Willow bark is actually quite similar to a common OTC painkiller because it contains salicin, a close relative of acetylsalicylic acid, i.e. aspirin. People have long chewed willow bark to relieve aches and pain, lower fevers, and reduce inflammation. These days, willow bark is sold dried for use in teas, or as a liquid or capsule supplement.
Because willow bark is so closely related to aspirin, it carries similar side effects. Prolonged use may upset the stomach, increase your risk of bleeding disorders, and slow down kidney function. It is not recommended for children and should not be mixed with other painkilling medications. Willow bark is not safe if you’re taking warfarin or any other anticoagulant medicines because of its association with bleeding disorders.