Background One of the most important formats to disseminate the evidence in health to different populations are Cochrane Plain Language Summaries (PLSs). PLSs should be written in a simplified language, easily understandable and providing clear message for the consumer. The aim of this study was to examine the extent to which PLSs are customized for lay persons, specifically by providing conclusive, comprehensible, and readable messages. Methods The study analyzed Cochrane PLSs of interventional studies (N = 4360) in the English language published from 1995 to 2019. We categorized the conclusiveness into one of the following categories: "positive", "positive inconclusive", "no evidence", "no opinion", "negative", "negative inconclusive", "unclear", "equal", "equal inconclusive". Language characteristics were analyzed using Linguistic Inquiry and Word Count (LIWC) software. The level of readability was measured by SMOG (Simple Measure of Gobbledygook) index, indicating the number of years of education required to read the text. For each PLS, we also collected the following data: Cochrane Review Network, year of publication and number of authors. Results Most of the PLSs (80%) did not have a conclusive message. In 53% PLSs there was no concluding opinion about the studied intervention or the conclusion was unclear. The most frequent conclusiveness category was "no opinion" (30%), and its frequency increased over time. The conclusiveness categories were similarly dispersed across Cochrane Networks. PLSs were written in an objective style, with high levels of analytical tone and clout above neutral, but a lower relation to authenticity and tone. The median number of years of non-specific education needed to read the PLSs was 14.9 (IQR 13.8 to 16.1), indicating that the person needs almost 15 years of general education to read the content with ease. Conclusion Most of the Cochrane PLSs provided no concluding opinion or unclear conclusion regarding the effects of analyzed intervention. Analysis of readability indicated that they may be difficult to read for the lay population without medical education. Our results indicate that PLSs may not be so plain, and that the writing of Cochrane PLSs requires more effort. Tools used in this study could improve PLSs and make them better suited for lay audiences.