Confusion of drug names is one of the most common causes of drug-related medical errors. A similarity measure of drug names, "vwhtfrag", was developed to discriminate whether drug name pairs are likely to cause confusion errors, and to provide information that would be helpful to avoid errors. The aim of the present study was to evaluate and improve vwhtfrag. Firstly, we evaluated the correlation of vwhtfrag with subjective similarity or error rate of drug name pairs in psychological experiments. Vwhtfrag showed a higher correlation to subjective similarity (college students: r=0.84) or error rate than did other conventional similarity measures (htco, cos1, edit). Moreover, name pairs that showed coincidences of the initial character strings had a higher subjective similarity than those which had coincidences of the end character strings and had the same vwhtfrag. Therefore, we developed a new similarity measure (vwhtfrag+), in which coincidence of initial character strings in name pairs is weighted by 1.53 times over coincidence of end character strings. Vwhtfrag+ showed a higher correlation to subjective similarity than did unmodified vwhtfrag. Further studies appear warranted to examine in detail whether vwhtfrag+ has superior ability to discriminate drug name pairs likely to cause confusion errors.