Const. Melissa Cliche of Quebec City police said a customer had ordered the prescription for her son at the store's pharmacy counter and then dropped it on the way out.
Another client found the medication on the floor and put it on the counter, next to a candy basket. "So, unfortunately, we don't know how, an employee just mixed it with the candy by accident and distributed it to the kids," Cliche said.
Seven pills, each wrapped in an individual blister package, were given to trick-or-treating children.
Police said the pills aren't dangerous, but if ingested they could produce minor side effects such as trembling, nausea and drowsiness.