Willow bark contains salicin, a substance that is stable only under acidic conditions. It’s a cousin to salicylic acid, but it’s not the same thing. When taken orally, salicin is converted by the digestive process to salicylic acid (beta hydroxy acid). The process of converting the salicin in willow bark into salicylic acid requires many steps and the presence of enzymes.
Willow bark extract is considered an “herbal” alternative to salicylic acid, but the likelihood that willow bark in the tiny amount used in cosmetics can mimic the effect of salicylic acid is likely impossible. On the other hand, willow bark appears to retain an aspirin-like composition and due to this may have some anti-inflammatory benefits for skin.
Vlachojannis J, Magora F, Chrubasik S. Willow species and aspirin: different mechanism of actions. Phytother Res. 2011;25(7):1102-4.