When willow bark is ingested, 80% of salicin content is absorbed. Once absorbed, the salicin is metabolized by the intestinal flora to saligenin. This is further metabolized by the liver to salicylic acid
Overdose from high quantities of salicin can be toxic, damaging kidneys, causing stomach ulcers, diarrhea, bleeding or digestive discomfort. Some people may be allergic or sensitive to salicylates and suffer reactions similar to those produced by aspirin.
The salicin inside willow bark works the same way as aspirin, by reducing inflammation and pain as it enters your bloodstream. Because of the anti-inflammatory properties of willow bark, it may be especially effective in combatting joint pain as well.