Omar Benson Miller -- Halloween Candy Can Be Deadly ... Just Ask My Bro

Some urban myths about dangerous Halloween candy are frighteningly true ... according to Omar Benson Miller, who doesn't trick or treat anymore because it almost killed his brother.

We ran into Omar leaving Hooray Henry's in West Hollywood on Monday night ... and he revealed how some bad candy messed up his younger sibling and several other friends when they were younger.

Watch ... it's a scary story, and the ex "CSI: Miami" star, and travel show host for Esquire TV, even explains why he still doesn't celebrate Oct. 31.