In 1829, researchers discovered that willow bark contains salicin, an active ingredient that is similar to aspirin, however willow bark does not cause damage to the liver the way that aspirin can. Some research has shown that willow bark is less likely to cause gastrointestinal side effects like ibuprofen and other nonsteroidal anti-inflammatory drugs. This is why it is used for pain stemming from headaches, muscle pains, cramps, rheumatoid arthritis, osteoarthritis, gout and some forms of spine disease.