A true “medicine tree,” the use of willow bark dates all the way back to the days of Hippocrates (400 BC) when his patients chewed the bark to help with fevers and inflammation. It’s a natural source of salicin, which is chemically similar to aspirin (and actually served as the inspiration for its invention in the 1800s). Cosmetically, salicin is also similar to synthetic salicylic acid, promoting exfoliation and smoothing of the skin – without the irritation often associated with its synthetic counterpart. All the good. None of the bad.