OBJECTIVES: In economic analyses, the size (volume per vial) of pharmaceutical products is usually taken as a fixed quantity. However, whilst the product is in development, there is scope for the size to be altered. The objective of this study was to examine how product sizes might be optimised for maximum efficiency. METHODS: Four drugs available in two product sizes were chosen as examples: pemetrexed, cetuximab, panitumumab and belimumab. Drug costs per administration were estimated based on general population weight and body surface area data from the Health Survey for England, using the most efficient combinations of the two available vial sizes. The sizes of the two vials were varied simultaneously to identify the lowest average costs achievable. RESULTS: Generally, smaller vials were associated with less wastage and lower costs. Within the size ranges analysed, the cost per administration could be reduced by an average of 4.4% (range: 1.12% to 6.39%). Over 1 year on treatment, this represents an average potential saving of £1,892 (range: £120 to £3,289). However, this was associated with an increased number of vials per dose. CONCLUSIONS: The cost effectiveness of new medicines could be enhanced by better planning in the manufacturing process and more efficiently sized vials. Costs were generally greater when the large vial was a multiple of the smaller vial (which is the case in three of the four example drugs). Non-divisible pairs are associated with more precise dosing and less wastage, as a greater number of unique quantities can be formed with same numbers of vials. Optimal vial sizing could reduce costs for health systems, improving patient access to new medicines as a consequence. Increases in the number of vials per dose may impact the time and cost of preparation, but additional costs may be offset by savings from reductions in wastage.