Recreational substance use and cognition in a large cohort of young adults with hair testing: Something is not nothing

Background
Chronic substance use is linked to cognitive impairments, such as cannabis affecting learning and attention (e.g., Scott et al., 2018), Ecstasy impacting declarative memory (e.g., Wunderli et al., 2017), and cocaine causing widespread cognitive issues (e.g., Vonmoos et al., 2013). However, prior studies primarily focused on highly selective samples of chronic users and relied on self-report measures of substance use with often uncertain validity. Additionally, polysubstance use was mostly neglected in previous studies. Thus, it remains unclear if substance use affects cognitive functions already at recreational and occasional use patterns and in less selective larger populations. 
To address this gap, the present study examines the associations between a wide range of substances and cognitive functions in a large community cohort of young adults and employing hair testing.
Methods
Data was obtained from 850 young adults (Mage = 24.4 years; SD = 0.4) in a large urban cohort study (z-proso) from Switzerland. Information on substance use was obtained through toxicological analyses assaying participants’ hair samples for substances such as cannabinoids, stimulants, opioids, and ketamine. The cognitive functions vigilance, working memory, and visuospatial declarative memory were assessed using the Cambridge Neuropsychological Test Automated Battery (CANTAB; Strauss et al., 2006). Further information was obtained on participants’ and their parents’ sociodemographic information.
To test associations between substance use and cognitive functions, we used multivariable linear regression analyses, controlled for participants’ sex, education, migration status, and household socioeconomic status. We also employed a Polysubstance Use Severity Index (PSUSI; Kroll et al., 2018) to estimate cognitive correlates of the use of multiple substances.
Results
Reduced vigilance was significantly associated with higher concentrations of cannabis (p = .005), cocaine (p = .038), codeine (p = .042), and higher levels of the PSUSI (p = .009). Impaired working memory was significantly related only to higher concentrations of codeine (p = .048). Lastly, impaired visuospatial declarative memory was significantly associated with higher concentrations of cocaine (p = .033), ketamine (p = .008), and higher levels of the PSUSI (p = .047).
Conclusions
Using hair testing in a large community-representative sample of young adults, we found that even occasional use of different substances may lead to negative effects on varying cognitive functions, such as attention, working memory, or declarative memory. These findings can inform future prevention and harm reduction measures as well as and drug policymaking.