Salicylic acid molecule    

Salicylic Acid (SA) is a β-hydroxyacid found in willow bark and sweet birch.

It is also a monohydroxybenzoic acid, a type of phenolic acid and a phytohormone (a plant product that acts similar to a hormone and regulates cell growth and differentiation).

SA functions as a desquamating agent that penetrates and dissolves the intercellular matrix of the stratum corneum.

It is commonly used in anti acne treatments as well as psoriasis, keratosis pilaris and warts.

It can cause hyperpigmentation on darker skin types (Fitzpatrick IV, V, VI) and when not used in combination with sunblock.