Sildenafil Citrate Imp.G(EP):5,5'-[1,4-Piperazinediylbis[sulfonyl(6-ethoxy-3,1-phenylene)]]bis[1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo[4,3-d]pyrimidin-7-one](Piperazine dimer impurity).