When I was growing up there were urban legends of evil people slipping razor blades into apples to give to kids on Halloween. Yes, kids, back in those days razor blades were paper-thin pieces of metal rather than plastic cartridges with multiple blades that cost more than a gallon of gas, and people actually gave out apples as candy. But as far as can be determined by people who've looked into it, the razor-blade-in-the-apple thing never actually happened. Nor have there been any child deaths or serious injuries caused by strangers poisoning or otherwise doctoring Halloween candy.
It's easy to see how such stories spread: People care about children and are eager to warn each other of dangers. And the fears aren't groundless. There are homicidal maniacs out there, and the rumors themselves might give some of them ideas. Besides, other things, such as medicines on store shelves, have been deliberately contaminated. There's nothing wrong with being careful as long as it doesn't go to paranoid extremes. There's nothing wrong with parents' carefully checking their kids' candy (including any apples that slip through the kids' anti-nutrition defenses).
Anyway, the latest twist on the legend is candy supposedly tainted with marijuana. Here's pediatrician Aaron Carroll MD on that subject and on supposedly tainted Halloween treats in general, and on a very real danger to children on Halloween:
More videos from the same series: https://www.youtube.com/user/thehealthcaretriage/videos
If you don't have time to watch this six-minute clip, the real danger is traffic. Children are two to four times as likely to be killed by a car on Halloween as on any other night. Reasons might include more kids roaming about, costumes that aren't easy for drivers to see, and masks that interfere with children's peripheral vision.