This study aimed to assess patient investigational medication knowledge and to identify factors associated with medication understanding by adult outpatients included in clinical trials. A cross-sectional prospectively designed survey was conducted on consecutive volunteers at 21 university teaching hospitals (in France) from February to December 2014. Investigational medication understanding was assessed at the time of the first dispensing using a structured interviewer-administered questionnaire based on information obtained from the literature that provided an 8-point score. Demographic and other baseline data were collected using structured interviews. Of the 236 participants, 139 (58.9%) of the respondents were male, and the median age was 54.9 years (range: 18-83 years). The mean understanding score was 6.24 and 72.5% of the patients had a score of 6 or higher. In univariate analysis, the medication understanding score was negatively correlated with age (r = -0.15, p = 0.0247) and positively correlated with the level of education (r = 0.25, p = 0.0002). In multivariate analysis, prognostic factors of a higher medication understanding score were: graduation from high school or a higher level of education; HIV infection; phase II/III/IV studies; mention of the drug on the prescription form, and the dispensing of a single investigational medication. Only a quarter of the adult outpatients included in clinical trials had a maximum possible investigational medication understanding score. Being old and having a low level of education were found to be important risk factors for inadequate medication understanding. This and other data suggest that sponsors should encourage initiatives aimed at improving investigational medication understanding in adults enrolled in clinical trials.