hi sir/madam, Thanks for your question on Healthcare Magic. An allergic reaction to vitamin D causes the immune system to develop a defense against the supplement because of a hypersensitivity to the vitamin. The immune system reacts by creating Immunoglobulin E, or IgE antibodies, that attack the vitamin supplement, according to the American Academy of Allergy, Asthma and Immunology. Various other chemicals are released in the body, causing inflammation in the throat, lungs, sinuses and skin. The primary chemical that causes inflammation is histamine, a hormone that helps fight off infection. After ingesting a vitamin D supplement, hives can develop within minutes or up to two hours. Hives can occur because of too much histamine released in the skin. The rash appears as welts that appear in clusters anywhere on the body. The rash causes the skin to swell and has defined borders, although the rash can appear in any shape or size. Eczema is a chronic skin disorder where the skin is hypersensitive to various triggers, much like an allergy, according to MedlinePlus. If you experience an allergic reaction to vitamin D and you’re pre-disposed to eczema, the allergic reaction can cause an eczema flare-up. Eczema appears primarily behind the legs, arms and on the face in blisters. The rash can crack, weep and crust-over if scratched. Consult your GP for further treatments. hope this was helpful. have a healthy day.