Lead investigator Alan Barber of Neurological Foundation and professor of clinical neurology at the University of Auckland found ischemic stroke and transient ischemic attack patients were 2.3 times more likely to have cannabis detected in urine tests as others matched for age and sex.
"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis (marijuana)," Barber said in a statement. "Cannabis has been thought by the public to be a relatively safe, although illegal substance. This study shows this might not be the case; it may lead to stroke."
The study involved 160 ischemic stroke/TIA patients ages 18-55 years who had urine screens upon admission to the hospital.
Among the patients, 150 had ischemic stroke and 10 had TIAs. Sixteen percent of patients tested positive for cannabis, and were mostly male who also smoked tobacco, while 8.1 percent of controls tested positive for cannabis in urine samples.
"These patients usually had no other vascular risk factors apart from tobacco, alcohol and other drug usage," Barber said. "It's challenging to perform prospective studies involving illegal substances such as cannabis because questioning stroke and control patients about cannabis use is likely to obtain unreliable responses."
The study provides the strongest evidence to date of an association between marijuana and stroke, but the association is confounded because all but one of the stroke patients who smoked marijuana also used tobacco regularly, Barber said.
The findings were presented at the American Stroke Association's International Stroke Conference in Honolulu.