From the Abstract: This paper argues that abstinence from nonmedical psychoactive substance use should be conceptualized not merely as harm avoidance but as a positive enabling condition for neurodevelopment and neurorehabilitation. Abstinence removes maladaptive plasticity pressures, facilitates recovery of brain structure and function, enhances responsiveness to lifestyle and cognitive interventions, and supports stable engagement in health promoting behaviours. Policy and clinical frameworks that embed abstinence as a central design principle are therefore best placed to protect and optimize brain health, especially in youth and in individuals recovering from substance use disorders.