the UK.
- Evidence of clinical response — typically a sustained weight loss of at least 5% from your starting weight while on treatment.
- Original starting BMI that met the criteria for initiation under Pathway 1.
- No new contraindications — your medical history will be re-reviewed by our prescriber as part of the transfer.
Pathway 3 — Maintenance Management
Continuing low-dose treatment to preserve weight loss
≥ 20BMI KG/M²
For patients who have successfully reached or are approaching their target weight, our maintenance pathway supports the long-term preservation of weight loss using a stable, lower dose.
- Current BMI of 20 kg/m² or above — this floor exists to protect against the risks associated with becoming underweight.
- Stable weight over the previous 8–12 weeks (i.e. plateau reached).
- Maintenance-dose prescribing — typically the lowest effective dose, rather than continued escalation.
- Ongoing clinical review at each prescription cycle, including weight, blood pressure, and any new symptoms.
Have you been diagnosed with any of these medical conditions?
The presence of any of the conditions below alongside a BMI of 27 kg/m² or above (or 24.5 kg/m² with the relevant ethnicity adjustment) means you may be eligible to begin treatment without needing to reach the higher BMI 30 threshold.
What to consider: Think about formal diagnoses given to you by a GP, hospital doctor, or specialist — not symptoms you suspect but have not had investigated. Past diagnoses still count, even if you are no longer receiving treatment for them.
Type 2 Diabetes — Whether managed through diet alone, tablets, or insulin. Please tick this if you have ever been diagnosed.
Pre-diabetes or insulin resistance — A blood test result showing your sugar level is higher than normal but not yet in the diabetes range, or a previous diagnosis of metabolic syndrome.
High blood pressure (hypertension) — A diagnosis made by a clinician, or any medication you currently take to lower your blood pressure.
Raised cholesterol or triglycerides — A diagnosis of high cholesterol, or currently taking a statin or other medication to lower your cholesterol.
Cardiovascular disease — Includes angina, coronary artery disease, a previous stent or bypass operation, or a diagnosis of heart failure.
Obstructive sleep apnoea (OSA) — A diagnosis confirmed by a sleep study. Please indicate if you currently use a CPAP machine at night.
Osteoarthritis of the hips or knees — Particularly where a clinician has suggested that losing weight would help to ease your symptoms.
Start Your Consultation →
Your answers are confidential and used only for the purposes of clinical assessment by a UK-registered prescriber.
Weight Loss Eligibility Check
Find out if you're eligible for our weight loss treatment in just 30 seconds.
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Your information is secure and confidential
This tool provides a general eligibility indication only and does not constitute medical advice. A full clinical assessment by one of our pharmacists is required before any treatment can be prescribed or supplied.
Ethnicity-Adjusted Thresholds
Why a lower BMI of 24.5 may apply to some ethnic backgrounds
BMI was originally designed using data from white European populations, but research has consistently shown that weight-related health problems can begin to develop at a lower BMI in people from certain ethnic backgrounds. To reflect this, NICE guidance (NG246, 2025) recommends a BMI threshold that is around 2.5 points lower for the following groups:
How This Differs From the NHS
NHS criteria are stricter than Happy Pharmacy's
NHS commissioning rules are designed around population-level cost-effectiveness and rationing, whereas a regulated private prescriber follows the licensed indication of the medicine and the relevant NICE technology appraisals (TA875 for semaglutide; TA1026 for tirzepatide). The thresholds for accessing treatment privately are therefore generally lower.
Why These Thresholds?
The evidence underpinning each pathway
These thresholds reflect the licensed indications of the medicines, the relevant NICE technology appraisals, and the long-term cohort and trial evidence on cardiometabolic risk in different populations.
South Asian
Chinese
Other Asian
Middle Eastern
Black African
African-Caribbean
If you are of mixed background, a prescriber will use clinical judgement, taking into account your full medical and family history rather than a single threshold figure.
NHS
Specialist Weight-Management Service
- BMI ≥ 35 kg/m² with at least one weight-related condition (BMI 30–34.9 only under very specific tier-3/tier-4 criteria).
- Referral required from your GP into a tier-3 multidisciplinary service.
- Typical waiting times of 6–18 months depending on local commissioning.
- Maximum treatment duration capped at two years under TA875.
- Requires evidence of prior structured lifestyle intervention.
- Tirzepatide being rolled out in stages — highest BMI groups offered it first.
HAPPY
Happy Pharmacy
- BMI ≥ 30, or ≥ 27 with a co-morbidity, or ≥ 24.5 with relevant ethnicity adjustment.
- Direct online consultation — no GP referral required.
- Typical clinical review within 24–48 hours of submission.
- Duration determined by individual clinical response, not a hard cap.
- Lifestyle support offered alongside treatment, but not a prerequisite.
- Switching and maintenance pathways available for continuity of care.
01BMI 30Obesity thresholdThe point at which all-cause mortality and the risk of type 2 diabetes, cardiovascular disease, and several cancers rise substantially.
02BMI 27Co-morbidity thresholdIn the STEP and SURMOUNT trials, BMI ≥ 27 was the inclusion criterion when a weight-related condition was present.
