Drug and substance abuse was a global problem and had been one of the major issues affecting the youth, both in school and out of school, due to drug and substance abuse. The study aimed to determine the effects of drugs and substance abuse in Mashimoni village, Kibra slum, Nairobi. It sought to provide additional information to the existing records about drug and substance abuse, which could have been of great importance to future scholars and those interested in researching the same issue, as well as relevant authorities like the Ministry of Health to identify effective measures to address this issue. The study had employed a descriptive cross-sectional study design, utilizing both quantitative and qualitative methods. The sample size had consisted of 87 respondents, selected through a Random Sampling method. Data had been collected via interviewer-administered questionnaires containing both open-ended and closed-ended questions. The data had been analyzed using the Statistical Package for Social Sciences (SPSS) version 20, and Microsoft Excel had been utilized for graphical presentation. The study had revealed that the majority of the individuals (77 out of 87 respondents), aged 13 to 24 years, had possessed adequate knowledge of the use and effects of drugs and substance abuse. However, despite this knowledge, some of them had still been abusing drugs and substances. Furthermore, the study had identified that the majority (50%) of the youth who were abusing drugs and substances had either completed secondary school education but not pursued tertiary education or had not completed secondary school education at all. Consequently, the study had recommended continuous awareness programs, the creation of opportunities, and direct intervention measures to address the challenge of drugs and substance abuse.