Curled on the floor retching, Tzvetanka Chiderova yelled for her mother to get the water pipe. Within minutes, the Montreal web-designer stopped gagging. The waves of nausea disappeared, she said.

“It was instantaneous,” said Chiderova, who turned to marijuana for medicinal purposes as a last resort while being treated for stomach cancer. Without it, she says, she could not have continued with life-saving chemotherapy.

Yet Chiderova’s personal experience does not count in the medical literature. Marijuana obtained on the streets is of unknown quality, the dosage is not standard, and there is no data on risk and safety. Also, few studies evaluate the claims of patients, while thousands of Canadians continue to seek relief from an array of symptoms — pain and inflammation — from migraines to multiple sclerosis, cancer to epilepsy.

But that is about to change. A Canadian consortium led by Montreal-based pain expert Mark Ware is to launch a massive, longitudinal study of medical marijuana, tracking the long term effects of patients using it...