But she didn't have cancer. She had an obscure syndrome called cannabinoid hyperemesis syndrome, a condition only recently acknowledged by the medical community. It affects a small population — namely, a subset of marijuana users who smoke multiple times a day for months, years or even decades.

There's no hard data on the prevalence of the illness. But in California and Colorado, which have loosened marijuana laws in recent years, some emergency physicians say they're seeing it more often. One study in Colorado suggests there may be a link.