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Scientists from New Zealand presented their data Wednesday at the American Stroke Association annual meeting, in Honolulu.
"Sixteen percent of stroke patients had positive cannabis screens, compared with only 8 percent of control participants," said study author Dr. Alan Barber, a stroke neurologist and professor of clinical neurology at the University of Auckland.
"We think the relationship between cannabis and stroke is certainly plausible," Barber said. "We know [from other studies] cannabis reduces the time to angina, that it's associated with myocardial infarction [heart attack] and heart rhythm problems. It can also cause vasoconstriction. If you constrict arteries in the brain, you can have reduced blood flow.
"We believe the association is not just a chance one, but worthy of further investigation," he added.
Barber's study involved 160 stroke patients and 160 healthy participants, all between 18 and 55, an age range during which people rarely suffer a stroke, he said.
Of the stroke patients, 150 had been admitted to the hospital for an ischemic stroke -- the most common kind of stroke, in which blood flow is blocked to the brain. The other 10 had been admitted for a transient ischemic attack (TIA or "mini-stroke"). All were given urine tests that screened for evidence of marijuana use.
Barber said the ingredients in marijuana show up in urine tests for three or four days if you only use it occasionally, but urine will test positive for the drug for 30 to 40 days in daily users.
The 160 control group patients had come to the hospital as internal medicine admissions with non-stroke diagnoses and did not know they were involved in the study. Their urine samples had been tested for other reasons and were about to be discarded.
"As a consequence, the controls were anonymous -- we only could know age, sex and ethnicity," Barber explained.
The results after the drug screens: Twenty-five (more than 15 percent) of the stroke patients had positive cannabis screens and were also more likely to be male (84 percent) and tobacco smokers (88 percent). Of the control urine samples, thirteen (8 percent) were positive for marijuana.
"There was a doubling for the risk of stroke," Barber said.
Because this study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
Two experts said the study is worthwhile, but doesn't lead to any concrete conclusions.
"It's not a strong study, not one you can hang your hat on, but it's better than others we've got," said Dr. Daniel Labovitz, director of the Stern Stroke Center at Montefiore Medical Center, in New York City.
"Even though it's tiny -- they did this assessment on a relatively small number of patients -- it's still possible to at least start talking about the topic. I don't think you can draw any hard conclusions," Labovitz said. "What we know from prior work is that some studies have shown an association with stroke. The finding of this article is not really news. It's another brick in the wall. It lends credence to the concept that smoking marijuana is a stroke risk."
Labovitz, as well as author Barber, also noted that the study didn't tease out the details of tobacco use -- how long and how much the young stroke patients had been smoking and how big a role that might have played in stroke risk.
Dr. Carl Lavie, a professor of medicine and medical director of cardiac rehabilitation and prevention at John Ochsner Heart and Vascular Institute in New Orleans, had similar concerns about the small study size.
"Obviously, this is a very small study and is only an abstract presentation, so we do not have the full details," Lavie said, but he added that the connection between marijuana use and stroke isn't far-fetched.
"A paper from a colleague of mine, Dr. Murray Mittleman, published in Circulation in 2001, showed that the heart attack rate increased by 4.8-fold during the hour after smoking marijuana," Lavie said. "Therefore, since the risk factors of heart attack and stroke are similar, it makes sense that both could be increased by pot smoking."
With drug studies, some also wonder if the researchers have an ethical bias, but the author said his concern lies with the drug's high popularity level.
"I don't have any moral position, but it's important people know there are potential risks," Barber said. "There's a perception out there that cannabis is a natural substance and what's the problem about? For medicinal reasons, it's different. For cancer, it stimulates the appetite. But if you use it for recreation, you need to know the risks," Barber said.