HP "under the tongue"?

I have read that taking any medication "under the tongue" makes it very effective as it directly goes in the blood stream. Putting it "under the tongue" is the next best thing after injection.

I dont know the basis of this .. But I have been spraying from HP under my tongue.

Does anyone know the reasoning why things are more effective "under the tongue"?? Lots of Homepathic medicins must be kept under the tongue till they dissolve.