Study Objective: The population of individuals in police custody constitutes a sentinel group that is underrepresented in the "standard panels" of public health studies. The aim of this study is to assess the extent of medication misuse among this group. Patients and methods: A retrospective study based on data regarding misuse in a sample of individuals in police custody examined between 2015 and 2016 at the forensic medicine unit of Hôtel-Dieu-Paris Hospital. Results: Among 5,149 medical examinations, 302 cases involved a substance use disorder or the misuse of medications. In 2016, the number of reports of clonazepam misuse increased (n = 65); the user population appears to be getting younger (mean age = 23.5 years) and primarily obtains the drug through street dealers (63%). Regarding opioid substitution treatments, indicators of abuse and misuse were confirmed, while morphine sulfate stood out due to a high proportion of street sales (>75%), IV injection (62%), and poly-drug use, particularly methadone and cocaine (62%). Conclusion: Based on these findings, national surveys of the general population, and studies conducted in settings of deprivation of liberty, individuals in police custody serve as a true barometer of the black market for street drugs. The clinical implications can be significant, and improved monitoring is necessary. For healthcare providers, this also involves better identifying these instances of misuse and substance use disorders to guide future patient care.