The analgesic action of willow bark depends on symbiotic or "friendly" intestinal bacteria to digest is components into painkilling forms. Aspirin does not require digestion by intestinal bacteria, and works more quickly. Willow bark, on the other hand, continues to provide pain relief longer than aspirin.

Unlike aspirin, the salicylates in willow bark do not increase the risk of bleeding. They do not usually irritate the lining of the stomach. For these reasons, willow bark may be useful for people who have chronic joint pain but cannot take NSAIDs or COX-2 inhibitors.