A high dose of B-12 (500 micrograms a day) can prevent B-12 deficiency, and it's safe. A high dose of B-6 (possibly as little as 200mg a day) on the other hand can cause (reversible) nerve damage. Niacin (about 500 mg a day or more) is considered a drug. While it lowers cholesterol, it can cause side effects like flushing and liver damage. Vitamin D may cause side effects at levels as low as 1,200 IU a  day.