Is the so called Niacin-flush (temporary skin redness) an allergic reaction?

Absolutely not. Some people misinterpret the Niacin flush (vasodilation) as an allergic reaction. There is a similarity but they are not the same. The flush caused by an allergy may be due to a local release of histamine into the epithelial tissues, triggered by the presence of allergens. The Niacin-flush instead is the result of a complex cascade of chemical reactions which, apart from activating some specific skin thermoreceptors, produce substances known as prostaglandins (PGD2 e PGE2), responsible for a temporary skin vasodilation.
The production of prostaglandins is the very factor that brings about a higher oxygenation and nourishment of tissues as well as a more effective and simultaneous elimination of toxins and cellular metabolic waste in the flushed skin area. There isn’t any allergen-related reaction in this mechanism. On the contrary, it is an oxygenating, nourishing and detox mechanism, initiated by Niacin.