A study by Auckland University researchers suggests cannabis use may double the risk of adults under 55 having a stroke.
Urine tests from 160 stroke patients show that 16% had recently used cannabis, compared with just over 8% of a control group.
The Neurological Foundation's professor of clinical neurology of the Centre for Brain Research says the study provides the strongest evidence to date of an association between cannabis and stroke.
However, Alan Barber says more research is needed, as all but one of the cannabis users also smoked tobacco and the only things known about the control group of hospital patients was age, gender and ethnicity.
Professer Barber says questioning people about their illegal drug use may not get reliable answers, so researchers asked the ethics committee whether they could test urine from non-stroke patients without their permission.
They may now ask the committee if they can repeat the tests and find out more about the smoking and socio-economic status of the control group, he says.