Since I have taken vitamins on and off for most of my life with positive results, I'm not too worried about long term affects in general. HOWEVER, I do think there is great value to cycling your supplements, and not constantly loading your system with a high concentration of one substance. I think its important to 'take a break' from things occasionally...
So to answer your question, I do think about the long term effects of taking a high concentration of one isolated substance (like biotin), and my way to mitigate any adverse affects is to not take it all the time, or cycle off for periods of several weeks. Other vitamins, like multi's and vitamins like C, I think are safe to take everyday.
