I have never found any papers in 10 yrs showing sublingual is a good way to provide B12 in supplement form. The oral mucosa is a very small surface area and the molecule needing absorption is very large.

If people insist on doing this, the only harm is causing some inflammation or reaction of the mouth membranes to the tablet and its other contents. If the pH of the tablet is acidic, then erosion of dental enamel may occur as well.

There ARE however many papers evaluating ORAL vs injectable forms and the oral has shown repeated success and value.

more here:
http://en.wikipedia.org/wiki/Vitamin_B12

B12 remains a sublingual because historically that is how it began orally. Now we know orally works (especially if used on an empty stomach). This is common in medicine...historical use. Magnesium oxide now known to not be absorbed at all orally is still prescribed by doctors for low magnesium levels, because historically it was the only magnesium supplement for a long time. Another remaining historical use is D2 on RX...which has been found about 80% less effective than OTC D3 in correcting D deficiency. But doctors continue to prescribe it anyway.

If anyone can find a good study showing sublingual B12 is better than oral, I'd like to see it.
There are many papers on PubMed now concerning ORAL administration, and its usefulness.

I chew up my tablets (which are cherry flavored) and swallow on an empty stomach. My tests showed a very robust response to my trial of the new Puritan's methylcobalamin. (level 1999 after 3 months of treatment--this is the top of the testing range and US labs typically cannot read higher).