"When trying any new skincare product, there's always a risk that you could react negatively to an ingredient. To prevent a full-blown catastrophe, be sure to patch test new products before you incorporate them into your routine. To test, apply the new product to your test area, and watch it over the next twenty-four to forty-eight hours to see how your skin react. The best place to test are behind your ear or no the underside of your forearm.
Some people will react fairly immediately to a product, while others take a bit longer to show negative effects. With so many variables at play, it's wise to patch test for at least a couple of days before committing a product to your entire face.
ONE PRODUCT AT A TIME
It's also best to introduce only one new product at a time. If you introduce multiple new products at once and have a bad reaction, you'll have no way of knowing which product is the culprit! Try to space your new product introductions apart by at least a week"
Excerpt from "Korean Beauty Secrets" Kerry Thompson