Azo compounds represent about two thirds of all synthetic dyes. Their usage in pharmaceutical industry has many purposes. One of the most important is coloring of pharmaceutical agents which improves their easy identification. Azo dyes often used in manufacturing of pharmaceuticals are: E102 Tartrazine, E110 Sunset Yellow FCF, Ponceau 4R (Cochineal Red A), Azorubine (Carmoisine), Amaranth, E133 Brilliant Blue and E129 Allura Red. Many azo dyes show carcinogenic and mutagenic activity, and they can provoke allergic reactions. Generally, toxicity of ingredients grows with the increase of benzene rings in their structure. Carcinogenicity of azo dyes directly depends on the structure of molecule and on mechanism of degradation. Products of degradation of azo dyes are mostly aromatic amines with different structures and they can also have carcinogenic properties. Carcinogenicity of many azo dyes is due to their cleaved products such as benzidine. Benzidine is known as carcinogen for the human urinary bladder. Except of carcinogenic and mutagenic activity, azo dyes can alter biochemical markers and they can provoke allergic reactions.