Recreational drug use is thought to harm the neurotransmitter communication systems that are important for cognitive processes. The previous studies on the effects of drug use on cognitive functions are rather inconclusive and suffer from methodological challenges, such as small sample sizes, unrepresentative sample types, short abstinence periods, and poor control for confounding factors. Therefore, this study aimed to investigate the effects of recreational drug use on cognitive functions, in particular PM, using mixed research methods while addressing those methodological difficulties. The study consists of two interrelated studies. In the first study, 53 drug users and 47 non-users were recruited and examined on executive functions (EFs), retrospective memory (RM) and prospective memory (PM), using questionnaire- and lab-based measures. The results revealed that drug users performed poorly in autobiographical memory and verbal learning tests. They also displayed PM deficits, but only in the lab-based measure. On the contrary, they were unimpaired in most EFs measures which might be associated with light drug use. In the second study, seven drug users were interviewed on different components of PM (e.g., RM, attention) to understand how they manage to remember and execute delayed intentions in everyday life from their point of view to unfold the observed discrepancy between the questionnaire- and lab-based PM measures in the first study. It was evident that RM, cues availability at retrieval, time awareness, and attention play a crucial role in PM, thus impairments in such domains might be associated with poor PM performance in drug users. The study also uncovered the cognitive factors (i.e., metacognition and motivation) that explain the observed discrepancy. Together, those impairments may affect the cognitive performance of drug users in a general manner as well as the core aspect of drug abuse-the propensity to continue using drugs despite their increasingly detrimental effects.