Salicylic acid (from Latin salix, willow tree) is a lipophilic monohydroxybenzoic acid, a type of phenolic acid, and a beta hydroxy acid (BHA). It has the formula C7H6O3. This colourless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin. In addition to serving as an important active metabolite of aspirin (acetylsalicylic acid), which acts in part as a prodrug to salicylic acid, it is probably best known for its use as a key ingredient in topical anti-acne products. The salts and esters of salicylic acid are known as salicylates. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.
In plants, salicylic acid is biosynthesized from the amino acid phenylalanine. In the industry, sodium salicylate is prepared by treating sodium phenolate with carbon dioxide at high pressure (100atm) and high temperature (390K) - a method known as the Kolbe-Schmitt reaction. The acidification of the product with sulfuric acid then gives salicylic acid.
Salicylic acid as a medication is used most commonly to help remove the outer layer of the skin. As such, it is used to treat warts, psoriasis, acne, ringworm, dandruff, and ichthyosis. Similar to other hydroxy acids, salicylic acid is a key ingredient in many skincare products for the treatment of seborrhoeic dermatitis, acne, psoriasis, calluses, corns, keratosis pilaris, acanthosis nigricans, ichthyosis and warts.
Salicylic acid is used in the production of other pharmaceuticals, including 4-aminosalicylic acid, sandulpiride, and landetimide (via Salethamide). Salicylic acid was one of the original starting materials for making acetylsalicylic acid (aspirin) in 1897. Salicylic acid is also used as a food preservative, a bactericidal and an antiseptic.