Smoking cannabis can cause potentially lethal damage to the heart and arteries of young and middle-aged adults, a study has found.
Researchers in France who looked at almost 2,000 patients with medical problems related to cannabis use identified 35 serious instances of cardiovascular complications.
Twenty heart attacks were recorded, as well as 10 cases involving arteries in the limbs, and three affecting blood vessels in the brain. Nine patients, around a quarter of the total, died.
Most of the patients were male, with an average age of 34.3 years.
Lead scientist Dr Emilie Jouanjus, from the University of Toulouse, said: "The general public thinks marijuana is harmless, but information revealing the potential health dangers of marijuana use needs to be disseminated to the public, policy makers and healthcare providers."
Previous research by her team found several "remarkable" cases of young cannabis users being admitted to hospital with cardiovascular complications.
"This unexpected finding deserved to be further analysed, especially given that the medicinal use of marijuana has become more prevalent and some governments are legalising its use," she said.
The findings are reported in the latest edition of the Journal of the American Heart Association.
The researchers looked at cases of serious health problems involving cannabis recorded by the French Addictovigilance Network, which monitors drug use, between 2006 and 2010.
They pointed out that health complications linked to smoking cannabis are likely to go largely unreported.
People with pre-existing cardiovascular weaknesses appeared to be more prone to the harmful effects of the drug, the scientists added.
"There is now compelling evidence on the growing risk of marijuana-associated adverse cardiovascular effects, especially in young people," said Dr Jouanjus. "It is therefore important that doctors, including cardiologists, be aware of this, and consider marijuana use as one of the potential causes in patients with cardiovascular disorders."