03BMI 24.5Ethnicity-adjustedNICE NG246 recognises lower BMI cut-offs for South Asian, Chinese, Black African, African-Caribbean, and Middle Eastern groups.
04BMI 21Switching floorReflects the clinical reality that established patients have already responded to treatment. Abrupt discontinuation correlates with rapid weight regain.
05BMI 20Maintenance floorSits safely above the underweight threshold of 18.5 kg/m². Below this point, continued GLP-1 therapy is not considered clinically appropriate.
06MHRA / GPhCRegulatory frameworkAll treatments dispensed are MHRA-approved. All prescribing follows GPhC standards. Side effects reportable via the Yellow Card scheme.
Frequently Asked Questions
FAQs
Answers to the questions we are most often asked about BMI and weight-loss treatment eligibility.
BMI (Body Mass Index) is a number worked out from your height and your weight, using the formula BMI = weight (kg) ÷ height (m)². It gives a quick estimate of whether your weight sits within a healthy range for someone of your size. The standard categories are: under 18.5 (underweight), 18.5–24.9 (healthy weight), 25–29.9 (overweight), and 30 or above (obesity).
BMI does not measure body fat directly, but at a population level it gives a reliable indication of how much body fat a person is likely to be carrying. As BMI rises above the healthy range, so does the risk of long-term health problems such as type 2 diabetes, high blood pressure, heart disease, stroke, certain cancers, and joint problems. The BMI thresholds used in prescribing also match the populations studied in the major clinical trials of these medicines.
For new patients starting treatment for the first time, you usually need a BMI of 30 or above. You may also qualify with a BMI of 27 or above if you have at least one weight-related health condition. If you are of South Asian, Chinese, Black African, African-Caribbean, or Middle Eastern family background, a lower threshold of 24.5 may apply. If you are switching from another provider or already in maintenance treatment, the thresholds are lower again.
No — a BMI of 27 alone is not enough on its own. To qualify at this BMI you must also have a documented weight-related health condition, such as type 2 diabetes, high blood pressure, raised cholesterol, cardiovascular disease, obstructive sleep apnoea, osteoarthritis, or another condition recognised as linked to excess weight. Eligibility is always confirmed during a clinician-led consultation.
For first-time patients, treatment is not usually recommended below a BMI of 27. However, if you are already receiving treatment from another UK-regulated prescriber and wish to transfer your care, we may accept you with a current BMI as low as 21. If you have already lost weight on treatment and are stable at or near your target weight, our maintenance pathway supports continued low-dose prescribing down to a BMI of 20.
If you enter your height and weight correctly, the calculator will work out your BMI accurately using the standard formula. However, BMI itself has limitations: it does not tell the difference between muscle and fat, and the standard thresholds may need to be adjusted for some ethnic backgrounds. For this reason, your BMI should always be looked at alongside the rest of your medical picture.
No — the calculator gives you an indication of whether your BMI meets the usual thresholds, but it cannot confirm eligibility on its own. Final eligibility is decided by a GPhC-registered prescriber after a full online consultation, which takes into account your medical history, any weight-related health conditions, current medications, and your ethnic background.
No — BMI is the starting point, but a clinician will always look at the whole picture. This includes your full medical history, any weight-related health conditions, any other medicines you are currently taking, and your ethnic background. If you are switching from another provider, your prescriber will also review evidence of your response to previous treatment.
NHS criteria are stricter and apply through a tier-3 or tier-4 specialist weight-management service. To access NHS treatment you usually need a BMI of 35 or above with at least one weight-related condition, a GP referral, and evidence of prior structured lifestyle intervention — with treatment capped at two years. Private prescribing follows the licensed indication of the medicine and is accessed directly through an online consultation with no GP referral and no two-year cap.
The next step is to complete our secure online consultation, where you will be asked about your weight history, any medical conditions, your current medications, and other relevant background. A GPhC-registered prescriber will then review your answers and decide whether prescription weight-loss treatment is appropriate for you. If it is, your prescriber will discuss the most suitable medicine and arrange ongoing reviews to monitor your progress.
References
This consultation is medically reviewed by GPhC-registered pharmacists and independent prescribers. Last reviewed: May 2026. Next scheduled review: November 2026. Medication is prescribed following a consultation with a qualified prescriber. Please click here to view full safety information.
- NICE. Overweight and obesity management. NICE guideline NG246. 2025. nice.org.uk/guidance/ng246
- NICE. Semaglutide for managing overweight and obesity. Technology appraisal TA875. 2023. nice.org.uk/guidance/ta875
- NICE. Tirzepatide for managing overweight and obesity. Technology appraisal TA1026. 2024. nice.org.uk/guidance/ta1026
- NHS. Body mass index (BMI) and healthy weight. nhs.uk
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). NEJM. 2021;384:989–1002.
- Rubino D et al. Effect of Continued Weekly Subcutaneous Semaglutide vs Placebo on Weight Loss Maintenance (STEP 4). JAMA. 2021;325(14):1414–1425.
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). NEJM. 2022;387:205–216.
- Lincoff AM et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT). NEJM. 2023;389:2221–2232.