Cannabis use may double the stroke risk in young adults, according to a recent study.
The study by Auckland University's Centre for Brain Research found stroke patients were 2.3 times more likely to have cannabis detected in urine tests compared with patients of the same age and sex.
"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis,'' Professor Alan Barber said.
"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.''
Prof Barber, the lead investigator for the study, this week presented the findings to the American Stroke Association's International Stroke Conference in Honolulu.
The study included 150 stroke patients and 10 who had suffered transient ischemic attack - when blood flow to a part of the brain stops for a brief period and results in stroke-like symptoms - aged 18-55 who had urine screens upon admission to hospital.
Of those patients, 16 per cent tested positive for cannabis and were mostly males who also smoked tobacco.
Researchers found no differences in age, stroke mechanism or most vascular risk factors between cannabis users and non-users.
In previous case reports, stroke developed hours after cannabis use, Prof Barber said.
"These patients usually had no other vascular risk factors apart from tobacco, alcohol and other drug usage,'' he 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.''
He said the study provided the strongest evidence to date of an association between cannabis and stroke.
However, the association is confounded because all but one of the stroke patients who were cannabis users also used tobacco regularly.
"We believe it is the cannabis use and not tobacco,'' Prof Barber said.
He plans to conduct another study to determine whether there's an association between cannabis and stroke independent of tobacco use.
"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,'' he said.
Physicians should test young people who come in with stroke for cannabis use, Prof Barber said.
"People need to think twice about using cannabis because it can affect brain development and result in emphysema, heart attack and now stroke.''