B vitamins are water soluble, so they are generally relatively safe to take in large amounts because they don't accumulate in the body, so you can't poison yourself like you can with some things. Some water soluble vitamins are associated with kidney stones if taken in high doses, so "unlikely to poison yourself" does not mean zero side effects.

I am seeing conflicting reports on B1 aka thiamine. If you don't have a compelling reason to try megadoses, I wouldn't do it. There's a lot we don't know.

But for many things, it's completely reasonable to increase your consumption of B vitamins. There's a lot of space between do nothing and megadoses that is safer to play in than skipping straight to megadoses.

/not a physician and don't play one on TV.