We have recorded blood flow velocity in the anterior and middle cerebral 
arteries by transcranial Doppler sonography in abstinent marijuana abusers (n = 
16) and control subjects (n = 19) to assess the effects of prolonged marijuana 
use of the cerebrovascular system. The pulsatility index, a measure of 
cerebrovascular resistance, and systolic velocity were significantly (p < 0.005) 
increased in marijuana abusers compared to the control subjects. These findings 
suggest that cerebral perfusion observed in 18-30 year old marijuana abusers is 
comparable to that of normal 60 year-olds. Thus, chronic abuse of marijuana 
might be a risk factor for stroke.