That was easy. "The data support a ‘drug residue’ effect on attention, psychomotor tasks, and short-term memory during the 12–24 h period immediately after cannabis use, but evidence is as yet insufficient to support or refute either a more prolonged ‘drug residue’ effect, or a toxic effect on the central nervous system that persists even after drug residues have left the body." -
http://www.sciencedirect.com/science/article/pii/037687169501097I