Is sugar absorbed into the bloodstream through the walls of the mouth?
Simple sugars such as glucose, fructose and galactose will be absorbed.


They are most probably transported by a common carrier, which is why if you present both glucose and galactose, the presence of one will inhibit the absorption rate of the other.
Uptake rate also seems to partly depend on the concentration of sodium ions in the buccal cavity. This was identified and explored subsequently.
It seems that at the cellular level, absorption is not the quickest but there is a lot of vascularization which allows it to enter the bloodstream quickly, regardless.




Here's a back-of-the-envelope calculation: about 10mg is absorbed in 5 minutes when you hold a solution of 3.6g/l in your mouth. A normal gulp volume is around 10ml. That's 36mg of sugar in a mouthful. Result: you can absorb 10mg of sugar from a theoretical maximum of 36mg. In other words, within 5 minutes, you absorb ~30% of all available sugar. That's pretty good, given that a sugar solution is not a more concentrated, solid sugary food!

Worthy to note that most complex sugars will not be absorbed. Note that many complex sugars actually won't be uptaken even in rest of the GI tract, like cellulose. 

See other posts already here on the topic.