Vit C is water-soluble, meaning any excess goes straight into the toilet. D3 is fat soluble, and there is a potential for liver toxicity at too high doses. Indiscriminately throwing mega doses of vitamins at problems because they are "natural" isn't always safe. A good multivitamin provides 100% of your daily needs, and unless you live in a third-world country where vitamin deficiencies abound due to malnourishment, the benefits of adding vitamins without being measured as actually deficient can be debated. Vitamin D deficiency due to an indoor lifestyle is very common, but megadoses of Vit D3 can do more harm than good.