Smoking cannabis may increase the risk of a stroke in young adults, a new study suggests.

New Zealand scientists looked at 160 patients who had suffered an ischaemic stroke or a transient ischaemic attack (TIA).

A stroke occurs when the blood supply to the brain is interrupted, depriving it of oxygen. As a result, some of the brain cells die and others are damaged. An ischaemic stroke is caused by a sudden blockage in the blood vessels supplying the brain by a blood clot. These make up the majority of strokes.

A TIA is also known as a ‘mini stroke'. It refers to a brief loss of blood supply to the brain, causing the person affected to experience brief symptoms of stroke, such as paralysis and trouble speaking. The blood supply is quickly restored and symptoms of the stroke completely disappear, normally within 24 hours.

The participants in the study were aged between 18 and 55 and all had their urine tested for drugs on arrival at hospital. Around one in seven tested positive for illegal drugs, the majority of whom were males who also smoked tobacco.

The scientists found that patients who had suffered a ischaemic stroke or TIA were 2.3 times more likely to have tested positive for cannabis compared to their peers.

"This is the first case-controlled study to show a possible link to the increased risk of stroke from cannabis. This drug 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," explained lead investigator, Prof P. Alan Barber, of the University of Auckland.

He noted that previous case reports have shown strokes occurring hours after cannabis use and these patients ‘usually had no other vascular risk factors apart from tobacco, alcohol and other drug usage'.

Prof Barbar acknowledged that while the study provides the strongest evidence yet that cannabis can increase the risk of stroke, the results are not clear cut because all of the stroke patients, bar one, smoked tobacco regularly.

However, he believes that ‘it is the cannabis and not the tobacco' that increases the risk.

He insisted that people ‘need to think twice about using cannabis' because it can affect brain development and lead to emphysema, heart attack and now possibly stroke.

Prof Barber called on doctors to test any young adults who have suffered a stroke for cannabis use.

Details of these findings are published in the journal, Stroke.