Polyphenols are of special relevance for the multiple mechanisms of action of the willow bark extract STW 33-I (Proaktiv®) - Kelber, O; Abdel-Aziz, H; Elstner, EF; Fiebich, BL; Heinle, H; Jaeggi, R; Khayyal, MT; Metz, J; Müller, J; Okpanyi, SN; Weiser, D (Planta Medica, 2006)