Ergocalciferol

Ergocalciferol (vitamin D2) is typically used topically at very low concentrations and functions as a lipid-soluble vitamin rather than an exfoliating or pH-dependent active, so it has a low inherent irritancy profile in patch-testing experience. Reported adverse events are uncommon and more often relate to the overall formula (solvents/penetration enhancers) than the molecule itself, but compromised eczema skin can still react to almost any additive. For patient safety in highly sensitive populations, I rate it as very gentle rather than inert. Safety Notes: Ergocalciferol (vitamin D2) is rarely used in mainstream skincare and, when present, is typically dosed at ultra-low levels as a label-active in leave-on creams/serums, often in the ~0.000001–0.0001% range due to potency and stability considerations. The lowest observed levels in commercial products can be around 0.0000001% (trace-level additions where the INCI is present but the dose is near the minimum practical for manufacturing), while the highest consumer-available “high-strength” leave-on products generally top out around ~0.01% (100 ppm), beyond which formulators more commonly use cholecalciferol instead and/or encounter solubility/oxidation and regulatory/claims constraints. Rinse-off usage is uncommon and typically at similarly trace levels because meaningful skin delivery is unlikely in short-contact formats.