Answer: essentially, aspirin. The actually medicine, is "salicin", which metabolizes into salicylic acid in the human body, which is very similar to aspirin, or acetylsalicylic acid (produced "artificially"); as such, the medicine functions as an NSAID similar to aspirin (e.g. blood thinner, anti-inflammatory (even for the willow plant itself), and chewing on the bark, or taking willow supplements, can help with headaches, back pain, joint pain, and can help people survive heart attacks.