This chapter examines the use of new psychoactive substances (NPS) among vulnerable populations and associated comorbidities. Adolescents, with developing brains, are particularly susceptible to NPS’ neurotoxic effects, impairing impulse control and decision-making, compounded by peer influence and misconceptions about safety. Prisoners face unique challenges, including aggression, psychiatric symptoms, and trafficking linked to NPS use, with young inmates often experiencing victimization and addiction amidst limited mental health support. Women, due to biological vulnerabilities and societal pressures, require gender-sensitive, trauma-informed care to address heightened mental health risks. The second section of the chapter explores NPS-related comorbidities, such as psychiatric, cardiovascular, infectious, and autoimmune diseases. NPS use exacerbates psychiatric episodes, cardiovascular issues (e.g., hypertension, arrhythmias), and infectious diseases like HIV and hepatitis C, often through injection practices and risky behaviors. This chapter underscores the critical need for targeted, specialized interventions to mitigate risks and improve outcomes for these high-risk groups.