Objective: The squeezing force and one-drop weight, suggested to directly influence adherence, were measured in 6 eye drop products containing a switch OTC drug, ketotifen fumarate, to investigate useful information for product selection.
Methods: The squeeze force, one drop weight, and pH were measured using a digital force gauge, analytical balance, and pH meter, respectively.  Information on additives contained in each product was collected from package inserts.  For the total number of drops, the number per 10 mL was calculated from the obtained value.
Results: The maximum squeeze force was 14.8 N of Irice AG Guard, and the one drop weight (33.2 mg) of Raferusa®AL was the minimum.  The total number of drops per 10 mL was 215 in Sutto eyes Z, being the minimum.  The pH was in the range of 5.2-5.7.  On comparison of additives among the products, a cooling agent was contained in only 2 products.
Discussion: Since the squeeze force was in the range of 5.3-14.8 N, it was less likely that the squeeze force reduces usability.  Since the one-drop weights of 2 products were more than 10 mg lower than the weights of the other products, the dose may be insufficient and the effect may not be attained.  The pH was within the acceptable range in all products.  Two products contain a cooling agent as an additive, and this has to be explained beforehand.  Information related to usability, actual feeling of the effect, and sense of the use of the products containing ketotifen fumarate was collected.