This study aims to contribute to the knowledge about lacking or contradictory effects of benzodiazepine on hyperaroused or aggressive behaviour in persons with intellectual disability (ID). We conducted a retrospective and natural case study in a person with mild ID and multiple adverse childhood experiences (ACE), using routine diary data consisting of 275 days including 113 consecutive days under benzodiazepine medication. The medication effects were documented as "calm/relaxed", "fretful/distressed" and "sleep/doze". Transitions between these were modelled using Markov chains. Differences in transitions were analysed using Chi-Square test for homogeneity. The results show a significantly reduced stability of mood and increased distressed behaviour under benzodiazepine. This is in line with reports about the effects of psychotropic medication in patients with ID and challenging behaviour. Besides other influences on unexpected medication effects, a possible dissociative identity disorder is discussed as an additional explanation.