Synthetic cannabinoids are sending increasing numbers of US users to hospitals, according to research published in the July 15 issue of the CDC’s Morbidity and Mortality Weekly Report.

Researchers analyzed data from the Toxicology Investigators Consortium, a registry established by the American College of Medical Toxicology. From 2010 to 2015, US toxicologists treated 456 cases of synthetic cannabinoid intoxication. Sixty-one percent of the patients said it was the only drug they had used.