As for making your hair shiny -- another often-suggested use -- "it might have an effect on the hair cuticle, closing it," she said, "but I don't know that I would use it in the hair." After all, coconut oil works better, she said.
Egg whites: Egg whites can provide a temporary tightening effect, a little relief for oily skin, Day said. But they come with a risk.
"You have to be careful with the egg white mask because egg whites sometimes have salmonella, and if you end up ingesting it by accident, you can actually get salmonella," she said. "So these days, unless you know the source of the eggs, I would be very careful with that one."
Spices: If a DIY facial scrub recipe calls for cinnamon, use it at your own risk, Day said. "I think that would be irritating. You wouldn't get enough of a concentration of cinnamon and you can probably even get blisters," she said. "It's a spice. If you put pepper on your skin, you can burn your skin."
But your skin can benefit from spices in your food, she said. Turmeric is anti-inflammatory, she said, and she often suggests adding it to meals.
"But it will stain your skin orange and you won't get enough absorption from using it on your skin to get the benefit," she said. "Over-the-counter products that contain turmeric use turmeric extract, and those are better on the skin."
Citrus: Citrus fruits, like lemons, can irritate skin, Day said. So if you're looking at a beauty recipe that calls for rubbing orange juice on your face or lemon wedges on your lips, stop reading. (Lips don't have oil glands, so they're especially sensitive," Day said.)
"Lemons have a chemical called psoralen, and the psoralen makes you exquisitely sensitive to light. It activates in about 10 to 15 minutes, and it takes about 24 hours to wear off. So if you do that, and go out in the sun, you can actually blister," Day said. "I see it on people at the beach if they're having a Corona or a margarita," she said. "Because they squeeze the lemon and get a rash on the back of their hand. It's the splatter pattern of how they squeezed the lemon, and the sunburn effect."
Hydrogen peroxide: Like lemons, peroxide is often suggested as a home remedy for lightening hair color. But Day warns against it.
"It can bleach, but it can irritate," she said. "Peroxide is toxic to skin cells. So if you have a wound and you keep putting hydrogen peroxide on it, it won't heal." Only use it on the first day of your injury to clean a cut or a wound, she advised.