After one of his patients broke out in hives and had trouble breathing after drinking marijuana tea, Dr. Gordon Sussman began asking his patients about their marijuana use, and recruited 17 people for a study. Sussman, acting division director of allergy and immunology at the University of Toronto, tested each patient with a skin-prick test using marijuana extracts.

All 17 developed skin reactions and other allergy symptoms; 15 experienced rhinitis and wheezing and 13 developed hives. “People have thought reactions were due to contaminants like mold, but I think marijuana itself is an allergen,” says Sussman.

He is writing up his study for publication; next, he’ll head to the lab in the hopes of isolating the allergen in the plant.

“With the use of medical marijuana, and with marijuana use just generally more widespread, I think people should be aware of this allergen,” says Sussman. “It’s not as uncommon as people think.”