What pain killing chemical does the inner bark of a white willow tree contain?

White willow (Salix Alba) is a species of willow, native to Europe and western Asia. Its name derives from the white undertone beneath its leaves.

The white willow is widely known as a source of salicylic acid, the precursor of one of the world’s most commonly used painkilling drugs, 'Aspirin', also known as acetylsalicylic acid (ASA). The medicinal power of white willow lies in its inner bark, which has been used therapeutically for several thousand years as 'Nature's Painkiller'.

Willow bark has been used throughout the centuries in both the West and the East. It continues to be used by herbal practitioners for the treatment of headaches, toothaches, and back pain, muscle and joint inflammation, pain and stiffness, as well as for conditions such as sports injuries and gout.

White willow bark causes few side effects and may be preferrable to aspirin type anti-inflammatories in conditions such as osteoarthritis that require long term use.