The study, carried out in New Zealand, identified that marijuana smokers were more than twice as likely than healthy adults to have suffered an ischemic stroke and transient ischemic attack (TIA).
Lead investigator of the study, P. Alan Barber, Ph.D., M.D., said:
"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis. 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 researchers assessed urine samples of a total of 150 ischemic stroke and 10 TIA patients aged 18-55. Close to 16 percent of the participants in the study had positive drug screens - most of whom also smoked cigarettes.
8.1 percent of those who came up positive in the urine samples smoked cannabis. There was no difference in age or stroke mechanism between those who smoked marijuana and those who didn't.
Previous studies have identified that ischemic stroke and TIAs can develop within hours of smoking marijuana.
"These patients usually had no other vascular risk factors apart from tobacco, alcohol and other drug usage. Questioning stroke and control patients about cannabis use is likely to obtain unreliable responses."
The regional ethics committee let the researchers access urine samples from patients who had been hospitalized. The researchers did not have full consent, therefore they only knew the sex, age, and ethnicity.
Although this study is the strongest piece of evidence that associates cannabis with stroke, it should be noted that the conclusions of the study have yet to be published in a peer-reviewed journal. In addition, the connection between cannabis and stroke may be confounded because all but one of the stroke patients who were marijuana smokers also engaged in tobacco use.
However, Barber believes that it is cannabis and not the tobacco that is causing the increased risk of stroke.
"We believe it is the cannabis and not tobacco. This may prove difficult given the risks of bias and ethical strictures of studying the use of an illegal substance. However, the high prevalence of cannabis use in this cohort of younger stroke patients makes this research imperative."
The authors stress the need to drug test young people who come in with stroke. They add that people need to be more aware of the very real health risks associated with using cannabis, especially as it can have a significant impact on how the brain develops, and now stroke.
A 2005 study published in the Journal of Neurology Neurosurgery and Psychiatry reported that regular cannabis use among young people could increase their risk of stroke.