Smoking cannabis can double the risk of stroke in young and middle-aged adults, a study suggests.
Scientists looked at 160 stroke victims aged 18 to 55 who had been admitted to hospital and undergone urine tests.
They were 2.3 time more likely to have traces of cannabis in their urine as other hospital patients who had not suffered strokes.
"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis," said lead researcher Professor Alan Barber, from the University of Auckland in New Zealand.
"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 stroke participants had either experienced an ischaemic stroke, caused by lack of blood to the brain, or a transient ischaemic attack (TIA), also known as a "mini-stroke".
In total, 16% tested positive for cannabis compared with 8.1% of the comparison group.
Studies involving illegal drugs which rely on questions and answers tend to be unreliable, the researchers pointed out. This study had the advantage of an objective measure of cannabis in the urine.
The scientists acknowledged that all but one of the stroke patients who were cannabis users also smoked tobacco. This called into question which substance was having the effect.
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But Prof Barber said: "We believe it is the cannabis and not the tobacco."
He hopes to conduct another study to confirm that cannabis was acting independently of tobacco.
"The high prevalence of cannabis use in this cohort of younger stroke patients makes this research imperative," said the professor, who presented his findings at the American Stroke Association's International Stroke Conference in Hawaii. "People need to think twice about using cannabis."