Why are stability studies a hurdle-race? There are many reasons why cosmetic formulations are prone to modify their composition as time goes by.4 Firstly, being a linear combination of different categories of substances, all cosmetics are a potential reaction pot. In theory, many chemical reactions could potentially take place without being easily foreseen.
Moreover, the concentration scales of components are spread over a wide range (i.e., from 101% to 10-4%), so the consequences of reactions might be striking or hidden.
Furthermore, while some ingredients are pure substances and have clearly defined compositions, some others are simply blends comprising large amounts of co-products. Others have mostly unknown compositions (e.g., many vegetal extracts, perfumes) at least for the formulator of finished cosmetics. In this last example, many labile chemical classes are included (e.g., aldehydes, esters, phenols). It must also be added that impurities may play a frequently hidden role in chemical reactions and skin compatibility.