don't think the sublinguals absorb much.

The whole technology for sublingual is very small. You notice there are
not many things available that way. Nicotine and nitroglycerin are the basic
only ones.

OTC products like we see for B12 don't have to provide bioavailability data
to sell them. Because B12 is a huge molecule, and the mouth surface is small,
I don't expect much gets in that way. I think most is dissolved in the saliva and swallowed. So just see that you take it on an empty stomach for best results.

I think the "sublingual" selling point is to make B12 seem more spiffy, increase patient believeability.

You know many decades ago there were buccal testosterone tablets for men.
They didn't work and were taken off the market.
We don't see other drugs that have minute doses used under the tongue.
Birth control pills for example.
Nitro is under the tongue for immediate angina treatment, but is very short acting and has side effects (it bypasses the liver that way). There are oral forms for longer duration. B12 doesn't need to bypass the liver, (this is called first pass--everything from the GI tract goes thru the liver first ) because the liver is where B12 is stored and converted. Studies have shown that oral swallow gives the same benefits. So I think the sublingual is just a marketing ploy to look "spiffy".