In order to address issues of generalizability across populations, more studies using a variety of populations and tissue types are needed. It is likely to be increasingly challenging to find early, subtle signs of exposure to illicit substances due to their lower rates of use and users’ wish to avoid detection. For some substances such as prescription psychostimulants, it may be helpful to obtain pilot data from children and adolescents chronically being prescribed these medications, as there are likely to be fewer confounding exposures at earlier ages.
