The chicken and the egg - a systematic review of the relationship between recreational drug use and mental health in the general population

Temporal relationships between the recreational use of alcohol, tobacco and cannabis with mental health challenges have been extensively reviewed. However, this is not the case for other widely consumed drugs. Between 7/6/23 and 3/12/23 we searched Web of Science, EMBASE, PsychINFO and MEDLINE databases for studies investigating temporal relationships between recreational drug use (RDU) and mental health (MH). From N=8,759 publications originally identified, we systematically reviewed 69 studies fulfilling four criteria; 1) studying the general population, 2) investigating cocaine, ketamine, psychedelics, opioids, 3,4-methylenedioxymethamphetamine (MDMA/ecstasy) or amphetamines, 3) prospective/longitudinal designs measuring RDU and MH on at least two separate occasions, 4) measuring symptoms of either depression or anxiety. Studies were placed into seven outcome categories: RDU was followed by worse MH (N1), worse MH was followed by increased RDU (N2), RDU was followed by better MH (P1), better MH was followed by decreased RDU (P2), as well as bidirectional, mixed or no relationships seen. Whilst the most commonly identified relationships were N2 (N=16, 23.2%) or N1 (N=15, 21.7%) these were seen in under half of the studies included. We identified a number of contributing factors, including patterns of drug use, socioeconomic factors, definitions of MH and RDU employed, and direction of relationship tested. Our results reveal a nuanced picture, suggesting blanket statements on RDU-MH temporal relationships may be misleading. We discuss these factors and offer suggestions that future studies may incorporate to better understand the relationship between these domains.