MDMA (3,4-methylenedioxymethamphetamine) has been widely studied for its potential therapeutic effects, particularly its ability to facilitate emotional openness and enhance psychotherapy outcomes. Recent research indicates that MDMA alters speech patterns, increasing emotional expressivity and social comfort, which may be crucial factors in therapeutic settings. This study aims to examine linguistic markers associated with authenticity, emotionality, and prosocial behavior under the influence of MDMA, specifically considering the role of familiarity with a conversational partner. Utilizing analysis of data collected from healthy volunteers in a randomized, double-blind, placebo-controlled study, we employed linguistic analysis techniques, namely the Linguistic Inquiry and Word Count (LIWC) tool, to quantify changes in speech content. Using a within-subjects design over four sessions, participants engaged in conversations while under the influence of MDMA or under placebo and in the context of a familiar or unfamiliar partner—allowing assessment of the interaction between drug effects and social context. We hypothesized that MDMA would increase linguistic markers of authenticity and emotional expression, and prosocial behavior. Additionally, familiarity with the partner would further amplify emotional expression, and have similar effects on the linguistic markers independently. Our analysis revealed statistically significant main effects of both the drug and familiarity with the conversation partner. Specifically, the drug was associated with increased emotional expression, prosocial behavior in speech, and negative tone. Additionally, familiarity with the conversation partner was linked to greater present-focus. However, we did not find a significant interaction effect between the drug and partner familiarity on these linguistic markers. Additionally, while there was no evidence that MDMA or familiarity altered the number of words per sentence, understanding other potential linguistic shifts could offer valuable insights for MDMA-assisted therapy (MDMA-AT), helping to inform therapeutic approaches and practitioner training.