Substance D is a fictional drug featured in the science fiction novel "A Scanner Darkly" by Philip K. Dick. It is known for its powerful and debilitating effects on users, leading to severe cognitive impairment and a split personality. The drug is depicted as highly addictive, causing users to lose their sense of reality and identity over time. In the story, Substance D is used as a tool for social control, reflecting the dangers of drug abuse and the impact of addiction on society. The narrative explores themes of surveillance, identity, and the consequences of substance dependence, making it a critical commentary on drug culture.