Doctors prescribe both natural and synthetic retinoids -- derivatives of vitamin A -- to treat cancer, eye diseases and skin diseases. Additionally, retinoids are popular topical anti-aging treatments for skin. Retinoid products have been classified into four generations, each increasingly potent.

Three types of retinoids fall under first-generation classification: alitretinoin, isotretinoin and tretinoin. Panretin is the brand name for alitretinoin gel, which is used to treat Kaposi's sarcoma. Brand names for isotretinoin include Accutane, Roaccutane, Accure and IsotrexGel. Doctors prescribe isotretinoins as oral medications for severe acne. Tretinoin, also known as Renova, Retin-A and Vesanoid, is a topical treatment for acne, wrinkles and darkened or damaged skin. It increases cellular turnover in problem areas of skin to make it smooth and uniform.

Second-generation retinoids treat psoriasis, psoriatic arthritis and icthyosis. Psoriasis and ichthyosis cause rough red patches and accumulation of scales on skin. Psoriatic arthritis is a rheumatoidlike arthritis. Etretinate, known as the brand name Tegison, and acitretin, brand name Soriatane, are second-generation retinoids taken orally as treatment.

Third-generation retinoids include tazarotene and adapalene. Tazarotene is sold as Tazorac, Fabior or Avage for use as a topical treatment for acne, psoriasis and skin discoloration. Adapalene is a topical gel known by the brand name Differin and its prescribed to treat moderate acne. Targretin is the brand name for bexarotene, a fourth-generation retinoid used to treat cancer and seletinoid G is the newest synthetic fourth-generation retinoid.