I find it interesting that the basis of methotrexate's success as a chemotherapy apparently was its ability to reduce a cancer tumor's uptake of folic acid (a vitamin!).
Well, yes. I think one of the first things they tell you when you've been diagnosed with tumors is the danger of vitamins.
Wow, I hope the marginal benefit to most of the population (ie those who are not very vitamin deficient) taking them is not less than the harm caused by people with undiagnosed tumors taking them willy-nilly.