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impaired performance in decision-making. This compared to 55
percent for short-term users and 8 percent for nonusers. The fre-
quent users also scored worse on tests of divided attention (the
ability to pay attention to more than one thing at a time) and verbal
fl uency (the number of words spoken within a certain time). “We
found that the longer people used marijuana, the more deteriora-
tion they had in these cognitive abilities, especially in the ability to
learn and remember new information,” says Lambros Messinis,
the lead author of the study. “In several areas, their abilities were
signifi cant enough to be considered impaired, with more impair-
ment in longer-term users than shorter-term users.” 39

Impairments to movement
and concentration mean
that people under the
in uence of marijuana put
themselves and others at
risk when they drive.