Doctors are warning young cannabis users that they could be at an increased risk of having a stroke.
A UK expert has urged caution over the Spanish study
Spanish researchers detailed the case of a 36-year-old patient, with no known risk factors for stroke, who had three following cannabis use.
The paper in the Journal of Neurology Neurosurgery and Psychiatry said the risk may have been underestimated.
But a UK stroke expert urged caution, saying more cases would have been seen if there was a significant link.
The Spanish researchers estimated perhaps 15 other cases of stroke had been linked to cannabis use. They called for further research to establish the actual extent of the risk.
Frequent cannabis use has in the past been linked to behavioural abnormalities and an increased risk of schizophrenia.
Bleeding on the brain
The patient who was studied was a primary school teacher, who had been an occasional user of cannabis.
He had no known risk factors for stroke, did not use other drugs, and drank only occasionally.
The first incident occurred after he had smoked a considerable amount of cannabis combined with three or four drinks at a party.
He lost his ability to speak, which was followed, a few hours later, by convulsions.
A brain scan revealed one patch of bleeding and another blood clot, but no evidence of narrowed/furred up arteries. He was treated and recovered.
Around a year later, after another bout of cannabis smoking, he again lost his ability to talk and experienced weakness on one side of his body.
A brain scan revealed a further small patch of bleeding as well as another blood clot, but in different areas from before.
After the second stroke, the man did not use cannabis for 18 months. However, he then smoked a "reasonable amount" in one go, which he combined with three or four drinks.
This was followed by an inability to recognise sounds, a condition known as auditory agnosia.
A further brain scan revealed a patch of bleeding as well as the damage left by the previous bleeds.
The researchers, led by Dr Juan Garcia-Monco, at the Galdacano Hospital, in Galdacano, Vizcaya, said the cardiovascular effects of cannabis - including a rapid heart beat and excessively high or low blood pressure - were often overlooked.
They said cannabis also quadrupled the risk of a heart attack within an hour of consumption.
Dr Garcia-Monco and his team wrote: "Cannabis is not as safe a drug as many believe.
"Future studies will be needed to clarify the role of cannabis as a stroke risk factor, as it could be underestimated."
They added: "If this was the case, cereberovascular risk may increase in the future, due to an increased consumption based on the potential therapeutic benefits of cannabis and its possible legislation in some countries."
In a commentary, also published in the Journal of Neurology, Neurosurgery and Psychiatry, Dr Dominique Deplanque, of the Department of Pharmacology at the University of Lille, said the study raised concerns.
"In that recreational cannabis appears not to be as harmless as was thought, there is a need to improve public information," she said.
"The therapeutic potential of cannabis and its derivatives should be rigorously evaluated and the benefit to risk ratio taken into account before authorising their medical use."
Dr Tony Rudd, a stroke specialist at St Mary's Hospital, in London, told the BBC News website: "Long-term cannabis use probably does increase the risk of stroke, probably because people use a lot of tobacco when they smoke cannabis."
But he said the link had not been made between short-term use and stroke risk.
"It would be a mistake to read too much into a single case report. It's clearly not a very strong association because, if it was, it would have come to our attention before now."