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 times more likely to have traces of cannabis in their urine as other hospital patients who had not suffered strokes.

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"This is the first case- controlled study to show a pos-sible 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."

In total, 16% of the stroke victims tested positive for cannabis compared with 8.1% of the comparison group.