Reasoning deficits among illicit drug users are associated with aspects of cannabis use.

Deficits in deductive reasoning have been observed among ecstasy/polydrug users. The present study seeks to investigate dose-related effects of specific drugs and whether these vary with the cognitive demands of the task. One hundred and five participants (mean age 21.33, SD 3.14; 77 females, 28 males) attempted to generate solutions for eight one-model syllogisms and one syllogism for which there was no valid conclusion. All of the one-model syllogisms generated at least one valid conclusion and six generated two valid conclusions. In these six cases, one of the conclusions was classified as common and the other as non-common. The number of valid common inferences was negatively associated with the aspects of short-term cannabis use and with measures of IQ. The outcomes observed were more than simple post-intoxication effects since cannabis use in the 10 days immediately before testing was unrelated to reasoning performance. Following adjustment for multiple comparisons, the number of non-common valid inferences was not significantly associated with any of the drug-use measures. Recent cannabis use appears to impair the processes associated with generating valid common inferences while not affecting the production of non-common inferences. It is possible, therefore, that the two types of inference may recruit different executive resources, which may differ in their susceptibility to cannabis-related effects.