It is true that certain vitamins can be harmful for you if taken in excessive amounts. This is particularly true for the fat soluable vitamins (A, D, E and K). This is due the fact that they will build up in the fat tissue and potentially to toxic amounts if consumed in too great of quantities. One must be careful with dosing particularly with vitamin A and vitamin D as these have the greatest potential for toxicity amongst the fat soluble family. I remember an interesting statement about this….”Make sure you never eat polar bear liver”. Sound odd but because polar bears eat so much fat from animals they concentrate huge amounts of Vitamin A in thier livers. So much, that if you ate polar bear liver you would die due to Vitamin A toxicity.
Now lets get to your question about B-vitamins. These are thought to be amoung some of the safest vitamins with relativly low potential for toxicity. Vitamin B6 can be toxic to the nerves if taken at over 300mg per day and Vitamin B3 (Niacin) can cause an upleasant reaction of flushing if taken in too high a dose although sometimes this is necessary for it to have theraputic action.
B12 is commonly given in doses much higher than the RDA (2.4 mcg (micrograms)) without known toxicity. Most B12 supplements will provide at least 2,000 mcg of B12 and some deliver as much as 5,000mcg. There has been no scientific evedence demonstrating any significant toxicty when given at this level. Currently no tolerable upper limit of B12 has been set by The Food and Nutrition Board indicating this lack of toxic effects. If you are considering taking a basic B12 supplement you should have nothing to worry about in regards to toxicity.