Originally Posted by Chairman Pow
Or perhaps the membrane under the tongue is thin enough to absorb stuff directly?
Correct- sub lingual adminstration is quicker than gastric and as quick as suppository. In both sub-lingual and rectal form the medicine passes through thin skin directly into inerstitial fluid surrounding venul;es and arterioles and hence straight into the blood supply. Additionally it also bypasses what is called first pass elimination- everything going into the stomach and absorbed into the bloodstream there goes directly to the liver which may work frantically to get the chemical out of the blood stream on its 'first pass' through the liver.
Glyceril Tri-Nitrate is given sublingually- fast adminsitartion and misses first pass, therefore going directly to the heart and having a quick and major effect there.
Note: Chewing tobacco works in the same way!