FIG. 2.

Standardization procedures employed in included studies. A substance was considered “withheld” if participants were instructed to avoid using it for ≥24 h (12 h for caffeine) or if abstinence was “verified.” Abstinence was considered “verified” if participants returned a negative “drug test”; that is, a breath test (alcohol), urine screen (other drugs), or blood test (caffeine). Pre-session cannabis abstinence was only “verified” if participants returned a negative blood or urine screen; furthermore, within-session cannabis abstinence was only “verified” if participants were supervised until the >8 h post-treatment assessment. Within-session alcohol and other drug use were also assumed to be “verified” if participants were supervised until this assessment (but could otherwise be demonstrated through a drug test). A substance was considered “continued” if participants were instructed to continue using it as usual. Sleep was considered “controlled” if participants were supervised until the >8 h post-treatment assessment or instructed to obtain sufficient sleep. Adherence to the latter was considered “verified” if an objective measure of sleep quality or duration was obtained. ^aUrine 11-COOH-THC concentrations were <20 ng∙mL⁻¹. ^bParticipants were instructed to avoid using “drugs”; however, it is unclear whether this included alcohol. ^cParticipants were retained in the laboratory for 2 weeks; however, the pre-session one standardization procedures were not specified. ^dShort-term cannabis abstinence cannot be verified in a population of regular users. ^eIt is unclear if this was verified. ^f“Recovery” was assumed to be unsupervised. ^gIndividuals with high habitual caffeine intakes were excluded. ^hIndividuals with sleep disorders were excluded. ⁱIndividuals with high habitual alcohol intakes were excluded. ^jIndividuals who used psychedelic drugs were excluded. ^kAlcohol intake was restricted to one glass. ^*Studies that observed significant negative effects of THC on “next day” performance (see Table 2).