Some limitations also should be noted. Firstly, not all included documents were written in English and simple analysis such as word count, page count, or readability calculation was not feasible. Although, all documents were carefully translated into proper English by experts then analyzed in the main analysis in a standardized way, we cannot rule out the possibility that some subtle nuance of original language was not delivered in translation. Secondly, the number of studies included in blinding and outcome analysis was too small compared to that of all included studies for content analysis, which might have made it difficult to detect statistically significant differences, if any, thus to draw any firm conclusion. Even so, the number of studies included in the main content analysis was larger than 16 of a previous study  and was similar to the suggested ‘medium’ sample size for integrated qualitative-quantitative research . More importantly, this study accomplished to collect the documents from many different countries of different environment and culture while the previous study had documents mostly from only few countries such as Germany, UK and USA. This diversity of documents in the present study made it possible to explore rich source of data. Last but not the least, the actual perception of the information on PIL by study participants may differ from degree of information disclosure judged in our study. Participants’ understanding and memory of the contents of PILs vary  and their behavior may be influenced by other multiple factors such as prior knowledge, health literacy, preference and expectation.