Vitamin D3 (cholecalciferol) is typically absorbed more readily than vitamin D2 (ergocalciferol). Some vitamins are toxic in high doses, including niacin (B), cholecalciferol (D) and tocopherol (E). The prototypical secosteroid is cholecalciferol (vitamin D). Vegan sources of vitamin D include ergocalciferol and synthetic or plant sources of cholecalciferol. Vitamin D (cholecalciferol) is produced industrially by exposing 7-dehydrocholesterol to UVB light, followed by purification. Previtamin D is an intermediate in the production of cholecalciferol (vitamin D). This means that they get less sun and therefore produce less cholecalciferol via UVB radiation. Today, Sorexa CD contains a 0.0025% difenacoum and 0.075% cholecalciferol combination. For possum control, cholecalciferol has been used as the active ingredient in lethal baits. The enzyme responsible for this, cholecalciferol 25-hydroxylase, was isolated by Michael F. Holick in 1972. It is then converted from cholecalciferol to calcifediol in the liver. As of 2019 the active ingredient in all bait stations has been changed to cholecalciferol. As a pharmaceutical drug it is called cholecalciferol (USAN) or colecalciferol (INN, BAN). Cholecalciferol (vitamin D) and ergocalciferol (vitamin D) are used as rodenticides. Like cholecalciferol, Ergocalciferol is inactive by itself.