Retinoic describes anything relating to retinol, the alcohol form of vitamin A, and most precisely refers to retinoic acid, the oxidised metabolite through which vitamin A exerts its profound effects on the body. Retinoic acid acts as a signalling molecule, switching genes on and off to direct cell differentiation, embryonic development, vision, and immune function. Because it is so potent, the body generates it locally in tiny, tightly controlled amounts from dietary retinol or provitamin carotenoids. In medicine, retinoic acid derivatives treat severe acne and certain leukaemias, but they are teratogenic, so their use in pregnancy is strictly forbidden. The term reminds nutritionists that vitamin A’s power lies not in the vitamin itself but in what the body makes from it.