Well, according to my own history it is actually better absorbed from the daily 5mg methylcobalamine than from the 3x/week 2mg injections I used to get (hydrocobalamine). But I agree there is a limit beyond which you basically just pee B12 .

I'd say 10mg in the morning and 10mg in the evening would probably make more sense if you want to take a high dose (but I'm not a doc etc.).

Most important I think is to have a steady supply. 5mg every day may be more valuable than taking high doses switched with not taking any for a while.

It's not likely to be toxic, but one can not be sure. It's like iron: most people are deficient or normal, so "fortified" foods were once the norm. Great when you are "normal" and potentially deadly when you have hemochromatosis. On the other hand: people who survive a house fire are often treated with up to 15 gram (yes, gram) of B12 over the course of a day (monitored obviously), so I guess it's safe to say it's safe in most cases.

Mind you, all this is based on my experience and reading over the years; there is very little hard science on these things it seems, so I might be proven totally wrong in the future no doubt. I've recently seen it happen with the LD50 of nicotine. For decades nobody ever doubted the established values, and now it appears to be off by a factor of 10 to 20.