Lash serums are similar to hair serums; they create fuller, longer lashes. The formulas stimulate lash growth, and some help condition your lashes. Most serums have concentrated formulas, and you apply the serum to the upper lash line. You have to use them properly to get the best results, but those results take time. Consistency and patience are the keys to seeing results from lash serums. Typically, you must wait for at least a month to see any noticeable results.