Colorado, one of four US states where marijuana is legal, has an increase in the number of cases heading the emergency room for smoking or consuming too much weed.
Twice as many out-of-state visitors wound up in the emergency room for marijuana-related symptoms in 2014 -- the year Colorado legalized retail sales of the drug -- compared to two years earlier, said the report in the New England Journal of Medicine.
Researchers recorded 163 marijuana cases per 10,000 visits by people from out-of-state in 2014, up from 78 per 10,000 visits in 2012.
The study was based on visits for marijuana-related symptoms at the emergency department of UC Health's University of Colorado Hospital.
"Anecdotally, we noticed that most out-of-towners were in Colorado for other reasons, such as visiting friends or on business," said lead author Howard Kim, an emergency medicine physician at Northwestern Medicine.
"They ended up in the ER because they decided to try some marijuana," added Kim, who began the study when he was a resident at the University of Colorado School of Medicine.
"This may indicate that out-of-state visitors are unprepared for the adverse effects of marijuana use."
Side effects of cannabis can include anxiety, hallucinations, high blood pressure, heart palpitations, stomach pain and vomiting.
Eating too much edible marijuana products, like cookies or brownies, can also lead to adverse effects.
"People eating marijuana products often don't feel any effect immediately, leading them to eat another edible," Kim said.
"Then they've ingested multiple products, so when the effect finally kicks in, it is much stronger."
Most of the cases were mild. Patients were given "supportive care and went home after a few hours, but some were admitted for further observation," the study said.
Researchers said other states in which recreational marijuana is legal -- including Alaska, Oregon and Washington -- may be experiencing similar spikes in emergency room visits.
Senior author Andrew Monte, assistant professor of emergency medicine at the University of Colorado School of Medicine, urged other states considering legalization to adopt pre-emptive public health education campaigns.