Cannabis use may more than double the risk of stroke in young adults, say Auckland researchers, who are convinced it is the illegal drug, and not the tobacco the victims are also smoking, that is to blame.

The study of 160 stroke and mini-stroke victims, aged between 18 and 55, was presented to a stroke conference in Hawaii this week.

It showed they were 2.3 times more likely than other patients to have cannabis detected in urine tests.

"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis," said study leader, Auckland University Professor Alan Barber.

Previous research showed strokes developed hours after cannabis use, he said.

But 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.

"People need to think twice about using cannabis, because it can affect brain development and result in emphysema, heart attack and now stroke."

He hopes 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."

It was also challenging to study the use of illegal substances because people were likely to lie about using them.