Stief C, Porst H, Sáenz De Tejada I, Ulbrich E, Beneke M; Vardenafil Study Group. Sustained efficacy and tolerability with vardenafil over 2 years of treatment in men with erectile dysfunction. Int J Clin Pract. 2004 Mar;58(3):230–9. doi: 10.1111/j.1368-5031.2004.00128.x. PMID: 15117088.