It is uncertain whether frequent marijuana use adversely affects human brai
n function. Using PET, regional cerebral blood flow was compared in frequen
t marijuana users and comparable, non-using controls after at least 26 h of
monitored abstention by all subjects. Marijuana users showed substantially
lower brain blood flow than controls in a large region of posterior cerebe
llum, indicating altered brain function in frequent marijuana users. A cere
bellar locus of some chronic and acute effects of marijuana is plausible, e
.g. the cerebellum has been linked to an internal timing system, and altera
tions of time sense are common following marijuana smoking. NeuroReport 11:
749-753 (C) 2000 Lippincott Williams & Wilkins.