Colorado parents are being told to be on the lookout this Halloween for marijuana-laced candy, NBC News reported.

The Facebook page of the Denver Police Department is warning that mass-produced candy can by sprayed with hash oil, and once the spray dries, there is no way to tell that the candy has been infused.

The station reported that the Denver PD Facebook page features Patrick Johnson, owner of marijuana shop Urban Dispensary. In the film, Johnson urges parents to inspect any candy their children bring home after trick-or-treating on Halloween to make sure it hasn’t been tampered with.

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According to Children’s Hospital Colorado, accidental exposures of marijuana products to children in Colorado have increased in the past three years, based on the rate of emergency department visits and admissions at the hospital

“Since 2005, states that allow some form of legal marijuana have seen a 30 percent annual increase in calls to poison control centers for marijuana ingestion, relative to a 1 percent increase in non-legal states,” the hospital states on its website.