Setting your makeup with cornstarch is a bad idea, but you can use it for other skin ailments.
Recently, a beauty video surfaced that suggested that you bake (set your makeup) with cornstarch. Turns out, according to a dermatologist that interviewed with Cosmo, baking your face with cornstarch is actually a terrible idea because it's "food for yeast that lives on your skin." that's derm-talk for this could lead to serious breakouts and other skin issues.
You can't use it for setting your makeup, but you can use it to treat a few other skin ailments like itchy skin, athlete's foot and bug bites. Simply mix cornstarch with cold water to form a thick paste and apply to affected area with a cotton swab and let it work over night; rinse with warm water in the morning.