OBJECTIVES: Objective of this study is to discover whether OTC package designs differ in their ability to transfer drug safety related information to pharmacy customers. The research was intended to answer two questions: 1) how well (in terms of speed and accuracy) do customers comprehend the relevant medical information on packages? 2) do alternative package designs cause significantly different levels of comprehension?. METHODS: Face-to-face interviews with 452 customers of 17 German pharmacies were conducted. In each of three sequential experiments (one OTC brand per experiment), each of the participants was shown two packages of the same brand (Paracetamol/Bisacodyl) and asked three drug-related questions per package. Multivariate linear regression analysis was used to identify the factors capable of explaining the accuracy of the answers (first outcome; quality) and the speed with which the answers were made (second outcome; time). The different abilities of the three package designs to transfer information correctly and rapidly were compared by descriptive statistics. The significance of the differences was tested by Wilcoxon tests. RESULTS: Older people, those with physiological problems, people not in a good mood, and those with a poor doctor-patient relationship, or a lack of trust in medicines in general answered the questions with a significantly higher number of mistakes and/or needed significantly longer to do so. When compared to two other package designs, one of the tested designs proved itself to be superior in quality and time. CONCLUSIONS: 1)   There exist significant differences between patient groups in their ability to comprehend medical information; specific patients do not understand medical information on packages in a sufficient way. 2)   The design of a medication package can measurably influence the quality and speed of information transfer to pharmacy customers. An optimized package design is a source of added value from a drug safety point of view.