The warning comes after a nurse found the blade hidden inside a party bag in preparation for children trick-or-treating on Halloween.
Peter McDougall, 28, who works at Royal Cornhill Hospital in Aberdeen, couldn't believe his eyes when he saw the knife wrapped up with one of the marshmallows inside.
He said: "I had bought the sweets as part of the collection I have for when the kids come round chapping on the door at Halloween.
"But I hadn't had any breakfast one day so I was hungry. I ate one of the marshmallows and I was about to have another one when I saw the blade.
"It's like a hacksaw blade, something similar to a stanley knife, that was inside the wrapping with the marshmallow.
"I was shocked and mortified when I saw it - who knows what could have happened if a child had opened it."
Peter, who bought the packet from Poundland's Berryden branch in Aberdeen, said: "It's longer than my finger. You can see all the rust in there. It looks like it's come off some machinery or something.
"I want parents to be aware that they need to double check what's in these bags of sweets when their children go trick-or-treating."
A spokeswoman for Poundland said: "An investigation is currently taking place into its Witches' Broomsticks sweets following a complaint from a customer that they had found a machinery blade inside.
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