It is disclosed a packaging material for a pharmaceutical product. The packaging material comprises a colored element which at about 38° C. discolors partially, revealing a marking. The element is formed by a first portion forming the marking printed with conventional ink, and a second portion printed with thermochromic ink. The thermochromic ink is colored below about 38° C. and becomes colorless when this temperature is reached or exceeded. The two portions are arranged so that the first portion is invisible below about 38° C., but becomes visible when this temperature is reached or exceeded.