vitamin c

Vitamin C is one of the least stable ingredients in skincare, which is why the packaging around it matters nearly as much as the formula inside. Exposed to light and air it oxidizes, and an oxidized product does progressively less of what it was bought to do, so opaque or tinted bottles and small openings are doing real work rather than looking serious. Keep it closed, keep it out of a sunny bathroom windowsill, and treat a product that has turned noticeably darker or browner as a signal that it has aged rather than as a cosmetic quirk.