Skin treatment should be approached the same way as exercise. You don’t run a marathon the first day out on the track and neither do you start a complete new skin care regimen all in one shot. Use one new product at a time until you are sure that you have no adverse reaction to it; then introduce another. That way, if you do react, you’ll know the culprit. Wait a week between each new product, just to make sure. You can also use this trick to narrow down which product is causing your skin problems—just start from scratch, one product at a time.