Treatment of genetic diseases which impair the absorption or utilization of specific vitamins may require megadoses of the vitamin throughout one's lifetime.
Pernicious anemia, homocystinuria, and biotinidase deficiency are three examples of genetic diseases which are treated with megadoses of vitamins.
In essence, that's vitamin-fortified urine you're letting out, which explains why paying good money on megadoses of vitamins is foolish but not really harmful.
Most B12 warnings arise when vitamin B12 is taken in megadoses that greatly exceed the recommended daily allowance of vitamin B12.
There is some association between megadoses of vitamin B12 and certain cancers.