Transdermal therapeutic systems (TTS) whose structure comprises a polymer matrix containing active substances, and a backing layer, and which contain at least one light-sensitive active substance are characterized in that the said TTS contain at least one colourless substance which absorbs light in the UV region, does not have any inherent pharmacological action, is dispersed or dissolved in the polymer matrix of the TTS, and/or is homogeneously distributed in the backing layer of the TTS.
