A flexible self-adhesive label comprising a radiation-sensitive indicator for a syringe, the syringe comprising a barrel part with a front part and a rear part, having a tip at the front part, a piston at the rear part, and an exterior surface with a marking thereon. The label is attached to at least one part of the syringe and does not overlap with the marking. Also disclosed is a labeling method, kit and dispenser featuring such labels.