The term retinoid captures a whole class of chemicals, all sharing one common structural motif (Figure 1, top) with many variations.

Figure 1: Top: the generic structural motif of retinoids; middle: the retinal molecule (involved in the vision process; retinal and some of its derivatives are embedded in a large protein molecule in the eye (rhodopsin); when exposed to visible light, the conformation of the double bonds in these small molecules changes and provides a ‘byte’ of information in the visual process); bottom: β-carotene (a precursor of vitamin A, found in the pigments of many colourful fruits and vegetables).

Apart from the natural role of retinoids in the vision process, as precursors for vitamin A, as agents in cell differentiation and proliferation (including bones) and an involvement in the regular function of the immune system, there are several uses of retinoids in medicine or as a component in cosmetic products.

Topical retinoid preparations are used to treat some skin conditions such as severe acne or psoriasis (dry, scaly skin patches), and are a component in some ‘anti-aging’ lotions and potions. Retinoids in such preparations generate superficial inflammation of the skin and activate collagen production which inevitably smooths out some fine lines and superficial wrinkles. For the effect to persist, the treatment has to be continuous (but may also cause severe skin irritation). Vitamin A and β-carotene, along with further retinoids and other substances, have been suggested as ‘chemoprevention agents’ to reduce the transformation risk of some persistent oral, pre-malignant lesions (for example, oral leukoplakia) into oral malignancies. The evidence for the efficacy of such agents is poor.