Hair is made up of protein, over 80 percent keratin protein to be exact.1 This is where amino acids come into play. Not only are amino acids the building blocks of protein (and your eyelashes), but they may also nourish and repair damage. This is why they’re the main ingredient in so many lash-enhancement formulas. But keep an eye out: Many brands include amino acids in their formulas that go by a different name: peptides. According to Kim, peptides “nourish, condition, and strengthen frail and brittle lashes.” Speaking of protein, collagen is the most abundant protein in the human body, and we know all about its skin and hair benefits. Because of this it is also an ingredient in many lash serums so your lashes get those same benefits. If used correctly, lash serums are safe. You should follow the specific instructions of the lash serum you are using, but typically they should be applied two to three times a day on clean lashes. Do not apply lash serum while wearing contacts. According to Frieling, if you experience any negative side effects, like itchy, red eyes, unwanted hair growth in areas the product repeatedly touches, darkening of the eyelids, reduced eye pressure, or permanently increased iris pigmentation, discontinue use immediately and see a doctor. According to Frieling, lash growth serums help grow new lashes, add thickness, length, and density. Lash conditioner, on the other hand, is for people who already have ample lashes and are looking to moisturize them. You may want to use a lash conditioner with lash growth serum to maintain and condition the new lashes that are growing. Remember to apply these formulas on clean lashes.