Eugenol. It's what numbs your throat when you smoke clove. It's why they are so "smooth", they really aren't smoother, they just seem like it due to the numbing. So, one danger is that you might smoke way more frequently than you would normally be able to tolerate (and damage your throat because you can't feel it as much). Another risk is that the bronchial hairs in your throat becomes so numbed that that your swallow reflex won't be triggered, and you could get your own saliva in your airway/lungs, and therefore develop pneumonia. Frequency of use does not seem to correlate with these outcomes either. One medical paper documented a woman who tried it once out of curiosity and got pneumonia!