The misconception that water soluble vitamins such as the B vitamins are harmless at high doses is far from the truth. Though they may not reach toxic levels in the body since they are flushed out by the kidneys, the added work load that they produce is what could be causing the harm to the kidneys themselves.
Canadian researchers studied 238 people with chronic kidney disease caused by type I or II diabetes. High doses of folic acid, B-6 and B-12 were given to see if the vitamins could protect the kidneys by lowering blood levels of homocysteine. After 3 years, the patients receiving the vitamins showed a greater decrease in kidney function than those who received the placebo. Also, more patients taking the vitamins had a cardiovascular event (such as a heart attack or stroke) than the placebo takers.
When it comes to vitamins, play it safe. More is not better! Stick to the recommended amounts. A typical multivitamin contains 400 mcg. of folic acid, 2 mg. of B-6, and 6 mg. of B-12.
~ Journal of the American Medical Assoc., 2010