Vitamins have a good reputation. Many people think they can do only good, and never harm. Sadly, this is false.
Biologically, the only difference between a vitamin and a medication is that some amount of the vitamin is necessary for life. Once you go above that amount, however, it is better to think of vitamins as pharmaceuticals, endowed with the potential for both benefit and harm.
In short, despite their positive-sounding name, it is better to think of vitamin supplements as medications, with all their attendant risks.
For example, not many years ago, there was enormous enthusiasm for vitamin E’s potential to lower the risk of coronary artery disease, and many physicians began recommending vitamin E supplementation. Later research has shown no such benefit and, rather horrifyingly, has raised suspicions that vitamin E supplements increase the risk of heart failure. Even a vitamin having no known toxic effects at any dose, e.g. vitamin B12, can cause harm by obscuring the diagnosis of a disease.
Possibly excepting women who are contemplating or experiencing pregnancy, any decision about vitamin supplementation should be undertaken with the same deliberation used in recommending a pharmaceutical. Many supposedly healthy people (discussion of the term “healthy” is a topic unto itself) will indeed benefit from vitamin D or other supplements, but it is far safer to rephrase the question “Do you recommend vitamin supplements for healthy people?” as: “Do you recommend pharmaceutical medications for healthy people?”