When used as directed, the products should not cause any physiological effects. However, every person is different and if you are concerned due to sensitivity, we recommend that you conduct a patch test as follows: For lashes, start with clean, dry eyelids. Apply a small amount of serum to the corner of one of your upper eyelashes. Dab the serum on the outer corner like you would a liquid eyeliner. For brows, start with clean, dry eyebrows. Apply a small amount of serum to one half of one brow. For hair and skin, follow the same suggestions. If after 24 hours you do not experience any redness or irritation, begin introducing the product as recommended.