I know that vitamin d is not a vitamin, but a secosteroid, what are the side effects of using this?

Not exactly.: Vitamin d actually has a dual identity; as a vitamin it helps regulate calcium metabolism, bone growth and mineralization. Structurally it is a secosteroid, which means it is metabolized to calcitriol, which in turn helps regulate a large number of genes, many of which are responsible for bodily repair and maintenance. A secosteroid is not a steroid, and does not produce steroid-like side effects.