Q&A: Are megadoses of B vitamins dangerous?

I take a multivitamin containing 1,000 to 3,000 percent of the daily value of vitamins B12, B6, thiamin, and riboflavin. Are such doses dangerous? —A.R., Riverside, Calif.

Probably not, but for healthy people there’s little or no evidence that megadoses of those B vitamins are useful, either. Only B6 has been linked to a possible side effect, nerve damage. And that was at higher doses—7,000 percent of the daily value (DV), taken long-term—than you’re taking. Megadoses of B12 appear to be safe, since studies of pernicious-anemia treatment with injections of more than 40,000 percent of the DV haven’t reported any side effects. Thiamin and riboflavin also appear harmless because the body absorbs only a fraction of the huge doses; the rest is excreted in the urine.

Read more on getting vitamins and nutrients from the foods you eat, and for more on the safety and effectiveness of B vitamins, take a look at our Natural Medicine Ratings (subscribers only).