Bendich: The U.S. Public Health Service recommendation is that all women of child bearing potential consume 400 micrograms a day of folic acid for the purpose of preventing neural tube defects. There are no data that indicate that this level of folic acid provided through multivitamin supplementation increases the risk of masking vitamin B-12 deficiency. In fact, a recent report showed that elderly who routinely took a multivitamin, which usually contained both folic acid (400 micrograms) and vitamin B-12 ( at 6 micrograms) had a significantly reduced risk of low vitamin B-12 status. Moreover, vitamin B-12 deficiency is rare in women of child bearing potential and the level of vitamin B-12 in the multivitamin would probably help ensure against vitamin B-12 deficiency in young women as well as other age groups.
I continue to recommend folic acid-containing multivitamin supplements as the most practical, inexpensive, and safe source of folic acid for all women of child bearing potential. Another important source is fully fortified breakfast cereals containing 100 percent of the daily value (DV) of folic acid and other essential vitamins and minerals..
I do not recommend that women choose a single supplement of folic acid alone because of three major reasons: first, the research showed added benefits with the multivitamin supplement, such as reduction in the risk of other birth defects in addition to neural tube defects. Second, many young women have numerous marginal deficiencies, including iron, vitamin E, and vitamin B-6 to name a few. The multivitamin can help to eliminate these deficiencies. Third, if a woman is going to change her daily habits and begin to take a "pill" every day, then wouldn't it be best that the "pill" include all the essential vitamins and minerals?
My answer is Yes!
Passwater: Does vitamin C cause kidney stones?
Bendich: Several studies have found no evidence that vitamin C increases the risk of kidney stone formation. 13-17] Most kidney stones are composed largely of calcium oxalate, and urinary oxalate levels are used as a marker for kidney stone risk. Recently, Dr. Theodore Wandzilak and his team at the University of California at Davis investigated the claim that vitamin C increases urinary oxalate levels. Previous studies suggesting that vitamin C may increase urinary oxalate levels have been flawed because vitamin C interferes with most of the previous methods used to measure urinary oxalate levels. Dr. Wandzilak's group used a new ion chromatography procedure in which vitamin C does not interfere. Their data show that the ingestion of increasingly large quantities of vitamin C did not cause an increase in the urinary excretion of oxalate. They concluded, "Therefore, the safety concerns raised about increased urinary oxalate level and, as a consequence, an increase in kidney stone formation in healthy subjects are not supported by our findings." 
Passwater: That information, plus the new evidence that calcium does not increase kidney stone formation, shows that the conventional wisdom is not always correct, and that tests must be conducted before valid conclusions can be made. Does vitamin C destroy vitamin B-12?
Bendich: In 1980, Dr. M. Marcus showed that an earlier report suggesting this possibility was due to an artifact of the analytical procedure. 
Passwater: Do people have rebound scurvy when they skip or stop taking vitamin C?