Influence of body mass index on prescribing costs and potential cost savings of a weight management programme in primary care.

OBJECTIVES: Prescribed medications represent a high and increasing proportion of UK health care funds. Our aim was to quantify the influence of body mass index (BMI) on prescribing costs, and then the potential savings attached to implementing a weight management intervention.

METHODS: Paper and computer-based medical records were reviewed for all drug prescriptions over an 18-month period for 3400 randomly selected adult patients (18-75 years) stratified by BMI, from 23 primary care practices in seven UK regions. Drug costs from the British National Formulary at the time of the review were used. Multivariate regression analysis was applied to estimate the cost for all drugs and the 'top ten' drugs at each BMI point. This allowed the total and attributable prescribing costs to be estimated at any BMI. Weight loss outcomes achieved in a weight management programme (Counterweight) were used to model potential effects of weight change on drug costs. Anticipated savings were then compared with the cost programme delivery. Analysis was carried out on patients with follow-up data at 12 and 24 months as well as on an intention-to-treat basis. Outcomes from Counterweight were based on the observed lost to follow-up rate of 50%, and the assumption that those patients would continue a generally observed weight gain of 1 kg per year from baseline.

RESULTS: The minimum annual cost of all drug prescriptions at BMI 20 kg/m(2) was pound 50.71 for men and pound 62.59 for women. Costs were greater by pound 5.27 (men) and pound 4.20 (women) for each unit increase in BMI, to a BMI of 25 (men pound 77.04, women pound 78.91), then by pound 7.78 and pound 5.53, respectively, to BMI 30 (men pound 115.93 women pound 111.23), then by pound 8.27 and pound 4.95 to BMI 40 (men pound 198.66, women pound 160.73). The relationship between increasing BMI and costs for the top ten drugs was more pronounced. Minimum costs were at a BMI of 20 (men pound 8.45, women pound 7.80), substantially greater at BMI 30 (men pound 23.98, women pound 16.72) and highest at BMI 40 (men pound 63.59, women pound 27.16). Attributable cost of overweight and obesity accounted for 23% of spending on all drugs with 16% attributable to obesity. The cost of the programme was estimated to be approximately pound 60 per patient entered. Modelling weight reductions achieved by the Counterweight weight management programme would potentially reduce prescribing costs by pound 6.35 (men) and pound 3.75 (women) or around 8% of programme costs at one year, and by pound 12.58 and pound 8.70, respectively, or 18% of programme costs after two years of intervention. Potential savings would be increased to around 22% of the cost of the programme at year one with full patient retention and follow-up.

CONCLUSION: Drug prescriptions rise from a minimum at BMI of 20 kg/m(2) and steeply above BMI 30 kg/m(2). An effective weight management programme in primary care could potentially reduce prescription costs and lead to substantial cost avoidance, such that at least 8% of the programme delivery cost would be recouped from prescribing savings alone in the first year.