Stark S, Sachse R, Liedl T, Hensen J, Rohde G, Wensing G, Horstmann R, Schrott KM. Vardenafil increases penile rigidity and tumescence in men with erectile dysfunction after a single oral dose. Eur Urol. 2001 Aug;40(2):181–8; discussion 189. doi: 10.1159/000049770. PMID: 11528196.