Most vitamin C products are a problem for anyone with sensitive or reactive skin – ascorbic acid oxidises quickly, can cause irritation, and doesn’t absorb particularly well. What interested me about this serum is that it uses a rare lipid-soluble form of vitamin C instead. That means it bypasses most of the usual downsides: no stinging, no yellowing, stable enough to use in the morning without worrying about photosensitivity. Because it’s oil-based, the active actually reaches the cells rather than sitting on the surface. The endocrine-disruption testing is what brought me to Skin Diligent in the first place, and this is one of their most awarded products.